More on the Static Bleed Unit

Returning to this project this morning, having had the night to dream up what exactly I wanted it to do, I decided that it had to do the following -

1. Allow for both ferruled and bare earth wires
2. Allow for bare antenna wire, ferruled wire, coax and 4mm banana plug antenna connections
3. Allow for coax, 4mm banana and ferruled wire connections to radio

I have managed to accomplish all but the ferruled radio connection. I will add this later, but for now ive run out of springy D10 terminals!

 

Internally, a bank of parallel 2W 1.5Mohm resistors form a high power 250k static bleed to ground. 250k is high enough to not affect tuning even on the most temperamental end fed wires, but low enough to quickly drain any static build up. Three extra antenna connectors have been added, so along with the Clansman stud connector, and the BNC socket, there is a 4mm banana plug socket, a screw type insulation piercing terminal and a sprung terminal. For the radio, there are options to connect via coax to the BNC, or 4mm plug. A further sprung D10 terminal will be added once I have some.

View from the side of the antenna terminals.  The BNC antenna socket can also be seen.

Radio end! The SURFs own little coax patch lead and holder remain, but a new 4mm socket allows for single wire connection. This socket was salvaged from an old IF injector and was a bit of a pig to install! 

View from the grounding side. The srpung terminal makes use of the hole forom the SURFs tuning control. Ive left the original meter cover in place, and found a rubber bung left over from something or other fits the hole left by removing the meter perfectly!

And thats it with the lid on! I just need to remove that god awful asset sticker!

Repurposing a 4W SURF into a Static Bleed Unit

One of the problems with lifting antenna wires using kites or balloons, is that wind blowing across the wire generates static. This can build up to tens of thousands of volts - enough to destroy modern front end transistors. If you want to play with kite antennas, you have to remove this static.

To do this, is really simple. You simply make a box with some high power, high value resistors in it, with connections for the antenna wire, the feed to the radio, and a connection to earth. Most people would do this using a little die cast aluminium box.

I wanted a design that did two things, 1. Used military connectors and would look the part next to other military radio equipment, and 2. wouldnt eat into my small stock of now very expensive die cast boxes!

Something that is much cheaper than a die cast box, is a Clansman 4W SURF unit!

As I had one spare, I decided I would remove the electronics, and repurpose it! The electronics, which include a beautiful silver plated air spaced variable capacitor, have been retained for spares.

The empty case has quite a few mouldings that make deciding exactly how to repurpose it a bit tricky! It turns out that the dehydrator plug hole is far too deep to insert anything in, and the meter hole far too wide. The hole for the tuning control though, was only a little too deep - but accessible! Its the little stub of aluminium that can be seen on the top left of the picture at the left end of the central divider. Ten minutes work with a file had this stub removed!

As I wanted two ground terminals, I decided the other should go below and slightly to the side of this hole, and so drilled a new hole for it. Here I made a little mistake as I didnt notice that internally this point in the case is sloped! Plus, it meant the nut was underneath the stud for the lid fixing, but I managed to get it tightened up successfully.

So here it is so far. Two ground terminals installed. The upper terminal is a sprung push type D10 terminal, salvaged from a defunct CRS/LR box. I had a bit of a shock when I opened this box and found it wet inside! Odd for a sealed box! I believe it to be water, but there were a lot of odd white crytsals inside as well! The other terminal is a screw type insulation piercing D10 terminal taken from a defunct PTC404 field telephone.

The electronics removed from that PTC404 are shown above. The build quality of these field telephones is absolutely superb!

The CRS/LR box and the PTC404 were part of a batch of kit I picked up today from Dominic at LRseries surplus. This batch included the antenna elements, base, and cables I need to get the TUAAM working, plus an EVHF antenna to play with, and a few spare audio plugs, to use to make up test leads.

Another item in the batch was a damaged battery extension cable. Much of the cable itself had failed, seemingly due to fuel or oil exposure. I found about 2ft of it that was usable, cut it out, and set to making it into something useful

The connector ends turned out to be filled with soft silicone rubber. This had to be picked out (easy enough) before I could access the terminals.

With the good cable reinstalled into the radio end connector, A couple of spare 4mm plugs later and I now have a very useful external power connector. I can now easily connect radios up tot he bench variable PSU for testing.

Despite everything, once I was home from LRseries, I thought of loads of other stuff I should have got! I also think I might want a couple more D10 terminals than I have! So looks like i'll have to go back again in a week or two!

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Inside the Racal BCC425 Initiate Box

Just in case anyone was wondering what was in the Clansman Initiate Box, well heres a photo

As you can see, its really very simple inside. Probably the easiest bit of Clansman to repair!



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Did I mention our charity walk?

Those of you who frequent this blog may also be aware of my other, seldom updated walking blog. Well, myself and Bob M1BBV are yet again taking on a rediculously long walk for charity!

Next month, we will walk 84 miles in just 3 days, to conquer the Hadrians Wall long distance path. This is to raise funds for Cancer Research UK.

If you like my blog, and if anything ive put in my ramblings has been helpful to you, why not consider saying thanks with a bit of sponsorship?

You can donate to our cause at our dedicated fundraising page here https://fundraise.cancerresearchuk.org/page/bob-and-martins-hadrians-wall-challange2019

Please help us hit our target and help find a cure for cancer!

I'll be adding the link to each blog post as well.

Up, Up and Away!

Ive finally got around to starting the work on fitting the ADS-B receiver system into a weatherproof housing for mast installation. I happened to obtain an ex-equipment outdoor unit housing, and realised I could repurpose it from its original job of line protection unit.

Ive used the same antenna, but with some blind grommets and silicone. Mounting the electronics took some thinking about to ensure good all round ventilation and airflow, and access to the SD card.

I need to get some better blanking plugs for the holes before it can go outdoors. and work out how im going to attach mast mounting brackets.

Ive also done the necessary repairs to the rescue box kite. This involved learning how to completely dismantle it in order to take off the wing fabric in order to attach the repair tape, and to fix the broken longeron.

Im now waiting for the tapes adhesive to cure and then can fold it back up again.

PRC344 low battery warning v.1.0 - 1st working

After a lot of fiddly work with these tiny 0603 sized SMT parts, and the retrofit of a standard 1/4w resistor to effect the required virtual ground, I now have the very first of the version 1.0 boards installed and tested in my PRC344.

Fiddly work - those black specks are the resistors!

The chosen resistors for Ra and Rb actually, in use, give an alarm threshold of 20.86v. Thats a little higher than I was aiming for, but not bad for fixed value 5% components.

Completed board

Insulated inside heat-shrink ready to install

The radio is now back together, tested on the variable bench PSU, and now sat monitoring North-East ICF on 270.050MHz, where there is some activity. Rather annoyingly, there is a dreadful sproggie from my PC right on that very frequency! So I have to move the radio quite a way clear of the computer.

I now need to let the radio drain down the battery until the alarm starts, and then check the battery voltage and the alarm function.

Project Delays

Its been some time since I posted on here, mostly due to being worked like a dog on overtime, but also some big household projects needed working on, and also training yet again for a charity walk.

Ive revisited the PRC344 battery warning project today. I'd forgotten about the modification needed to the v.1 PCBs! This is because I was let down by a supplier and am still waiting for the 100k and above resistors.

Needing to get at least some of these v.1 boards completed and up running, I tested the circuit on breadboard again tonight using 10k range resistors instead, just to prove that it works that way, which it does. I also tested various combinations of Ra and Rb in the 1-10k range and found a couple of combinations that give a 20.2v cut-off (4k7 and 6k8) or 18.7v cut-off (3k9 and 6k8). I'll have to check the minimum terminal voltages of the batteries and decide which to fit. I think though that 1v/cell really is best as a maximum discharge, so the 20.2v values will probably be chosen.

This will allow me to get at least a few of the boards working, at least until I run out of 0603 10k resistors! But I should at least be able to demonstrate one! Then, when the rest of the resistors arrive, the remaining boards can be made up with either 10k or 100k series.

Its now however almost certain that I will redesign for a v.2 board using 0805 size parts, this will allow the use of a better voltage rated and higher value capacitor for the timing, but I will also add the duty cycle control diodes as well.


Ive also, since last posting, acquired a WW2 rescue box kite (as used with the famous 'Gibson Girl' lifeboat transmitter) which I have repaired and restored. I am now in the process of building an antenna connection and protection unit, which will drain wind static and protect against electrical discharge. Once that is done, it will be time to attach an antenna wire, fly the kite and load up the Clansman!

Fatal Flaws!

Another nasty realisation looks like it might be the death of the first version of the PRC344 battery warning module!

Yesterday it came to light that a further 100k resistor was required. Not a difficult thing to add in itself. But while shopping for the 4M7 resistors needed for the timing, and realising that they are rather expensive, a consideration of the alternative, that is going to a 10uF capacitor and dropping the resistor down to 470k, led to the discovery that the 1uF ceramic capacitors im using are only rated at 16V!

I really dont know how far I could safely push that rating! A quick test shows that the voltage on the capacitor peaks at about 16V for a 24V supply. But, a freshly charged battery could easily be up to near 28V! And there are also single use 33V Lithium batteries out there! A check at 28V showed the capacitor to have over 19V on it!

There are 10uF 35V SMD tantalum capacitors available at good prices, but these are considerably bigger than the 1uF 0603 ceramic! No chance of a retro-fit mod here!

So, although I have a fair bit of cash tied up in the parts stock for these v.1 modules, it is looking risky to sell them as capable of being reliable in a 24V Clansman system. One option might be to add a potential divider on the supply line to keep the voltages safe of course! I will test that out. The whole module takes only a couple of mA so it should be a safe method, if a little untidy.


But another issue is that a non 50/50 duty cycle is preferred, and to do that requires extra parts anyway. I will test this out on breadboard, but whether I then invest in a v.2 module for sale will have to depend on if there is likely to be enough demand. I am also however going to test out whether or not I can do away with the switching transistor.

So the v.1 warning module looks like it might be a bit of a hybrid SMT/through hole unit, with fixed 3sec on/3sec off warning. This would at least save the project!

Touchy circuits!

Having received the LM358s, I set about checking the circuit for the PRC344 battery warning would behave as required on the breadboard.

Working now with just a single 8-pin DIL device meant that much of the circuit was scrunched up on the breadboard, making it tricky to see if everything was connected properly, so it was no surprise when it didnt bloody work!

After some time fiddling and testing, I discovered that the battery level detector side of the circuit did work just fine, but the astable multivibrator side didnt!

This now was where it got strange! The astable would work, but only if I had the tip of the multimeter probe touching pin5! Or, even weirder, if I myself touched pin5 or the leg of the resistor connected to it!

I checked everything! I tried different positions on the breadboard, different feedback resistors, battery supply, capacitors bypassing the pin to ground, capacitors bypassing supply to ground, even THREE different ICs! The damn thing just would NOT work unless I touched it!

By now, I was starting to think the worst! Ive got around £20 of PCBs and components tied up in this! What the heck can be wrong? Are the ICs junk?

No. It turns out, with thanks to MrBungle on the UK Vintage Radio Repair and Restoration forum who pointed this out to me, that pin5 in this case requires a 'virtual ground', made up of the two feedback resistors I had, plus another to the supply rail! I nipped out and added another 100k resistor, and lo and behold the bugger is oscillating again!

This means of course that my shiny PCBs are already out of spec and need modding! But, an additional resistor is hardly a big mod and wont make much difference to the space requirements. One aspect of this though is that the circuit now operates correctly, which it seems it wasnt doing with the 741 op-amp, and gives a 50/50 3sec duty cycle. Making it now do the 10sec off/2sec on that I previously had, requires even more modification, with the addition of at least another resistor and a diode, most likely two diodes. I can of course do this on the existing run of PCBs but its going to start getting a bit messy! If the board does prove to have some demand for it, then a mkII version will have to be made with the extra parts.

For now, im just glad that the damn circuit does in fact work! And yes, the big problem of the optocoupler being always slightly on has been solved by using the LM358!

Dual Dual Op-Amps!

The LM358 dual op-amps finally arrived today - both types! So I now have oodles of both DIL-8 and SOIC-8 dual op-amps to play with!

The first three of the battery warning modules now have their IC mounted. Im still waiting for the higher value 0603 chip resistors though, so cannot complete and test them just yet.

Nearly...

What I can do though now is rejig the breadboard prototype with an LM358 and test the system on that, which will allow me to find the most appropriate values for the sense resistors RA and RB. I do expect the values to be a little different than the LM741 circuit, but the biggest variation will be down to which of the standard values gives the closest threshold to the desired 20V.

Some of you reading this may also be aware of the sad passing of Rev George Dobbs G3RJV recently. The G-QRP club have set up a tribute fund on behalf of the Alzheimers Society, if you wish to make a donation https://george-dobbs.muchloved.com/