mcHF - Making A Start

 Ive spent some time recently on just feeling my way around the physical construction of this radio. Its a very compact unit, so it makes sense to take the time to really understand how it all goes together.

Im now in a position as to confidence to make a start. Since I have been learning the construction of the case, it made sense to install the loudspeaker first, since that requires the absolute least work! But even this was a learning experience, as it taught me that there are a few of the Japanned machine screws that are just that little bit longer, and seem to be intended for mounting the power components. 

Having watched several peoples videos, read as much of the .io group files and posts as I can, and found and joined a Facebook user group, Ive got a good idea how it all fits, and a number of pitfalls. For instance, it looks as though one of the holes in the RF screen will need widening to accommodate a tall capacitor. Theres a little trickiness as well in installing the ESD protection neon lamp, as its board holes are underneath the BNC connector!

So, later, I will install the neon, the BNC, and the various connectors and switches. With these in place I will be able to see better where and if the RF screen requires adjustments!

1:48 Merlin Helicopter Build Restarts

 With so many long term projects, Ive finally pushed myself to bring out the Merlin. My boys bought me this as a present a couple of years ago now, and for various reasons it was shelved awaiting better times.

Part of the problem is with the paints. I wouldnt normally do a 1:48 scale, I much prefer 1:72, which are large enough for me, without too much fiddly detail. On a 1:48 kit, especially one of an aircraft with multiple projections (antennas, scanner optic, lamps, Pitots etc) there is an immense amount of tiny parts each with their own painting requirements. Its possible to spend a lot of money on paint to do no more than one or two brush strokes! But since it was a present from my boys, I couldnt skip anything!

Much of today has been spent doing very fine exterior parts, antennas etc, and building the tail rotor. Dozens of parts painted, assembled, installed, and apart from it now standing on its own wheels - it looks no further on!

Clocking On to Kit Building

 Two packages arrived for me today. One was my long awaited (ok, not long at all, ordered last thursday!) mcHF Transceiver kit. More, probably much more, on that later!

The other contained two items, both simple, low cost electronic project kits to build with Tom. In this case, they were the two different clock kits. Hence the post title...

As with most of these cheap Chinese kits, documentation is rather sparse, consisting of a parts list and schematic, and in the case of one of them, a layout diagram as well, plus a fare bit of 'instructions' in the form of ideograms!

But, these kits have come on in quality terms recently, now, the PCBs are generally excellent, if sometimes a little unconventional (who else has built Pixie CW transceivers with all the tracks on the same side as the components?) and the silkscreening on both boards shows not only the part number and placement, but also the value. Armed also with the schematic.... sorry, circuit diagram (im appalled with myself for the level of Yankish I find myself talking these days!) building these things is a doddle, ideal for a beginner.

On the subject of instructions, or lack thereof, we come to the mcHF. This open source (and hence much copied) Software Defined QRP HF Transceiver is of course intended to me a foundation from which modifications and developments can be made. Trouble with that of course, is that it means there is no one, single, immediately relavent set of instructions for building it!

Thta doesnt bother me really, but I am a little apprehensive as to how to be sure that some essential error correction of modification hasnt occurred that im unaware of! Much of this though is just the need for me to trawl all the available data, of which there is considerable, and find the pertinent info.

The photo above shows the mcHF kit removed from the box but not yet the rest of the packaging. One the left is the anti-static packaging containing the two PCBs. As these are pre-populated with the SMT components, I have not yet opened this up to look, and will only do so when I have them on the anti-static bench. The two packets in the center contain the 'though-hole' parts, the top pack the parts for the RF board, which comprises, hardly surprisingly for a transceiver, a heck of a lot of toroids and binocular cores to wind, plus connectors, relays and the like; the bottom pack is the parts for the User Interface (computer/AF) board, and is mostly connectors and switches. To the bottom right is the LCD display, and above right the metal case and hardware pack.

I intent to construct this sloooowly! Although there are not that many parts involved, the boards are complex, and although I expect that the pre-population should be good, my first two tasks will be to carry out a complete inventory, to ensure I have all the supplied parts, and a thorough detailed inspection of the PCBs, looking for any defects, errors, dry joints, solder splashes etc. This is not intended to detract from the kits production - i dont expect to find anything! - But with such a complex system, an hour spent with a magnifying glass ans continuity tester could easily save many many hours of confused fault finding later!

New Fence Challenge

 After serving 20years of my Arqiva sentence for crimes against communications technology, I was awarded a long service payment, which has gone almost in its entirety on the cost of materials for replacing the 30ft run of fencing alongside the patio.

I wont go into the rigmarole of actually purchasing the materials, but suffice to say i was rather miffed to be told that Wickes wouldnt deliver them because 'they are heavy', yeah no shit!

So after roping my brother in law Chris and his van into the job, im now armed with around a tonne of concrete posts, gravel boards, postcrete and fence panels.

And so the build begins! The first task, was to remove the old fence. This was made from pallet wood nailed to 3x3 posts attached to those gawd awful 'metpost' fence spikes. Most of this was rotten, so it was just case of pulling it all to bits. This was when I discovered that the previous job had been done by Randolf Scott on a rare day off, or at least by some other bloody cowboy. The post spikes had been concreted in!

Most of yesterday was used up in hammering, chiseling, pick-axing etc the block for the first post of the run. To make the job much more technically challenging, the fence runs in a narrow raised  area between a single thickness retaining wall, and a lower, panel fence owned by my neighbour - neither can risk being damaged! On top of this, the first post needed to go right at the side of the house wall.

Having eventually removed the concrete block and the post spike, and dug the hole to the required 2ft depth, the next task was to transfer, on my own, a 45kg 8ft concrete post from the drive, around the corner, down a step, to the hole. Knowing I couldnt lift the post entirely by myself, I eased it up one end at a time onto a barrow! It was then slid directly off the barrow and into the hole.

Because of the location of the house wall, I have no easy access to the other side of this post! So, the first pour of postcrete was done with the post leaning out at 45 degrees and held up on the shaft of the pickaxe. As soon as the concrete mix was in, one of the old timber posts was positioned on the other side, one on the nearside, and the concrete post pushed up into position, checked for level and plumb, and the timbers adjusted to support it.

This post will need another pour of concrete at some point, and some of the old fence planks have been cut down and placed in the hole to act as shuttering for the next pour to prevent the excessive sized hole taking up too much concrete. Exactly how im going to get it around the back, behind a 6ft high post next to a 10ft high wall, in a space only accessible from next-doors garden and then only if I remove some of their fence, well, i'll improvise that job later!

And so today, with one post in place, the job continues, in the intermittent rain, of removing the rest of these bloody awful post spikes and the concrete surrounding them!

Binoculars and other Disappointments

 Well, we finally had a clear enough night to allow me to properly test the Celestron Skywatcher 20x80 binoculars. Sadly, they have proved to be faulty.

Having worked out that to mount them on the threaded fitting of the telescopes tripod required the removal of the 70mm 'scope's OTA (Optical Tube Assembly) and the rotating of the binos onto the mount, as neither the binos threaded fitting or the tripods fitting could be actually 'screwed', i was finally in a position to take a look at some suitable astronomical targets - Mars and the Moon.

Well, the collimation error is such that the two images were out of alignment by around a third of the width of the moons image! To make things worse, this was the vertical alignment - horizontal collimation can (on older, not brand new!) binos be sorted with Conditional Alignment. Mars was two totally separate planets!

So, these are going back! I guess this is where I find out just how good the customer service at Carmarthen Cameras is!The Amazon return system only gave me a refund option, not a repair/replace, and no pre-paid shipping, so im going to be out yet another tenner (on top of the extra tenner that the bloody binos cost in the first place!) in sending them back.

Another disappointment this week was to find that my missing bottle of Zolcal-D to treat Grace for he soft shelled eggs, is clearly never going to arrive! The seller hwes-758 racked up almost 400 negative feedback in the last month alone, and most of their positives were actually complaints! They are now ive just seen no longer a registered user. I can only think that they have been running some sort of interest rate scam - hold the money until forced to refund, if you do that with enough peoples money you make on the interest, although i cant see it being an effective scam at the present bank rates! Looking at other listings for this product, it looks like they are playing this game with several user names!

So, while I await ebay refunding me (have to wait until wednesday!) I ordered it from Amazon for a couple of quid more and it arrived next day!

Automotive Wiring Is A Pain In The Arse!

 Given a normal bit of desk or rack mounted 12v powered equipment, fitting a simple fused changeover relay would take me around 5min, and that includes working out the wiring route and tying it into the loom.

Exactly the same task, but on my car? Well, after 2 hours I'd found somewhere to mount the relay, and got the control wire through the bulkhead! and thats with the wire running parallel with the already routed bloody thick high current cable for the transceiver!

The purpose of this relay is to kill power to the transceiver when the ignition is off. As it currently stands, I can easily leave the radio on without noticing, with the result that I go to set off for work after 4-5 days off, and the battery is flat!

So, a 30A relay and a suitable tap into the switched supplies is needed. A few quid. But far more hassle! First of course, I had to find a suitable switched supply! The dashcams are fed from the switched feed to the wiper motor controller, which being an 'add-a-circuit' type piggy-back fuse holder connection, actually covers up several fuse positions! Luckily, Ive found that F44, a 3A feed to the cars audio system, is accessible. An extra 100mA or so on this wont hurt, and it keeps all the audio/comms systems on the same main circuits.

Then, where to mount the relay? This needs to be close to the battery, but of course cannot be on it! The nearest available secure place I could find was a hole on the corner of the EMU housing! Turns out the aluminium housing of the EMU is floating! Neither grounded nor as best I can determine isolated! Luckily the mounting bracket of the relay is isolated, but it means a longer wire run from the relay coil to ground at the battery negative terminal.

So with this selected, the tedious and trying task of getting a guide rod through the bulkhead rubber gasket was next. The rod in question being a 1.5mm stainless steel antenna rod. Thin and flexible to do the job, but none the less a real pain when single manned. Without an assistant, it was a case of push a few centimeters, climb out the footwell, into the engine bay, look but mostly feel for the end of the rod, and repeat, until such a time as I could finally grip and pull the rod far enough to attach the wire, without damaging anything else close by - cables, hoses etc! With the single 5A wire (blue) taped to the end of the rod, and an ample estimated length payed out from the spool, came the nerve wracking task of pulling the rod back though the bulkhead into the footwell beside the fusebox, hopefully without losing the end! Well, it took considerably more force than expected to get the bulge of the joint through!

So now, I have the relay fitted in place, hand tight, the control wire fed through, crimped and connected to one side of the relay coil. The ground connection wire is crimped and connected to the other side of the relay coil, has the eyelet crimp attached, but it tied up and taped ready to be attached to the battery negative terminal later. The fusebox end of the control wire is unterminated, and the positive high current connections are currently a loop of 30A wire crimped and attached to both of the relay contacts! This is because A - I dont have the bloody 'add-a-circuit' fuse module to complete the control connection, and B - Ive run out of bloody butt-joint crimps to complete the high current wiring with!

And so the circuit will stay in this state, as will the passenger side footwell and seat, which are littered with tubs of crimps, fuses, wire, tools etc, as well as the temporarily removed, but still full, glovebox itself, until such a time as I can get to my automotive parts supplier. Which could be hours, might be days.

And all this came about because im stuck in the house awaiting a delivery, and I also had a blown brake light to replace!

Conditional Alignment

 Gawd, that sounds like some form of sexual psychoanalytic technique! But, its actually a way to fix buggered binoculars!

Binoculars, like other optical systems, have a number of parameters that have to be in tip top alignment  for them to work properly. Some, such as chromatic aberation, coma, etc, are inherent to the glass itself and can only be solved by buying the best. But others are a physical alignment issue. In particular - collimation.

Ive come this because I decided to give away one of my pairs of 10x50 binos. Back-yard astronomy is a great and relatively simple hobby to get into for a youngster, but many people make the mistake of jumping straight to a telescope and are disappointed when the images they see are nothing remotely like what the press tell us they should be. Sorry, but nothing you can buy at amateur level is going to give you views like what are published in magazines and on the web from the Hubble space telescope or the myriad of astrophotographs created by allowing software to 'stack' hundred upon hundreds of long time exposures! Buy the biggest possible mirror in your budget, lets say thats a 8" dobsonian - and Andromeda will still be a vague fuzzy oblong! Sorry, thats how it is! Combined with the tiny field of view which makes actually finding the thing you want to see difficult in the first place, and its no wonder so many give up!

But it shouldnt be like this. The night sky is amazing - even if all you can see are the brighter navigational starts from a light polluted city. Its the way people are introduced to it that makes all the difference! Dont go out and buy a telescope! Go out, and buy - a planisphere! Or a simple, beginners level star atlas. Then - simply go out, and look up!

Of course, just looking denies you some of the more easily found wonders. But a pair of binoculars will reveal a plethora of amazing sights. Even for me, the sheer background of the starfield when using a pair of simple binos to look into the Milky Way is astonishing.

So when a work mate mentioned their daughter was interested in the night sky and wanted a telescope, I asked if they had any binos. With the answer being no, I decided my spare pair would perhaps give the young person a much better start into the hobby.

Now, This left me with my ancient Miranda 10x50s, that I bought when I was about Sams age, and still at school, and the cheap Bressers of Sams. This is when, in deciding to check them over, I found that Sams had been sat beside a bottle of aftershave, and all the rubber had gone sticky! I also found that the collimation was so far out that there may as well have been two of everything! So, how to fix?

Well, the sticky rubber was solved by rubbing a load of talc into it! Its not perfect, but pretty much the best I can do without the impossibility of replacing the rubber parts.

The collimation is something that I dont have the equipment to do fully, but it is relatively simple to do what is called 'conditional alignment', this is where the collimation is corrected, for the specific interpupilary distance in use. In other words, it fixes it for anyone whos eyes are the same distance apart. This is done by very carefully adjusting the prism adjustment screws hidden just under the rubber covers! I have done a 'rough and ready' job on these, entirely by trial and error with them focused on a pylon some 10 miles away. A more accurate alignment could be done using specialist Bahtinov masks, which are a form of grating, but I cant be bothered with the expense or difficulty in making my own, for a cheap pair of binos.

Anyway, these are the pair Tom was using up on the hill, and he assures me the images were sharp and not doubled, so it seems to be good enough for now.

The Sky At Night

 is, generally, cloudy. Sods law really. With my interest in most things waning, i find myself coming back to what has long fascinated me - the night sky. Despite a rigorous scientific background and a moderately deep understanding of the heavens, im still able, perhaps more so simply because i understand them so well and dont have to resort of mindless fantasy fiction (otherwise known as 'religion'), to be amazed by what can be seen.

To this end then, I decided now was a good time to improve my astronomy capabilities. Having decided on a budget and considered long and hard as to what kind of instrument I wanted to go for - do I go for the greater light gathering of a big Dobsonian mounted reflector, or the easier aiming of a slightly smaller set of optics on an equatorial mount, etc - I made a choice. Only to find it out of stock, in all variations, everywhere!

It seems, which people unable to go out and about as they used to due to the pandemic, that people are choosing to try out the more solitary pastime of amateur astronomy, and the shops are simply running out of stock! Such strange knock-on effects are starting to become apparent in many widely diverse activities, such as the famous flour shortage as everyone took to home baking!

So the telescope has not yet been upgraded, but I did manage to get my 2nd choice instrument. I had made a selection of the Celestron Skymaster 20x80 binoculars. These were a great price from my preferred supplier. Now, im going to briefly mention these and my recent experience. This is simply a statement of my experience and I dont wish it to be taken as anything but. My preferred supplier is Rother Valley Optics. When i dealt with them before in the shop i found them very helpful, and being a great price and a 'local' indepenant supplier wished to use them again. Having to do so online though, I found my enquiry regarding suitability, which included a very basic 'beginners' question, to be answered rather briefly, with none of the personal insight I would have expected from a friendly amateur astronomer. Also, my order for the binoculars was subsequently cancelled and refunded with no more than a note to 'check stock levels next time', no query as to whether I wished to hold on back-order until the stock came in. All in all I was somewhat disappointed by the experience. I do appreciate that these are difficult times, but as they say "civility cost nothing, young man!"

Instead, my order was re-placed through Amazon, with Carmarthen Cameras. This cost me £8 more, but at least they had stock! Having no direct contact with the store, I can only report on the fact that they arrived quickly, and the packaging while not substantial was adequate.

Now, these binos are not pocket eyeglasses! They are huge! Ideally they should be mounted on a tripod, but my only spare one is a lightweight one for the SLR and rather flimsy. Nevertheless, myself and Tom took a stroll up the local hill yesterday to test them out. My main concern was ensuring that they were properly collimated, the slightest error in collimation and they would be on their way back to the shop! Much testing on very distant objects and I can say, at least within the limits of my own horrendously myopic and astigmatic vision, that the collimation is good. The arm strain as a result of hand holding them for an hour not so good! A couple of targets proved remarkable for testing these, one was a lucky chance of a hot air balloon over York, which to the naked eye was just noticeable as a dark balloon shape, in Toms 10x50s as a dark blue balloon, and in these as a dark blue balloon with a yellow and blue checkered top section! The other was a pair of aircraft warning lights (this of course was mid-afternoon) somewhere in Leeds city center. Not even the building they were on was visible in the 10x50s!

On the night these arrived the sky was clear and I did have a chance to play for a few minutes before it clouded over. Its been overcast since! But the main joy of the trip up the hill yesterday was teaching Tom how to use binoculars! With the light weight cheapo Bresser 10x50s that i'd just repaired (so far out of collimation that there was literally two of every object! and the rubber sticky from alcohol degradation that needed talc rubbing in), I spent some time to teach Tom properly how to set them up, starting with Interpupilary distance, followed by eye relief, and dioptre setting, then focusing. I think the excited shouts and comments can be taken as confirmation that he enjoyed the experience!

One thing ive realised though, is that I dont have anything I can comfortably sit/lay out in the garden on to observe! Im hoping that with it now being autumn, cheap sunloungers might be on discount somewhere!

No Enthusiasm

 Ive not posted on here for quite some time. What with the Covid-19 lock-downs, the unprecidented and unbelievable scene of the control of the nations radio and TV being effected by dispersed engineer-operators working from home (myself included - I carried out one of the initial tests of the system), the travel restrictions, the cancellation of fairs, rallies, and other events, and then the sheer confusion and inconvenience of returning to working from the office....

...I just cant be bothered!

This is true, I seem to have lost all enthusiasm for not just radio and electronics, but pretty much everything other than reading and taking long baths. (this is itself is nothing unusual - ive lost interest many times before, however those times have usually coincided with life events, for instance losing interest in radio during the years I serviced public safety radio systems) The only odd thing is that this time, the loss of enthusiasm is making me anxious.

I still have many projects on the go, but I just cant bring myself to get on with any of them. I do recognise that this is a detrimental aspect of my mental health, although im rather glad that it isnt manifesting itself in the form of panic attacks! Oddly, as a result of working from home using telephony headsets and with without people close by, my telephobia has actually improved. I seem somewhat more confident and able to use the phone with less hesitation and worry, even, I feel, to perhaps having a bit of time out portable with a nice bit of HF combat net radio!

Of course, much of my current state may be attributed to the drugs im using to control other symptoms. These include some rather potent controlled substances, plus a few 'special' molecules, and are helping me overcome the insomnia, and settling down the Willis-Ekbom. Unfortunately they are also allowing me to observe and analyze life from a 3rd person perspective.

I do feel that I need to get back into some level of radio though. Not from the shack though I feel, but perhaps portable, a little SOTA maybe. Little by little myself and Bob M1BBV have managed to find slots to get a little walking in, so a few SOTA activations may be on the horizon. If nothing else, I need to get out and have some time in the wild open countryside.

I just need the motivation and inspiration to get started!

There will be more interesting posts eventually, I assure you (whoever you are, although I doubt I have many 'followers') but for now, like almost everything in life, its a case of keeping distance, and waiting.

No go on the 5V SLAB

Well, the SLAB that was reading 5V has definitely had it! Initially, it looked good, and I was able to transfer it to my Accucel charger. But then, the voltage started to drop, even on charge.  It was always unlikely that this one would recover.

The one reading 10V, I was much more conservative with the water (about 2ml per cell), and have left this on the slower charger. Its still only taking a very low current, just a handful of mA, but this is steadily increasing.

So, who knows?

Refurbishing dead SLABs?

I use quite a few 12V 7Ah Sealed Lead Acid batteries (SLABs), which I normally obtain as surplus due to routine replacement in UPS equipment. Recently, this source hasn't been available due to the pandemic, and I find myself with one usable battery and two dead ones.

The two dead ones are reading 5V and 10V. Shaking the one reading 5V, it audibly rattled! Clearly, it wasnt as wet in there as it should be!

So, I thought i'd try and refurbish it! The lid of these is lightly glued on, and it was fairly easy to prise it of using a sharp chisel. Under the lid, the cells are closed off using pushed on rubber caps. These easily lifted off, revealing the plates of the cells, all but dry!

As it happens, I had a bottle of de-ionised water on the shelf in the workshop. Using a syringe, ive refilled the cells up to the top, and then gently sucked the excess out of the filler holes. This proved to be a rather messy job, so a roll of paper towel was kept handy to mop up any spills and splashes.

With each cell refilled, the caps replaced, and the lid put loosely back on, it was time to try it on the charger. For this, I am using my 'semi-intelligent' SLAB charger, and monitoring the current into the battery using a multi-meter. This showed the current rapidly ramping up! Ive left it for now as the current was passing 60mA and climbing. A check of the battery voltage showed it to have risen to well over 7V at this point.

Whether the battery will recover enough to be useful, who knows. Its certainly worth a try.

Now, one thing to note - I have added more water to this first test battery than required. There is a reason for this - I dont know yet how easily the glass matting absorbs the water. It may be that it takes several hours to do so. Supposedly, the correct amount of water is just enough for the matting to appear wet.  I will top up the second battery more slowly and observe.

The first battery had dropped to about 6.5V over the 7h since I stopped charging. On restarting the charge, the current input was up at 400mA and rising. The charger can supply 1.5A. As it is now daytime and I can keep an eye on it, I will allow it to charge during the rest of the day and see how it does. For safety, it is in a plastic box and covered with paper towels.

Off-Grid BOINC research computer proof of concept

Quite some time ago, after a mobile phone upgrade, I hard wired a Buck converter module to the battery terminals of a Samsung Galaxy S7, removed the SIM card, and left it crunching numbers as part of the BOINC project, sat in the corner of the shack powered by the main station PSU. In its steady operating state with no display lit, it consumes about 130mA, which is probably less than the operating current consumed by the big linear PSU powering it!

My plan has always been that my main BOINC contribution should be independent of mains supply, so, after a slight debacle involving a fraudulent sale on ebay of a 100W solar panel with 30A charge controller, which when tested was shown to be a 20W panel and the controller hardly built for 10A, I have a solar panel and charge controller with which to set up a proof of concept... and half my money refunded!

The concept is simple. The BOINC phone and its Buck converter connect to the charge controller load terminals, a 12V 7Ah SLAB to the battery terminals, and the 20W panel to, well, the panel terminals of course! Unfortunately, the only battery of this size I have remaining in an operational state is the one used to power the under stairs lighting! This has been borrowed temporarily. It will be replaced for the time being with a 25Ah beast that just happens to be doing nothing.

The whole lot is in a position where it will receive a decent amount of south facing sun through the day - on my eldest boys windowsill! He wont mind - its already covered in IT and computer parts!

Some tests on the phone showed a peak current at start up of about 450mA, and a steady operational current (computing but display off) of about 130mA. This with a fully charged and 100% capacity 7Ah battery should last several days. At full sun output, the 20W panel should be capable of charging a 7Ah battery in around 5h. So in theory, this set-up should be able to operate 24/7. Of course, this battery is not new and far from peak condition!

If it operates without shutting down in the middle of the night using this battery, then I can have confidence in it operating well with a better 2nd hand unit, which I will have to scrounge up another time!

NAVTEX Antenna repair - bypassed

The bypassing is now in place. A 100nF cap from pin 1 to pin 11 of the mixer ICs pads (remember that the IC isnt there!), this effectively connects the secondary of L3 to the supply rail. Since this system uses a combined power/signal line, that should now be feeding the receiver. A temporary link takes the other side of the secondary to ground. One leg of a 680 ohm resistor has been lifted to isolate the DC from the secondary, and a final temporary link jumps the missing L4 secondary to re-establish the DC supply to the front end transistors.

With the NAVTEX receiver in spectrum test mode, I can now see 'noise' where-as before this was totally blank, unless I put a finger on the supply wire. I can get a peak to appear on the display if I crank up a 518kHz signal from the Marconi 2955 test set. This is air coupled from a whip antenna, and had to be cranked up to about -20dBm output, but moving the antenna and its feeder around near the whip does suggest that the pick-up is by the antenna and not via the feeder or any other means. So far so good, but the proof of course will be whether it actually receives a NAVTEX transmission!

I have the station reserve receiver tuned to 518kHz as well to compare. Sadly, at the time of writing, were in a quiet part of the transmission schedules!

NASA Active Dual Channel NAVTEX Antenna Repair

Despite what the title says, its unlikely this will ever be a dual channel antenna again!

For a start, the PIC 12C508 IC, which provides the switching control and the local oscillator for the channel conversion, is not working. Whether it is just physically damaged but the code can be read, will have to be discovered. Even if the code will read, chances are that its been protected and cannot be used.

The mixer IC MC1496 has also been removed from the board. I have no easy way of testing this, so have to assume for now that it is faulty. Ive removed it to prevent any damage from causing problems with the rest of the circuit.

And so we come to L4 (as I have designated it). This is the output tuned transformer. And its knackered! I had noticed some corrosion under it, as there was with the PIC and the previously rotted off crystal. On removing it from the board I found that the fine wires of the primary had completely corroded away.

This seems at first a major problem, but in reality probably not. The frequency its tuned to is known, 518kHz, and its series capacitance also known, 150pF, so its a simple matter to calculate the secondary inductance needed, which is 630uH. Replacing it is just a matter of finding a suitable 10mm can transformer with that impedance secondary and the same winding configuration.

But for now, I have the trouble of bypassing all this to try and get the direct 'pass-through' 518kHz channel working. All the transistors tested out OK off board on their DC parameters, so hopefully its just a case of getting the supply to them and the signal from them!

The secondary of L3 is the problem. This would normally feed the mixer inputs, but also has a DC level on it for one of those input pins. So, I need to isolate that DC level from the transformer, provide a DC block from the transformer to the output/supply wire to allow the signal out, and temporarily ground the other side of the winding. I 'think' that I can isolate the DC by lifting one leg of one of the 680 ohm resistors, and a 100nF cap should work for coupling the signal.

NAVTEX Active Antenna

Ive finally come back to the antenna for the NASA Target NAVTEX receiver.

Ive mapped out the circuit diagram, and can now start working out how to fix it.

My suspicion of the lack of 518kHz was with the transistors, but ive had them off the board today and tested, and they check out ok.

Now, I know the PIC oscillator is not working. The original crystal was corroded, so has been replaced by one close but not exactly right. I would have expected it to work but give perhaps slightly off frequencies. I suspect that at some time the antenna feed has been reverse polarised, and this has killed the chip. It might well have killed the MC1496 as well.

So, the plan now is to remove the MC1496 and the PIC 12C508 from the board, plus lift any associated components that might affect the signal path, and see if I can get it working just on 518kHz, by bypassing the conversion circuitry. Im also going to remove the transformers and test them off-board to ensure their windings are intact, as some corrosion/dirt can be seen under one of them. Testing them on board is pointless as the DC circuitry is routed through them/across their pins.

If I can get that working, then I can work on the conversion side another time. 

Nasa Marine Target Navtex Pro Plus

Purely because I simply wanted one, I've eventually picked up a purpose built NAVTEX receiver. This is a Target NAVTEX Pro Plus unit from Nasa Marine. Second hand, and faulty, of course!

This is an older model, with external channel switching, as the people at Nasa Marine have been kind enough to supply me with a hand drawn copy of the receiver circuit diagram proving this. So the receiver itself can only handle the 518kHz broadcasts. To cope with the 490kHz National services, the active E-field probe antenna has frequency changing capability.

The receiver works - I have it connected to a short indoor length of wire, which allows it to receive some of the strongest signals. Its internal back-up battery is not in a very good way, but that's not much of a worry.

The antenna and Local/International switch box have issues. There is some corrosion on the antenna PCB - one tantalum bead capacitor has lost a leg - but it doesn't look too bad. There is a PIC micro-controller that looks to be serving as local oscillator, which has a lot of green gunk around and under it. One of the PCB traces looks rather poor as well. So long as the corrosion has not got into the transformers, repair shouldn't be too tricky. There is a destroyed track in the switch box.

So, a bit of linking out and cleaning up required!