A drunken challenge accepted means that I am in the process of developing a controller that will give the Clansman PRC344 radio a scan capability. This is pretty frivolous to say the least! - the radios 50kHz steps means many active frequencies will never be heard anyway! But, its a fun thing to try and make happen,
The idea is to develop a means of controlling the tuning of the radio in timed steps, with received signals halting the scan. Initially, the system will be 'blind', that is, entirely self contained within the radio with no functions other than to start and stop it, and to override the halt to continue scanning. But later, by sacrificing one of the radios audio connectors, and external controller could be attached for more sophisticated operation.
This is no easy task. For a start, I have to take control of the synthesiser, this means 14 control lines plus a means to break the ground connection to the frequency dials. But, I also need to override the radios RF front end tuning, which means taking control of another 18 lines, and switching over a further three! I also need to accept control inputs from 3 different parts of the radio! Plus, I need an RS232 port. All in all, that means arranging for a minimum of 38 control lines on my controller!
And all this has to not only run on 3V to be compatible with the synthesisers logic, but switch tuning voltages up to 70V! And, when not selected to scan - have zero current drain!
If all thats not bad enough, the entire thing must fit within the space left by the removal of the Tx control module!
Yet, I think I have the answer! And its all down to modern high performance microcontrollers and sophisticated logic chips. A string of resistors, a couple of 24v DPDT relays, two MOSFETs, a zener, a 3v regulator, three high voltage 8-port multiplexers, two 16-bit I2C port expanders, and a tiny little biddy PIC microcontroller! All built with SMT parts.
Just waiting on the prototyping samples to arrive. In the meantime, I have to do the daunting part that will form the heart of the system - Ive to learn to program in C...
Sunday, 27 January 2019
Saturday, 26 January 2019
QRPlabs Si5351A VFO kit
Once again today I have had the pleasure of building one of Hans Summers kits. Hans designs and sells, as QRPlabs, incredibly versatile and high quality radio kits, mostly based around a core Atmel processor control unit. Having previously built a six-band version of his Ultimate 3S QRP WSPR beacon transmitter (which can do so many more modes than that!) and the QLG1 GPS receiver to govern it, and having a need for an effective way to replace crystals in vintage military radios in order to test conversion to amateur bands, I decided to buy the QRPlabs Si5351A based VFO/Sig Gen kit.
The kit comprises a 2x16 backlit LCD module, the Si5351A DDS module, and the QCV control module. This is exactly the same as the U3S, the only difference being the firmware version of the processor chip, and some of the discrete components. The DDS module is capable of working up to at least 150MHz, and in this kit, produces two independent outputs. One output is fixed frequency, set up in presets in the firmware, the other the VFO output. The above photo shows the PCBs after populating.
This kit comes with a rotary encoder with a push button switch action. This was the only place I went astray during the build, as I managed to mix up one of the encoder lines with one of the switch contacts, resulting in some rather odd things happening when trying to test!
After a good rummage around, I came up with a metal enclosure for it. I had hoped to use a die-cast box, but all my stock were either too small or way too big! But I had this sheet aluminium box kicking about Its a slightly odd enclosure as it has the seams of one half on the outside of the other, rather than hidden inside.
What took most of the build time was the marking out, drilling and cutting of the box.
Unfortunately I havent been able to complete the build today, as I discovered that I dont have any panel mount push buttons in stock, nor any suitable stand offs to mount the unit into the case with! Ive now ordered these, but they may take several weeks to arrive!
A quick test showed all to be well. One thing I discovered though which I hadnt expected, was that the firmware includes backlight brightness control. I had already installed the link for full brightness as I didnt expect this feature, which is used in the U3S, to be in this firmware. So, that link was removed and the correct link for processor control of the brightness added.
Im so pleased with how well this has gone together that I decided I just had to build another of Hans' kits! So, since I needed to measure the QLG1 GPS units dimensions (as ive asked a work mate to 3D print me a box for it!) and this meant pulling out the U3S, I decided that what I really need is a touch more power for WSPR ... so now the QRPlabs 5W HF PA kit is on order, along with an upgraded firmware chip for my U3S, which will control the PA and also give me the ability to control an antenna changeover relay. The HF PA kit has the same PCB form factor as the rest of the kits PCBs, so effectively just 'bolts on'.
Tuesday, 22 January 2019
HT Chain issues
I have a requirement to replace the mechanical switch shown here, that selects one of 18 different HT levels 0-80v from a potential divider chain, with electronic switching controlled by a 3v logic port expander.
Im currently thinking p-channel MOSFETs for the HT switch, but these need biasing from the HT line they are on, which I think would kill the logic gates! So, I also need a driver device that will isolate the 3v logic control (MCP23017) from the HT and the switch.
Sunday, 20 January 2019
PRC-344 Alignment - Working Fully
The Clansman PRC344 is now probably as good, electrically, as it will get.
Having removed the Direct Current Amplifier module3, in order to disable the radios ability to transmit (since there is absolutely no legal way of using the transmitter, yet lots of ways to inadvertently send a signal that at best could get you prosecuted and at worse endanger an aircraft!), I could happily ignore all the alignment process relating to the transmitter.
So this evening I took the time to run through the alignment processes for the power supplies and the receiver. The only part of this that gave any problem was the AGC alignment. This required setting one control for 350mV peak waveform on the oscilloscope, and 0v on the voltmeter. However, although I could hear the receiver being affected by the adjustments, I couldnt get anything like the specified waveform levels!
*I have now realised that I was monitoring for the 0v on the wrong test point! It may be that I have to do this section again sometime!
But, the radio does now seem to be performing as specified. One downside to this being that with the mute setting as per the manual of -108dBm, the background level of QRM within my QTH is enough on many channels to open the mute!
The final test required, ive just completed. This is rather a frivolous aspect, but I wanted it to work, so ive tested the REMOTE and I/C settings using a Remote Handset and a few meters of DON10 telephone wire. But, with this working, it means that I could stick the radio up on the workshop roof and run telephone wire into my warm comfortable work position to monitor while I get on with something else.
Having removed the Direct Current Amplifier module3, in order to disable the radios ability to transmit (since there is absolutely no legal way of using the transmitter, yet lots of ways to inadvertently send a signal that at best could get you prosecuted and at worse endanger an aircraft!), I could happily ignore all the alignment process relating to the transmitter.
So this evening I took the time to run through the alignment processes for the power supplies and the receiver. The only part of this that gave any problem was the AGC alignment. This required setting one control for 350mV peak waveform on the oscilloscope, and 0v on the voltmeter. However, although I could hear the receiver being affected by the adjustments, I couldnt get anything like the specified waveform levels!
*I have now realised that I was monitoring for the 0v on the wrong test point! It may be that I have to do this section again sometime!
But, the radio does now seem to be performing as specified. One downside to this being that with the mute setting as per the manual of -108dBm, the background level of QRM within my QTH is enough on many channels to open the mute!
The final test required, ive just completed. This is rather a frivolous aspect, but I wanted it to work, so ive tested the REMOTE and I/C settings using a Remote Handset and a few meters of DON10 telephone wire. But, with this working, it means that I could stick the radio up on the workshop roof and run telephone wire into my warm comfortable work position to monitor while I get on with something else.
Pye PF8 - Stumped by the filters
With the band specific capacitors replaced, Ive managed to peak the transmit output of the PF8 up to about 170mW. This is still a far cry from the specified 500mW. Likewise, the receiver barely peaks higher than -85dBm.
In order to make certain this was a filter issue, I have bypassed the 3-pole input helical filter - and the receive sensitivity shot up!
But now I have a big problem. Not only are the correct filters simply not available, but the ones fitted cannot be modified to work better either. In fact, its near impossible to remove them anyway without destroying them (as I found when attempting to take one off my 'donor' unit!)
So, what to do? Well, pretty much all I can do is either find some 2- and 3-pole helical filters for 430MHz that will fit, probably impossible, or at least prohibitively expensive, or think up some way to replace the helical filters with small home brew band pass filters.
All this just so I can own a working, genuine Bodie and Doyle CI5 walkie-talkie!
In order to make certain this was a filter issue, I have bypassed the 3-pole input helical filter - and the receive sensitivity shot up!
But now I have a big problem. Not only are the correct filters simply not available, but the ones fitted cannot be modified to work better either. In fact, its near impossible to remove them anyway without destroying them (as I found when attempting to take one off my 'donor' unit!)
So, what to do? Well, pretty much all I can do is either find some 2- and 3-pole helical filters for 430MHz that will fit, probably impossible, or at least prohibitively expensive, or think up some way to replace the helical filters with small home brew band pass filters.
All this just so I can own a working, genuine Bodie and Doyle CI5 walkie-talkie!
Pye PF8 Conversion Progress
It has taken me a little longer than anticipated to get to this stage, as I replaced the crystals and did the initial tune up for 70cm way back in 2016! But today I finally embarked on the task of changing the band specific capacitors,
In order to replace these caps, the side of the radio needed to be removed, this involved also desoldering the 'call' switch, as well as removing the bolts through the PTT buttons. Whenever I work on something as delicate as this, I take copious photos - so I can see where everything goes back to!
The first two capacitors are along the very edge of the PCB, and up against one of the filters.
If fitting these was fiddly, then the next three were a nightmare! These three were in-board, and surrounded by other much taller, and very delicate parts. Replacing these took all the skill I could muster, and a lot of will power to ignore the pain of burning fingers!
A minor disaster awaited me - in desoldering C11, I inadvertently also desoldered C13 and R16! Both of which fell out. It took a lot of desoldering to clean up and open the PCB holes in order to be able to get them back in!
All this took over two hours! I then had to reposition and secure the wiring loom on the track side, refit the screening plate, and ensure that the tiny coax that goes to one leg of C47 was correctly resoldered.
So now im in a position to test again. All being well, the set will power up! If I get past that milestone, im hoping that at the very least I will now be able to bring the Tx power up closer to the designed 500mW. Whether I get any improvement on receive we'll have to wait and see.
In order to replace these caps, the side of the radio needed to be removed, this involved also desoldering the 'call' switch, as well as removing the bolts through the PTT buttons. Whenever I work on something as delicate as this, I take copious photos - so I can see where everything goes back to!
The first two capacitors are along the very edge of the PCB, and up against one of the filters.
 |
| C36, 2nd from left (brown) and C35, 4th from left (orange) fitted |
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| C45 black left of power transistor; C47 above transistor; C11 left of green cap |
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| PCB holes cleared to replace C13 and R16, and fit new C11 |
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| The wiring loom and shielding |
Saturday, 19 January 2019
PRC344 is alive
After repairing the 6v and 12v regulator modules, and proactively replacing the same capacitor in the 3v module, I now have a working PRC344 UHF AM transceiver.
Thanks to Alan G8LIT, I now have the EMERs for this radio, so further repair and alignment should be much easier. Ive already set up the various voltage rails correctly (most were a little low), but the receiver is still somewhat down. Further alignment will wait until I have printed out the instructions. There is also still an issue with the fit of the battle antenna, which refuses to rotate into the locked position.
One thing I have been able to do, now I have the information to work it out, is disable the transmitter! I had thought I would have to unsolder the supply rails etc to the transmit section modules in order to ensure none of the various ways that the radio can be put into transmit would operate, but it turns out that all the transmit controls are dealt with by module3, simply pulling this out totally disables the transmitter with no other detriment.
A few design flaws show themselves with this radio. The first is with its carrying method. It was intended to be fastened to a carrying harness using an adapter plate that used straps to hold the radio, and as such there are no bolt holes for the normal GS adapter plate as used with the PRC351. But, the radio is also very heavy and very square - which means just picking it up is awkward as theres nothing to easily get hold of! Second, internally there are several adjustable parts which have the same designation as each other! For instance, all the regulator modules are adjusted by an R7. This means that if you accidentally pick the wrong module you could cause quite some damage by adjusting the wrong supply rail!
Ive spent some time today clearing and tidying the RF bench, which really was cluttered up. This was mostly in order to find a place for my new variable bench power supply. In doing so, I decided to also rationalise and maximise the space used by the test equipment, so the test bench is much tidier now and more compact, less wasted space ('scope now on top of the Marconi, PSU on top of high power test load, frequency counter on top of the spectrum analyser). This created enough space that ive been able to put a couple of stackable bins next to the test kit to hold short coax leads and test leads/scope probes etc.
Ive already laid out the next job on the bench, which is the band change modifications to my Pye PF8. This task involves swapping five very small ceramic capacitors, to swap the radio from the U0 band to the T1 band, which covers 70cm. With these capacitors changed, I should hopefully be able to peak the radio up on 70cm without having to hit the bottom of the coils!
Thanks to Alan G8LIT, I now have the EMERs for this radio, so further repair and alignment should be much easier. Ive already set up the various voltage rails correctly (most were a little low), but the receiver is still somewhat down. Further alignment will wait until I have printed out the instructions. There is also still an issue with the fit of the battle antenna, which refuses to rotate into the locked position.
One thing I have been able to do, now I have the information to work it out, is disable the transmitter! I had thought I would have to unsolder the supply rails etc to the transmit section modules in order to ensure none of the various ways that the radio can be put into transmit would operate, but it turns out that all the transmit controls are dealt with by module3, simply pulling this out totally disables the transmitter with no other detriment.
A few design flaws show themselves with this radio. The first is with its carrying method. It was intended to be fastened to a carrying harness using an adapter plate that used straps to hold the radio, and as such there are no bolt holes for the normal GS adapter plate as used with the PRC351. But, the radio is also very heavy and very square - which means just picking it up is awkward as theres nothing to easily get hold of! Second, internally there are several adjustable parts which have the same designation as each other! For instance, all the regulator modules are adjusted by an R7. This means that if you accidentally pick the wrong module you could cause quite some damage by adjusting the wrong supply rail!
Ive spent some time today clearing and tidying the RF bench, which really was cluttered up. This was mostly in order to find a place for my new variable bench power supply. In doing so, I decided to also rationalise and maximise the space used by the test equipment, so the test bench is much tidier now and more compact, less wasted space ('scope now on top of the Marconi, PSU on top of high power test load, frequency counter on top of the spectrum analyser). This created enough space that ive been able to put a couple of stackable bins next to the test kit to hold short coax leads and test leads/scope probes etc.
Ive already laid out the next job on the bench, which is the band change modifications to my Pye PF8. This task involves swapping five very small ceramic capacitors, to swap the radio from the U0 band to the T1 band, which covers 70cm. With these capacitors changed, I should hopefully be able to peak the radio up on 70cm without having to hit the bottom of the coils!
Wednesday, 16 January 2019
New test kit
Ive been informed that my new bench PSU is awaiting me in the works postroom. My current bench supply is ok, but its an ancient 0-30V 500mA unit, that severely restricts what I can power!
So, ive taken a punt on a new unit from China, although supplied out of a UK based drop-shipper as its taken only 2 working days to arrive!
This is a 0-30V 0-5A unit, so should be far more versatile! Of course, the old one will stay on the bench, always good to have a choice.
Ive also, after reading lots of reviews and recommendations, ordered a new multimeter. Hopefully this will be more reliable than the cheapo little DVMs ive been using.
In light of the fault finding ive been doing recently I think these are probably a good investment! Now, if only I could find a 100MHz dual trace 'scope for under £50...
Also, ive ordered something in light of the work ive been doing on the Clansman PRC-349 conversion. The key to the conversion is the synthesiser mixer crystals. Ive scoured every supply I know of old units, and no one has anything suitable! Vince at IQD has some crystals that will do for testing initially, and I will talk to him about these in the next couple of days, but ultimately the correct frequencies will be needed, both for testing, and for final conversion. As custom crystals as one offs start at around £15 each, this is something I need to avoid until I can prove the conversion works. The, if theres enough interest, a bulk purchase at much lower unit cost could be arranged.
So, I need a way to simulate the crystals. This led me back to Hans Summers and QRPlabs.
QRPlabs do a Si5351A based VFO kit http://qrp-labs.com/vfo.html
This will work to a couple of hundred MHz, and has two outputs, one can be selected as a fixed frequency, and one variable as VFO. This would provide both conversion frequencies for the synthesiser. It will also be a very useful little stand alone unit on the bench for building receivers etc.
Theres little more I can do with the PRC-349s until I get either crystals or the VFO, but I can at least work on how best to open the modules to access the parts that require modification.
So, ive taken a punt on a new unit from China, although supplied out of a UK based drop-shipper as its taken only 2 working days to arrive!
This is a 0-30V 0-5A unit, so should be far more versatile! Of course, the old one will stay on the bench, always good to have a choice.
Ive also, after reading lots of reviews and recommendations, ordered a new multimeter. Hopefully this will be more reliable than the cheapo little DVMs ive been using.
Also, ive ordered something in light of the work ive been doing on the Clansman PRC-349 conversion. The key to the conversion is the synthesiser mixer crystals. Ive scoured every supply I know of old units, and no one has anything suitable! Vince at IQD has some crystals that will do for testing initially, and I will talk to him about these in the next couple of days, but ultimately the correct frequencies will be needed, both for testing, and for final conversion. As custom crystals as one offs start at around £15 each, this is something I need to avoid until I can prove the conversion works. The, if theres enough interest, a bulk purchase at much lower unit cost could be arranged.
So, I need a way to simulate the crystals. This led me back to Hans Summers and QRPlabs.
QRPlabs do a Si5351A based VFO kit http://qrp-labs.com/vfo.html
This will work to a couple of hundred MHz, and has two outputs, one can be selected as a fixed frequency, and one variable as VFO. This would provide both conversion frequencies for the synthesiser. It will also be a very useful little stand alone unit on the bench for building receivers etc.
Theres little more I can do with the PRC-349s until I get either crystals or the VFO, but I can at least work on how best to open the modules to access the parts that require modification.
Tuesday, 15 January 2019
Bugger!
Refitted module 6 and the (now identified) Schottky diode, attached a handset and battery. Checked 24v at cathode of the diode. All looking fine... turn on...
...sound and stench of another component somewhere in the radio expiring!
At least the dial light works now!
Pulling the modules, and performing a classic 'sniff' test indicated the 12v regulator module 4. A check of the voltages on its connections showed 25v, 1.2v, 0v. Compared with the other regulators these are correct, except that the 0v should be 12v! (that 1.2v is on the connection that goes to the regulators 'ref bypass' pin, whatever the heck that does!)
The actual electronics in the regulator modules, at least the 12, 6 and 3v units, is actually the same! Its a 1-40v adjustable regulator. Sadly, a quick metering doesnt show a short this time. So this one is going to take some testing!
Edit -
Well, it didnt take much testing! The ferrite bead inductor in the supply rail, before the 22uF capacitor, was open circuit! It had burnt out. Why? Because the bloody 22uF tantalum leaks like a sieve! So thats two of these caps dead! It didnt test short because A) it only shows short one way, and B) the leg had melted off!
...sound and stench of another component somewhere in the radio expiring!
At least the dial light works now!
Pulling the modules, and performing a classic 'sniff' test indicated the 12v regulator module 4. A check of the voltages on its connections showed 25v, 1.2v, 0v. Compared with the other regulators these are correct, except that the 0v should be 12v! (that 1.2v is on the connection that goes to the regulators 'ref bypass' pin, whatever the heck that does!)
The actual electronics in the regulator modules, at least the 12, 6 and 3v units, is actually the same! Its a 1-40v adjustable regulator. Sadly, a quick metering doesnt show a short this time. So this one is going to take some testing!
Edit -
Well, it didnt take much testing! The ferrite bead inductor in the supply rail, before the 22uF capacitor, was open circuit! It had burnt out. Why? Because the bloody 22uF tantalum leaks like a sieve! So thats two of these caps dead! It didnt test short because A) it only shows short one way, and B) the leg had melted off!
PRC-344 Module 6 repair
With module 6 removed from its shield, the next task was to track down the failed component(s). The large TO-5 can is the regulator IC, an LM100H. Very much obsolete and available only at extortionate prices!
Armed with the datasheet for the LM100H, so I knew which pins connected where, I first traced and drew out a rough circuit diagram. There are four connections to this module. Three make immediate sense - raw 24v, regulated 6v, and Ground. But the fourth im not sure of! But it does seem to rule out the plainly obvious repair of simply replacing the thing with a modern three-terminal device!
The first suspect, and the easiest to isolate and test, was the 22uF capacitor across the raw 24v input line. I lifted the +ve leg of this and metered it...
Bingo! Bloody thing was dead short! I lifted the other leg to take it totally out of circuit and confirmed this was the case.
Now, im not at all sure what type of capacitor this is! But its value, voltage rating, polarity, size and the fact it was across a supply rail leads me to suspect a tantalum. My next job was to physically remove it. Although these modules look normal, they are in fact washed over with a tropicalizing varnish, and the capacitor was stuck down solid. It required a careful application of a wood chisel to prize it off!
A modern standard electrolytic capacitor fits neatly in its place. I would have liked to have fitted an axial device, but the only 22uF 50v devices I have are radials, so I had to bend the legs around a little!
At this point, I havent yet put the module back in the radio and tested it. First, ive to check this beastie over -
This is mounted in fuse holder contacts in the supply line. My suspicion was that it was a varistor, but a bit of research online suggests it might actually be a Schottky diode!
Armed with the datasheet for the LM100H, so I knew which pins connected where, I first traced and drew out a rough circuit diagram. There are four connections to this module. Three make immediate sense - raw 24v, regulated 6v, and Ground. But the fourth im not sure of! But it does seem to rule out the plainly obvious repair of simply replacing the thing with a modern three-terminal device!
The first suspect, and the easiest to isolate and test, was the 22uF capacitor across the raw 24v input line. I lifted the +ve leg of this and metered it...
Bingo! Bloody thing was dead short! I lifted the other leg to take it totally out of circuit and confirmed this was the case.
Now, im not at all sure what type of capacitor this is! But its value, voltage rating, polarity, size and the fact it was across a supply rail leads me to suspect a tantalum. My next job was to physically remove it. Although these modules look normal, they are in fact washed over with a tropicalizing varnish, and the capacitor was stuck down solid. It required a careful application of a wood chisel to prize it off!
A modern standard electrolytic capacitor fits neatly in its place. I would have liked to have fitted an axial device, but the only 22uF 50v devices I have are radials, so I had to bend the legs around a little!
At this point, I havent yet put the module back in the radio and tested it. First, ive to check this beastie over -
This is mounted in fuse holder contacts in the supply line. My suspicion was that it was a varistor, but a bit of research online suggests it might actually be a Schottky diode!
This (soldering) guns for hire, even if were just faultfinding in the dark...
I have a certain amount of sympathy with the lyrics of the song from which I have paraphrased this post title - as a night shift worker the first few lines are very familiar! So apologies to Mr Springsteen.
Earlier today, I popped along for a nosy at Johnsons of Leeds, and came away with a Clansman PRC-344 UHF manpack and antenna. I always knew I was taking a big gamble on an untested set, so was only minimally disappointed to find it doesnt work. Physically it is complete, and no damaged parts other than the lamp/call switch, which does still work but the toggle is loose.
So, with nothing more than a basic block diagram and 25years of experience, I set about trying to find the problem...
Firstly, the fact that it is totally dead (not even the dial light works) is actually a good thing! A part working set is always more of a worry, but a totally dead unit immediately points to a power supply problem.
Opening it up, I discovered a few of the plug in modules to be loose, but seating them back in correctly didnt cure anything. It may be that someone had previously started fault finding? Who knows!
What I can say, is that the build quality of these is amazing!About half of the electronics are in the form of plug in modules, but the interconnecting wiring harness looks a nightmare!
The first job was to ensure that the battery supply was actually reaching the electronics. So with the set off, a check at the incoming battery terminal showed 19v. Hmm, ok so the power is there, but thats a recently charged 24V battery! So, battery off and trace the supply wire, expecting to find a fuse blown.
What I found, in a fuse holder, was not a fuse. I think its some form of Varistor, but either way, there was no voltage after it. Having removed it from the circuit, I tried metering out the supply wiring to other parts of the radio, and made an interesting discovery - there was a dead short!
Now, in a radio as complex as this, a dead short could be a real basket to locate! But, here we have the advantage of plug-in modules! So, after making a sketch of the module positions and numbers, I pulled all the modules I could. Checking again showed no continuity to ground, so it was reasonable to assume that the short was in one of the modules.
Putting them back in one by one, and checking the continuity after each, the short returned when I replaced module 6.
Module 6 is the 6V stabilised power supply. I suspect that much of the radios systems run on 6V, and so this being faulty would almost certainly account for a dead radio!
Earlier today, I popped along for a nosy at Johnsons of Leeds, and came away with a Clansman PRC-344 UHF manpack and antenna. I always knew I was taking a big gamble on an untested set, so was only minimally disappointed to find it doesnt work. Physically it is complete, and no damaged parts other than the lamp/call switch, which does still work but the toggle is loose.
So, with nothing more than a basic block diagram and 25years of experience, I set about trying to find the problem...
Firstly, the fact that it is totally dead (not even the dial light works) is actually a good thing! A part working set is always more of a worry, but a totally dead unit immediately points to a power supply problem.
Opening it up, I discovered a few of the plug in modules to be loose, but seating them back in correctly didnt cure anything. It may be that someone had previously started fault finding? Who knows!
What I can say, is that the build quality of these is amazing!About half of the electronics are in the form of plug in modules, but the interconnecting wiring harness looks a nightmare!
The first job was to ensure that the battery supply was actually reaching the electronics. So with the set off, a check at the incoming battery terminal showed 19v. Hmm, ok so the power is there, but thats a recently charged 24V battery! So, battery off and trace the supply wire, expecting to find a fuse blown.
What I found, in a fuse holder, was not a fuse. I think its some form of Varistor, but either way, there was no voltage after it. Having removed it from the circuit, I tried metering out the supply wiring to other parts of the radio, and made an interesting discovery - there was a dead short!
Now, in a radio as complex as this, a dead short could be a real basket to locate! But, here we have the advantage of plug-in modules! So, after making a sketch of the module positions and numbers, I pulled all the modules I could. Checking again showed no continuity to ground, so it was reasonable to assume that the short was in one of the modules.
Putting them back in one by one, and checking the continuity after each, the short returned when I replaced module 6.
Module 6 is the 6V stabilised power supply. I suspect that much of the radios systems run on 6V, and so this being faulty would almost certainly account for a dead radio!
Saturday, 12 January 2019
Converting the PRC-349 for amateur band use?
The Clansman PRC-349, a huge 'hand-held' 1/4W low-band FM transceiver, is the British forces version of the Racal BCC349. These are regarded as useless for amateur purposes as the 10MHz band they cover, from 37 to 46.975MHz, in 25kHz steps, doesnt cross any of the amateur bands.
But - Racals own advertising of the BCC349 clearly states that it can be supplied covering ANY 10MHz segment between 30MHz and 76MHz. Since the frequency control element is the most expensive (other than the RF Power device), and Racal wished to sell to as many markets as possible, its very unlikely that a major amount of work was required to change the band. Racal would surely have used a standard set of modules, with minor component changes.
So, knowing the operation of these sets, there are a number of necessary steps to convert them to amateur use -
1. Change the Rx and Tx frequency references
2. Disable or otherwise overcome the 150Hz tone squelch control
3. Re-align the filtering/oscillators
4. Modify LOUD and WHISPER settings to have the same mic gain on Tx
Sounds easy? Well, possibly, or possibly not. Studying the frequency synthesis system in use however throws up an interesting aspect - the VCOs do not feed directly to the divider chains! Instead, each (Tx and Rx VCO) is down converted with a fixed reference crystal oscillator, and the resulting roughly 2.5 to 12.5MHz band is fed to the dividers. It makes for a reasonable assumption then, that so long as the reference crystal frequencies are selected to mix the required VCO frequencies down to this range, and the VCOs are modified to cover the desired frequencies, that it should be possible to make the synthesiser work in a segment that covers an amateur band.
However, this wouldnt be all. The VCOs would need modifying certainly to oscillate as required, but also the Tx PA tuning, Band Pass filtering, and Rx front end filtering/tuning will all require modification to the desired frequencies.
It just so happens though that I have a copy of the Racal BCC349 technical manual, and all the necessary circuit diagrams! If we aim for moving up in frequency, to perhaps the 6m band (at least to start with, 4m would be better but lets go easy at first!), then I can see from the diagrams there isnt really too much to modify, and since we are going up, modifications to tuning coils is by removing turns, which is far easier than adding them to resonate lower! It might be that some parts will even align to 6m as is, or perhaps with some simple changes to fixed capacitors.
The likely difficult, or perhaps easy but expensive, part will be replacing the reference oscillator crystals. Custom crystals are in the order of £35 a pair! But, it might be possible to find stock channel pairs from older PMR equipment that will work - at least for testing.
In fact, probably the hardest part of a conversion will be dealing with the tone squelch! This might be defeated the way it is in the PRC-351, with an internal 160Hz oscillator, the difficulty here is that there is almost NO free space inside the PRC-349 AT ALL! So where such an oscillator would be fitted ive no idea!
What I can do though, if find out of my stock of spares a full set of working modules, and arrange a bread-board set-up, so that the various sections can be experimented on to see if the adjustments are in fact possible - I just need to get hold of a suitable pair of 3rd overtone crystals for the reference oscillators now!
But - Racals own advertising of the BCC349 clearly states that it can be supplied covering ANY 10MHz segment between 30MHz and 76MHz. Since the frequency control element is the most expensive (other than the RF Power device), and Racal wished to sell to as many markets as possible, its very unlikely that a major amount of work was required to change the band. Racal would surely have used a standard set of modules, with minor component changes.
So, knowing the operation of these sets, there are a number of necessary steps to convert them to amateur use -
1. Change the Rx and Tx frequency references
2. Disable or otherwise overcome the 150Hz tone squelch control
3. Re-align the filtering/oscillators
4. Modify LOUD and WHISPER settings to have the same mic gain on Tx
Sounds easy? Well, possibly, or possibly not. Studying the frequency synthesis system in use however throws up an interesting aspect - the VCOs do not feed directly to the divider chains! Instead, each (Tx and Rx VCO) is down converted with a fixed reference crystal oscillator, and the resulting roughly 2.5 to 12.5MHz band is fed to the dividers. It makes for a reasonable assumption then, that so long as the reference crystal frequencies are selected to mix the required VCO frequencies down to this range, and the VCOs are modified to cover the desired frequencies, that it should be possible to make the synthesiser work in a segment that covers an amateur band.
However, this wouldnt be all. The VCOs would need modifying certainly to oscillate as required, but also the Tx PA tuning, Band Pass filtering, and Rx front end filtering/tuning will all require modification to the desired frequencies.
It just so happens though that I have a copy of the Racal BCC349 technical manual, and all the necessary circuit diagrams! If we aim for moving up in frequency, to perhaps the 6m band (at least to start with, 4m would be better but lets go easy at first!), then I can see from the diagrams there isnt really too much to modify, and since we are going up, modifications to tuning coils is by removing turns, which is far easier than adding them to resonate lower! It might be that some parts will even align to 6m as is, or perhaps with some simple changes to fixed capacitors.
The likely difficult, or perhaps easy but expensive, part will be replacing the reference oscillator crystals. Custom crystals are in the order of £35 a pair! But, it might be possible to find stock channel pairs from older PMR equipment that will work - at least for testing.
In fact, probably the hardest part of a conversion will be dealing with the tone squelch! This might be defeated the way it is in the PRC-351, with an internal 160Hz oscillator, the difficulty here is that there is almost NO free space inside the PRC-349 AT ALL! So where such an oscillator would be fitted ive no idea!
What I can do though, if find out of my stock of spares a full set of working modules, and arrange a bread-board set-up, so that the various sections can be experimented on to see if the adjustments are in fact possible - I just need to get hold of a suitable pair of 3rd overtone crystals for the reference oscillators now!
Friday, 11 January 2019
Clansman Handset cable repair
Well, I ended up cracking on with it.
One good thing about these cables is the messenger cord. By tying this the two halves of the cable were secured, and it was just the tricky job of stripping, joining, soldering, and sleeving the individual wires, then enclosing the whole repair in heat-shrink.
Of course, making extra sure that the heat-shrink tubing was on the cable before making the connections!
One good thing about these cables is the messenger cord. By tying this the two halves of the cable were secured, and it was just the tricky job of stripping, joining, soldering, and sleeving the individual wires, then enclosing the whole repair in heat-shrink.
Of course, making extra sure that the heat-shrink tubing was on the cable before making the connections!
Clansman Cable Chomped!
Since ive got the 320s in bits, I thought i'd also try and find out why my spare handset wont key the transmitter.
After much metering, cable flexing at both ends, and scratching of bonce, all of which got me nowhere, since the metering showed good and the flexing didnt cause anything.. I happened to move the middle of the cable!...
...and it keyed up!
Examining the cable I discovered a cut in the outer sheath, moving this exposed the damaged interior! It looks like at some point the cable has been trapped and cut. Really annoyingly, the cut is about six inches from the plug end! These are not easy plugs to repair! Had it been the same distance from the handset I could have just cut it back and soldered it back on.
So, thats another job for another day! Ive a cable on a pair of DT100 cans to repair first!
After much metering, cable flexing at both ends, and scratching of bonce, all of which got me nowhere, since the metering showed good and the flexing didnt cause anything.. I happened to move the middle of the cable!...
...and it keyed up!
Examining the cable I discovered a cut in the outer sheath, moving this exposed the damaged interior! It looks like at some point the cable has been trapped and cut. Really annoyingly, the cut is about six inches from the plug end! These are not easy plugs to repair! Had it been the same distance from the handset I could have just cut it back and soldered it back on.
So, thats another job for another day! Ive a cable on a pair of DT100 cans to repair first!
PRC320 slow tx/rx recovery - fixed?
Having spent a long time investigating the slave relay 6RLA (the relay on the unit 6 motherboard that switches the tx/rx 6v line and the IF signal), using tacked on LEDs to observe the DC rails, and 'dry' testing of the IF contacts using continuity testing and an external supply to activate the relay, I started to come to the conclusion that whatever was causing the fault was not associated with the physical switching. I couldnt positively rule it out, but my suspicion began to fall elsewhere.
In particular, it fell on module 6b - the Receive Audio module. This contains the demodulator/detector, audio stages, and AGC circuitry. I noted in particular the presence of one of the suspect tantalum capacitors in its supply rail, isolated from the rest of the radio by a 100 ohm resistance.
So, today I embarked on the task of swapping this module for the one from the test-bed radio. Not a simple task!
There are 18 connections, plus 3 bolts, to module 6b. Each is made via a link wire, looped around the module pin. Each had to be desoldered and lifted, the pin cleaned, the bolts unfastened, and the module withdrawn, first on the spare module from the test-bed radio, then the one from the faulty radio.
Since I had the test-bed open and was removing 6b, I decided to also remove module 6a - the Receive IF stage. This is located beside module 6b, and would be the next suspect in the investigation!
After removing these from the test-bed, they were carefully put aside. The same was then done to the faulty radio, only this time removing just module 6b. With that put safely away from either the radios or the previously removed modules, to ensure they didnt get mixed up I immediately installed the spare 6b module into the faulty radio.
With the module replaced and reconnected, and a couple of the chassis screws replaced to ensure ground continuity - I made a brew and a sandwich!
Refreshed, I reconnected unit 2 (the rear section), the handset and the battery, and began testing...
At present, I am testing with a dummy load connected of course. Carrying out several key up/down tests, with various times in Tx mode, on various modes, and with some modulation, it does seem that there is no longer a delay in returning to receive, at least no longer than the momentary pause as the relays drop out.
Next, I need to completely reassemble the radio, and ensure it continues to operate like this. Then, I will need to replace the dummy load with an antenna connection and test the receiver with live signal. Only then will I really know if the set is fixed. But im moderately hopeful!
If it does indeed turn out to be working now, then the fault is with the 6b module, which I will need to repair before putting it back into service in the test-bed. The photo below shows the inside of module 6b... im not looking forward to fault finding this thing!
In particular, it fell on module 6b - the Receive Audio module. This contains the demodulator/detector, audio stages, and AGC circuitry. I noted in particular the presence of one of the suspect tantalum capacitors in its supply rail, isolated from the rest of the radio by a 100 ohm resistance.
So, today I embarked on the task of swapping this module for the one from the test-bed radio. Not a simple task!
 |
| 18 soldered connections and 3 bolts! |
There are 18 connections, plus 3 bolts, to module 6b. Each is made via a link wire, looped around the module pin. Each had to be desoldered and lifted, the pin cleaned, the bolts unfastened, and the module withdrawn, first on the spare module from the test-bed radio, then the one from the faulty radio.
Since I had the test-bed open and was removing 6b, I decided to also remove module 6a - the Receive IF stage. This is located beside module 6b, and would be the next suspect in the investigation!
 |
| module 6b (left) and 6a (right) |
With the module replaced and reconnected, and a couple of the chassis screws replaced to ensure ground continuity - I made a brew and a sandwich!
Refreshed, I reconnected unit 2 (the rear section), the handset and the battery, and began testing...
At present, I am testing with a dummy load connected of course. Carrying out several key up/down tests, with various times in Tx mode, on various modes, and with some modulation, it does seem that there is no longer a delay in returning to receive, at least no longer than the momentary pause as the relays drop out.
Next, I need to completely reassemble the radio, and ensure it continues to operate like this. Then, I will need to replace the dummy load with an antenna connection and test the receiver with live signal. Only then will I really know if the set is fixed. But im moderately hopeful!
If it does indeed turn out to be working now, then the fault is with the 6b module, which I will need to repair before putting it back into service in the test-bed. The photo below shows the inside of module 6b... im not looking forward to fault finding this thing!
 | |
| Inside No. 9, sorry, 6b... |
Wednesday, 9 January 2019
Inside the PRC320
With the initial tests inconclusive, im having to delve into the guts of this set.
The first photo here shows the faulty set. The relay ive been testing is located in the bottom right corner of the PCB, which is unit 6. In this state, with unit 2 (the rear section) attached, the radio operates, and I can check at least a few voltages.
Above is the test-bed radio with unit 6 in the 'service' position. How your meant to service the radio like this when the back doesnt attach the manuals dont say. The relay can just be seen behind the pink wires at the bottom. There are three components associated with this relay, a 100 ohm resistor, a 68nF capacitor, and a 180uF 6.8v tantalum capacitor. All are suspect! The tantalum isnt part of the relay switching like the other two, but is a smoothing capacitor on the 6v Rx rail. If I have a suitable part then this might get replaced anyway! The various audio module cans are shown here, of particular suspicion is the top one, 6b, as this has another big tantalum in its 6v Rx rail!
From the other side. The other slave relay is part of the turret tuning mechanism, which is the big metal block in the middle! You can just about see the relay in the middle of the side plate, part hidden by the tuner unit.
And the view from the other side, which shows just how difficult it will be to work on the relay in the turret tuner!
So, my plan is thus - tack solder an LED to the 6v Rx rail and see if I can discern any link between the delay in return to Rx and the switching of this rail. If so, or if inconclusive, I will replace the associated components, and if the fault remains, will exchange the relay.
The first photo here shows the faulty set. The relay ive been testing is located in the bottom right corner of the PCB, which is unit 6. In this state, with unit 2 (the rear section) attached, the radio operates, and I can check at least a few voltages.
Above is the test-bed radio with unit 6 in the 'service' position. How your meant to service the radio like this when the back doesnt attach the manuals dont say. The relay can just be seen behind the pink wires at the bottom. There are three components associated with this relay, a 100 ohm resistor, a 68nF capacitor, and a 180uF 6.8v tantalum capacitor. All are suspect! The tantalum isnt part of the relay switching like the other two, but is a smoothing capacitor on the 6v Rx rail. If I have a suitable part then this might get replaced anyway! The various audio module cans are shown here, of particular suspicion is the top one, 6b, as this has another big tantalum in its 6v Rx rail!
From the other side. The other slave relay is part of the turret tuning mechanism, which is the big metal block in the middle! You can just about see the relay in the middle of the side plate, part hidden by the tuner unit.
And the view from the other side, which shows just how difficult it will be to work on the relay in the turret tuner!
So, my plan is thus - tack solder an LED to the 6v Rx rail and see if I can discern any link between the delay in return to Rx and the switching of this rail. If so, or if inconclusive, I will replace the associated components, and if the fault remains, will exchange the relay.
The Hunt Commences - Tx/Rx switching fault PRC320
For some time ive been meaning to fault find an issue with my Clansman PRC320 HF manpack radio. This set has a slow return back to receive after transmitting, a delay that seems to increase the longer the pressel is held (i.e. longer in Tx mode), up to a maximum of about 2 seconds. This doesnt seem much, and indeed under proper R/T procedure in a military net would not be noticeable, as proper R/T procedure includes time between key up and speech to allow for stabilization of the equipment. But for amateur use, this delay is a nightmare! It means I miss the start of callsigns, and in contests or fast paced contacts such as SOTA - its possible to miss the entire transmission!
So, how to tackle fault finding such a problem? What can cause a transceiver to be slow returning to receive?
Several possibilities come to mind -
A. Sticking relays
B. Failing capacitors
C. Failing power supply
D. AGC faults
E. Semiconductor breakdown
Anyone who has ever been inside a PRC320 will know just what a maze they are! (if youve never seen inside one - piccies to follow!). They are a modular system, which on the face of it sounds easy to work with, but the modules are soldered in and bolted down - they are not plug-in units like on other Clansman equipments. Essentially, the radio breaks down into six sections, each consisting essentially of sealed or otherwise very awkward modules.
Part of unit 2, the rear section, is the reflectometer module 2b. As well as sensing the RF levels at the antenna, this also provides a master relay, which switches the antenna between Tx and Rx paths, and also 24V out to a pair of slave relays on other modules.
The whole rear section can be removed, this comprises the audio inputs, bandpass filtering, PA, and reflectometer sections. Because of this, it was the best place to start, as I could simply swap unit 2 with the same from my 'test-bed' radio, thus proving or eliminating the master relay...
So, I swapped the rear sections between the radios - and the fault stayed put. So not the master relay then!
The next, and in fact last, part that could easily be checked was module 5, the PSU. In the test-bed radio, this is an original unit that has had its horrid 1970s tantalum capacitors replaced with modern electrolytic units. In the faulty radio, it is a complete modern rebuild using DC-DC converter modules. But, this module at least connects with a plug and socket!
So, I swapped module 5... and the fault stayed put.
This leaves me with two relays, the slaves, that might be at faults. One of these is in a module hidden away within the tuning turret, so I decided to start with the other, which is located on unit 6, the motherboard - the only directly accessible circuit board in the whole radio!
Unit 6 consists of a PCB with various discrete components, upon which is mounted and soldered about five screened modules - each soldered in and bolted on. It can be raised into a 'service' position, to access the modules, but in this position the radio cannot be operated as unit 2 cannot be connected! It is also awkward to do this as the LSB mod gets in the way!
So I decided to meter out the relay and see if the change in voltages from the contacts matches the delay, indicating a sticking relay. I had already found just from the sound of the relays clicking that this relay seemed ok, but on metering the Rx 6v contact, discovered that the response times of my meters isnt fast enough to outrun the delay, and so the test proved inconclusive.
Im at a bit of a loss now. Unless I can locate a fault with the relay on unit 6, then im into the realm of module swapping! A rather daunting task! A last ditch method I can try is to tack-solder an LED and series resistor to the 6v Rx line from the relay. The LEDs response time should be near instantaneous, and so if the relay is at fault it should show.
If that doesnt work, then since I will have to start wielding the soldering iron, I may as well just swap the relay, and its associated components, out anyway, in exchange with those of the test-bed radio.
So, how to tackle fault finding such a problem? What can cause a transceiver to be slow returning to receive?
Several possibilities come to mind -
A. Sticking relays
B. Failing capacitors
C. Failing power supply
D. AGC faults
E. Semiconductor breakdown
Anyone who has ever been inside a PRC320 will know just what a maze they are! (if youve never seen inside one - piccies to follow!). They are a modular system, which on the face of it sounds easy to work with, but the modules are soldered in and bolted down - they are not plug-in units like on other Clansman equipments. Essentially, the radio breaks down into six sections, each consisting essentially of sealed or otherwise very awkward modules.
Part of unit 2, the rear section, is the reflectometer module 2b. As well as sensing the RF levels at the antenna, this also provides a master relay, which switches the antenna between Tx and Rx paths, and also 24V out to a pair of slave relays on other modules.
The whole rear section can be removed, this comprises the audio inputs, bandpass filtering, PA, and reflectometer sections. Because of this, it was the best place to start, as I could simply swap unit 2 with the same from my 'test-bed' radio, thus proving or eliminating the master relay...
So, I swapped the rear sections between the radios - and the fault stayed put. So not the master relay then!
The next, and in fact last, part that could easily be checked was module 5, the PSU. In the test-bed radio, this is an original unit that has had its horrid 1970s tantalum capacitors replaced with modern electrolytic units. In the faulty radio, it is a complete modern rebuild using DC-DC converter modules. But, this module at least connects with a plug and socket!
So, I swapped module 5... and the fault stayed put.
This leaves me with two relays, the slaves, that might be at faults. One of these is in a module hidden away within the tuning turret, so I decided to start with the other, which is located on unit 6, the motherboard - the only directly accessible circuit board in the whole radio!
Unit 6 consists of a PCB with various discrete components, upon which is mounted and soldered about five screened modules - each soldered in and bolted on. It can be raised into a 'service' position, to access the modules, but in this position the radio cannot be operated as unit 2 cannot be connected! It is also awkward to do this as the LSB mod gets in the way!
So I decided to meter out the relay and see if the change in voltages from the contacts matches the delay, indicating a sticking relay. I had already found just from the sound of the relays clicking that this relay seemed ok, but on metering the Rx 6v contact, discovered that the response times of my meters isnt fast enough to outrun the delay, and so the test proved inconclusive.
Im at a bit of a loss now. Unless I can locate a fault with the relay on unit 6, then im into the realm of module swapping! A rather daunting task! A last ditch method I can try is to tack-solder an LED and series resistor to the 6v Rx line from the relay. The LEDs response time should be near instantaneous, and so if the relay is at fault it should show.
If that doesnt work, then since I will have to start wielding the soldering iron, I may as well just swap the relay, and its associated components, out anyway, in exchange with those of the test-bed radio.
Thursday, 3 January 2019
DMX512 Line Tester
Its been over 28 years since I last had any real involvement with stage lighting control, but an occasion has arisen whereby I need to do some basic tests on a DMX512 'universe' - as a DMX controller/slave network is known.
Testing DMX protocol properly, that is, by decoding the data packets, requires an expensive DMX analyser. But, as with most electronics handled by 'amateurs' (as opposed to professional engineers and technicians - but certainly not ruling those out!) the commonest faults are likely to be due to wear and tear and bad handling of cables and connectors. So, a simple device that can prove the flow of data, and diagnose simple connection issues, is all I require.
Now, proper, compliant systems use XLR-5 connectors, and I will be making up a tester for this once the plug arrives. But, many cheaper or older DMX lighting controllers use the XLR-3 connector. The build shown here is for a 3-pin XLR-3 line tester.
Im not going to include a circuit diagram here, as these can be found on the net easily. In the XLR-3 the DMX512 signal is on pins 2 and 3, and ground pin 1. DMX uses the RS-485 hardware protocol, and is a differential pair signal. All that is needed for basic testing is an inverse-parallel pair of LEDs across the differential line, i.e. between pins 2 and 3, along with a suitable series resistor to prevent the tester taking much current from the line (which is rated 250mA max, but any current draw from the line can induce noise, so this tester should not be left connected when the system is in use). As the DMX512/RS-485 standard requires a line termination, a second resistor should also be used across the pair to match the cable impedance. This is about 120 ohm.
Rather than a separate pair of LEDs, a single bi-colour device is preferred. These have two pins but contain back to back chips, usually one red and one green. I didnt have any of these, but instead had tri-colour LEDs, which are the same chips but a common cathode, and hence three legs. So, I had to use a connection to ground on pin 1. A pair of 270 ohm 1/4w resistors act as Rseries and Rload. Im not actually sure this is the right approach, as these were specified for the 2-leg bi-colour LED, and I think Rload should be a 120 ohm unit, and Rseries whatever gives best compromise between current draw and brightness. I'll modify them for the XLR-5 build once I get the plug, and have chance to test the XLR-3 on a live system. It shouldnt matter much though as the hardware protocol is rather robust.
The photo below shows the tester wired up. Care was taken to ensure that the distance from the pins to the LED matched that as from the pins to the top of the cable strain relief when the plug is made up
With the pins, circuit and cable securing gland inserted into the barrel, and the cable relief screwed on to complete the device, the LED protudes nicely out of the end of the rubber!
A quick calculation shows that for this type of LED, with Vled of 2-2.2v, and an expected line voltage of 6v, this tester should draw about 14mA. Thats not too bad for a line with 250mA max, but personally I would prefer 10mA, which would be a 380 ohm series resistor.
For the XLR-5, I will use Rseries 380 ohm, and Rload 120ohm. 120ohm is a standard value in the E24 series, but 380 ohm is not. I will have to decide whether to use 390 ohm (dimmer) or 360 ohm (a touch more current)
Testing DMX protocol properly, that is, by decoding the data packets, requires an expensive DMX analyser. But, as with most electronics handled by 'amateurs' (as opposed to professional engineers and technicians - but certainly not ruling those out!) the commonest faults are likely to be due to wear and tear and bad handling of cables and connectors. So, a simple device that can prove the flow of data, and diagnose simple connection issues, is all I require.
Now, proper, compliant systems use XLR-5 connectors, and I will be making up a tester for this once the plug arrives. But, many cheaper or older DMX lighting controllers use the XLR-3 connector. The build shown here is for a 3-pin XLR-3 line tester.
Im not going to include a circuit diagram here, as these can be found on the net easily. In the XLR-3 the DMX512 signal is on pins 2 and 3, and ground pin 1. DMX uses the RS-485 hardware protocol, and is a differential pair signal. All that is needed for basic testing is an inverse-parallel pair of LEDs across the differential line, i.e. between pins 2 and 3, along with a suitable series resistor to prevent the tester taking much current from the line (which is rated 250mA max, but any current draw from the line can induce noise, so this tester should not be left connected when the system is in use). As the DMX512/RS-485 standard requires a line termination, a second resistor should also be used across the pair to match the cable impedance. This is about 120 ohm.
Rather than a separate pair of LEDs, a single bi-colour device is preferred. These have two pins but contain back to back chips, usually one red and one green. I didnt have any of these, but instead had tri-colour LEDs, which are the same chips but a common cathode, and hence three legs. So, I had to use a connection to ground on pin 1. A pair of 270 ohm 1/4w resistors act as Rseries and Rload. Im not actually sure this is the right approach, as these were specified for the 2-leg bi-colour LED, and I think Rload should be a 120 ohm unit, and Rseries whatever gives best compromise between current draw and brightness. I'll modify them for the XLR-5 build once I get the plug, and have chance to test the XLR-3 on a live system. It shouldnt matter much though as the hardware protocol is rather robust.
The photo below shows the tester wired up. Care was taken to ensure that the distance from the pins to the LED matched that as from the pins to the top of the cable strain relief when the plug is made up
With the pins, circuit and cable securing gland inserted into the barrel, and the cable relief screwed on to complete the device, the LED protudes nicely out of the end of the rubber!
A quick calculation shows that for this type of LED, with Vled of 2-2.2v, and an expected line voltage of 6v, this tester should draw about 14mA. Thats not too bad for a line with 250mA max, but personally I would prefer 10mA, which would be a 380 ohm series resistor.
For the XLR-5, I will use Rseries 380 ohm, and Rload 120ohm. 120ohm is a standard value in the E24 series, but 380 ohm is not. I will have to decide whether to use 390 ohm (dimmer) or 360 ohm (a touch more current)
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