Codar AT5 completed project!

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This has been an enjoyable and nostalgic project.  My vision was to have a rebuilt Codar AT5 and Power Supply working fine on Topband AM, just like the old days. My biggest problem was the power supply, it is not easy to find components to give an output of a 300v, 150v and 6.3v. After an appeal on Facebook and Packet Radio, Dave G4JLG came up with a suitable transformer. I went to see Dave and he showed me around his shack, his antennas are impressive to say the least! Next I needed a suitable aluminium case to house the PSU and transformer. I had an old 12v PSU case that I didn’t want any more, so I stripped it down and used the case, just the job. One problem that I had was the smoothing choke, I found one on e-bay which was ideal as well as a nice 32mf / 32mf 500v smoothing capacitor.  The PSU also has the switching for the AT5, so I bought a suitable 4 pole 3 way switch and PL239 sockets for the antenna feeds. The OA2 stabiliser valve and socket I got from the Rochdale Rally. I added a couple of indicator bulbs (actually LEDs) to show NET and TX, these are powered from an extra 6.3v winding on the transformer.

The completed Power Supply and case.

Now for the Codar AT5 itself, I didn’t want to try to power up the transmitter without doing some renovating. The inside of the transmitter is in very good condition, some of the old resistors had already been replaced. The 160m / 80m switch had fallen to pieces as well as the AF gain potentiometer, so these needed replacing which I did without any problem. A couple of high power resistors also needed changing and I tested all the other components with my test meter. Now the big problem…on the back of the AT5 is a B9A socket which provides the power to the transmitter. I needed a B9A plug to connect the PSU to the Codar AT5. They stopped making these years ago and nobody stocks them when you look on line including e-bay. Another appeal on Facebook was needed, this time helped by Malcolm G3ZNU who kindly sent me one. Now I was ready to power up this little beast and see what it can do!

It works fine! Now got an HT voltage of 250v on load from the power supply and giving a carrier output of 7 watts. The AM modulation seems to be working and sounds ok, but I need some reports now to see what it really sounds like. Last night I came on the Topband Bolton Wireless Club 1.953Mhz net and got reports from Dave, G4JLG, Ian G0CTO, John G4NTY and Geoff G4DZK. Nice reports, audio sounds fine, but modulation level a bit low. The audio gain is turned right up, so I decided to investigate the problem. I looked at the circuit diagram and realised that there are two 10mf decoupling capacitors in the cathodes of the audio stage. I don’t remember seeing any small electrolytics when I was rebuilding parts of the transmitter, I had a look and sure enough, hidden beneath the wiring were two old electrolytic capacitors. I changed them and tested the AT5 again, WOW! what a difference, the modulation level shot up, so much that I had to back off the audio gain control. Fantastic!

My vision…just like the old days, including the Acos Microphone!


Rochdale Rally Nov. 2017

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This time I went to this rally with a list of bits that I wanted, mainly for my Codar AT5 and Power Supply that I am building. Parts are becoming difficult to find now. I needed an OA2 stabiliser valve and B7G valve base, a B9A plug to connect to the AT5, some 2 or 3 watt resistors, grommets, rubber feet, and other bits and pieces.  Amazingly, I managed to get all except the B9A plug!

I really enjoyed this rally, lots of second hand stuff and junk to sift through, but this one was different…I have never seen so many morse keys in all my life!  There seemed to be morse keys of one sort or another at most of the stands, but one stand had literally scores of various keys…CW Heaven! Also an abundant number of Z-Matches, you can see at least three in the first photo.

I’m surprised that nobody bought the Stolle Rotator in this photo. I had one on the chimney for over 25 years and it was a brilliant rotator cost me £15. I have just bought a Yaesu rotator and it cost almost £300!

Acos Microphone

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To go with the Codar AT5 I need a high impedance crystal microphone.
Eventually, I found my old favourite Acos Microphone on e-bay!
My original one that I used with my old topband transmitter was grey, the audio quality was superb. I used it for years until one day I left it hanging on my anglepoise lamp. The heat destroyed it!
I looked around for ages to find a replacement, then one day I passed a second hand shop in Radcliffe and there in the window was an Acos microphone. The shop was closed, so I asked my then girlfriend (now wife) to buy it for me. She did.  It was black rather than grey, identical to the one I bought on e-bay last week.

Codar AT5

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At last I have managed to get hold of a Codar AT5 Topband AM Transmitter!

I built a copy of the At5 back in 1971 when I first got my licence (and before) It was my first major valve project. The Codar AT5 was built in the late 60’s and cost about £16, a lot of money then. A friend of mine had one and I loved going to his house to play with it and work some stations around the Walkden area. It amazes me how they managed to get all the valves and components into such a small box.
My latest project is to clean and repaint the case and chassis and hopefully bring it back to it’s original state. One problem though…I need a suitable power supply!  The AT5 has it’s own matching power supply, but they are even more difficult to get hold of now. Looks like I will have to build one from scratch!

WSPRLite for lower bands

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WSPRLite is a fascinating little device.  It send out a WSPR beacon on 10Mhz or 14Mhz supported by DXplorer to see your results and plot graphs, see maps of spots and even compare your own antenna to others in your area. It is quite addictive trying out my antennas on different bands.  To be able to use the lower bands, I bought the matching SOTABEAM filter board for WSPRLite. I am now testing my antenna on 40m,80m and 160m. The results of both my antennas and propagation are amazing to see.

First I wound the filter toroids.

Then built up the rest of the filter board.

Connected to WSPRLite…now awaiting some results!

First results. The great thing about WSPRlite is that you can compare your own antenna to someone else. In my graph mine is blue and G8LIK is red. Looks like his antenna is better than my longwire. The amazing part is that you can see the change in propagation conditions. During late morning and early afternoon 80m completely died, I listened on the band and could hear almost nothing.  Around 3.30pm onwards the band improved very fast, notice how the band died earlier and then picked up later compared to G8LIK.  No DX today, just spots around Europe with the furthest being EA8BFK at 2929Km.

Radio Astronomy

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This month’s Sky at Night program was all about meteorites and how to find and detect them especially during the Perseids meteor shower. In the program was a section that showed some Radio Astronomy which detected incoming meteorites. This involved listening to a powerful VHF radio station out of range normally, in this case France (GRAVES) . When a meteorite enters the atmosphere it leaves an ionised trail which reflects radio waves back down to the ground. This is heard as a ‘ping’ and can be viewed on a computer. Impressive, but I thought it looked complicated.

Then the following night I went to a meeting of the Bolton Wireless Club and Ross (G6GVI) told me how he had re-created the set up on Sky at Night by using a piece of software called ‘SpectrumLab’ and a receiver tuned to 143.050Mhz to monitor the GRAVES VHF station in Dijon, France.  I decided to have a go myself with some amazing results! I used my Yaesu FT857D tuned to 143.049Mhz and set to USB with just my collinear as an antenna.

The first image shows a larger ‘ping’ along with some smaller ones recorded over about half an hour just after midnight shortly after the Perseids peak.

The second image is captured over a longer period, from around midnight to 8:00am, it shows lts of small pings along with some quite big ones. Remember that these are taken after the Perseids maximum, but still a lot of activity.

Easy Topband Transmitter by G3OGR

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At last…I managed to find the original article in Short Wave Magazine!

In 1970 just before I got my Amateur Radio licence I built my very first transmitter for Topband. The circuit came from Short Wave Magazine and was produced by F.G. Rayer G3OGR. My first attempt was built on an aluminium chassis and my Dad made me a front panel out of mild steel. At the time Neil, G3ZPL had built the same circuit and it worked great! The circuit consisted of an EF91 valve in the VFO and another in the buffer. The Power Amplifier (PA) was a 5763 valve which could run about 15 watt input.
I have never been able to trace the original article, I remember that it was in SWM and had a green cover with TRIO on the top. I even wrote to SWM to see if they could trace it in a back issue, the guy managed to find a ‘similar’ circuit by G3OGR, but it was not the original with all the construction details. There is a nice website which has scanned copies of  the archives of many of the old radio magazines. I managed to find the magazine that I was looking for…. April 1970!   (You have to scroll down a bit to find the beginning of the magazine)


After spending weeks of painstaking drilling, filing and soldering I finally got the transmitter to work, but it had a problem. The VFO seemed to react with the PA. When I tuned the transmitter to the antenna the VFO would pick up spurious signals and shift up the band! Although I had an OA2 stabiliser valve this did not solve the problem. Maybe this was caused by the power supply dropping in voltage when the PA was taking more power.
Later I borrowed my transmitter to George G3ZQS and his antenna got hit by lightning which welded together the vains on the tuning capacitor! Not to mention the Power Supply which ‘blew-up’ – one of the smoothing capacitors exploded leaving a permanent dint in the roof!
When I got my licence I rebuilt the transmitter with a screened VFO in a separate aluminium box, and an aluminium front panel, this worked really well and was completely stable. The modulator used an ECC83 which had a high gain, I seem to remember using an ECC82 which had a lower gain, but same valve pin configueration.
F.G. Rayer produced loads of circuits and designs for transmitters and receievers in the 1960’s and 70’s. Almost every month had a design from this guy in Practical Wireless, Short Wave Magazine or Radio Communication. Most were variations of a theme and at the time he was the Enid Blyton of radio. I have to admire Francis G. Rayer, his designs were always well written and easy to follow. I recently found out that he also wrote science fiction books!
Thank you G3OGR!