Uniden 2020

Last year a friend of mine was having to downsize his radio collection and he gave me a number of items he did not wish to sell because they had minor problems and he did not have time to look at them. Some of them I passed on to new Hams if I thought they could make use of them. One item was of particular interest to me, a Uniden/Tempo 2020 HF transceiver. I soon discovered it had several faults and I thought these faults were well documented on various web pages. How wrong I was !.

My first job was to insulate the high voltage area to keep my fingers and anyone helping me, out of the high voltages, remember this is a BoatAnchor rig and the valves (tubes) operate on a very high voltage ! While working on the little beast, I dropped something on the power supply area on the under side and was greated with a BIG blue flash and a splat. Seems like no damage done, did not even soil my jocks. My guess the stray wire or capacitor pigtail fell on some mains wiring.

As I would be working on this thing for some time now I decided to make the under side much safer. I cut up a document folder to get a BIG sheet of clear plastic and fixed it in place with thin double sided tape.

The next problem to fix was the broken AGC switch, a three position toggle switch. I rummaged on the web and searched forums but could not find a replacement. I had several Micro brand centre off three position switches and soon worked out I could solder a mounting plate over the mounting lugs for the original switch and attach the new switch via its mounting hardwere. I will continue to look for a replacement for the original switch.

I had to remove the synthesiser and the VFO to replace components in these. Thanks to two web sites for the "heads up" on these faults. I have also obtained a new set of gears from a supplier on Ebay, these rigs seem to have more gears than any other radio I have worked on. The original gears crack and eventually may jam the mechanism.

The first two days of working on this rig, I made a little progress and wasted quite a bit of time chasing a couple of "red fish". I looked at the VFO signal into the board where it is mixed with the off-set oscillators for each band. It looked far too small so I took the VFO apart to see what was happening in that and it's external board. The VFO seemed to be all good, plenty of signal. Should have looked at the circuit diaram a bit closer, the three output from the VFO are coupled through 2 pF caps and a CRO probe, even on 10X, attenuates these signal so you see nothing. I Replaced the 2 pF caps just in case and put that part back together. Then I checked the RF path from the antenna socket to the RF board, seemed to be open circuit.

"Red fish" number two, the RF gain pot is a pot accross the antenna input and when I wound it fully clock wise I could get a signal through. I then applied a BIG RF signal into the antenna socket and yes I did receive but at very low sensitivity. Needed about 10 mV to hear anything at all. Tried various bands and was able to tune the signal with the VFO just as Uniden and the Japanese Gods intended. Next step was to go back to the RF board and do more tests on the dual gate FETs, several web sites mentioned these were a problem. Luckily they were in sockets and I replaced the RF front end FET's and still no improvement. Did not look at the conventional mixer FET, a big mistake ! The last picture above will give you an idea of how difficult it is to remove the RF board to do any repairs.

After struggling trying to find more faults for a few days I was mentioning this project at our North East Radio Club Wednesday morning coffee club at the local library when Colin, VK5ACE, happened to mention he was the local "Mr fixit" for these many years ago and he would like to take over the project for me "just for old times sake".

Here is the list of faults Colin found:

  1. Receiver very deaf when signal put straight into the RF board, Required 5 to 10 mV RF input to get any output. Traced this fault to traces of tin whiskers across the tuning gang. Removed the whiskers and sealed with conformal coating.
  2. Not receiving when RF when put into the RF antenna connector. Traced to a broken wire from the relay to the socket. Removed the old PL259 socket and fitted a new Teflon socket.
  3. Receiver still deaf, traced to the local oscillator board and the mixer FET. Fitted a new FET and sensitivity was now better than 0.63uV
  4. Turning to the transmitter, no RF output on the 80M band. Found coil L259 open circuit. Luckily Colin had a spare coil from his days repairing these rigs.
  5. Screen resistor burnt, fitted a new 15 K resistor.
  6. Preselector out of alignment, adjusted RF driver to receiver preselector.
  7. Digital display intermittent, reworking solder joints on the input connector to the display fixed this problem.
  8. No ALC showing on the meter - found two resistors out of tolerance, now ALC working. Suspect the output tubes may requite replacement as the output power was a bit low

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Written by and Copyright, Phil. Storr © Last updated 30th March 2019