Marconi 2022 Repair

Marconi 2022 Signal Generator repair and testing.

I recently bought a used Marconi 2022 signal generator. Having had a 2019 for some years. I was happy to have something which was considerably smaller. The 2019 is a real beast. Physically large and heave with it.

Once it arrived and was unpacked I was very impressed, size and weight were excellent but I needed to get it on the bench and tested. That’s when I discovered, Huston we have a problem. It powered on and I quickly connected it to a VHF/UHF hand held transceiver and dialed up 145Mhz. Yes we have a signal and its as expected. As i changed output level I could hear the attenuator relays clicking away. and all looked good. I changed frequency to 433Mhz and that where the problems started. No signal.

First think. if you are working on old Marconi kit do join the channel. A great bunch of people there and very knowledgeable. I logged in and checked for service manuals and downloaded them. I also found some very interesting YouTube channels on the 2022. One from a chap called RFMAN RFMAN. Check out his very informative video.

I opened the 2022 and started to carry our some checks. I bought a thermal camera attachment for my phone a few years ago. Its a great asset to fault finding and as you can see below. I found the fault very quickly and only needed the meter to confirm my findings. There are at least 2 x 100 ohm resistors which feed voltage to the VCO’s. There are 3 of them on the board. One resistor was showing signs of over heating and when I did voltage checks it had 12 volts on the rail side and 0 volts on the VCO side. Something is shorting it out. I removed the PCB and UN-soldered the resistor. Using the meter on ohms range I checked on the VCO side and the DC rail was at 0 ohms. Likely candidate was the electrolytic capacitor, a 220uf at 16volt unit. There are two on the board. I removed both and replaced them. I think someone had replaced the second one as it was a different manufacturer to the faulty one. The one shown below was a dead short. once the PCB was back in the unit I retested and it was back to normal. VCO getting voltage and all frequency ranges working. I refitted the screen can on the VCO’s and the shielding and case and retested. All working correctly and as expected.