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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 16th Dec 2020, 7:04 pm   #1
G6ONEDave
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Default Troublesome Roberts RT22

This set landed on my bench recently. It used to belong to my father and my sister had expressed an interest in the old portable radio. So after an inspection, I found that the volume control was seized up. It was corroded down the shaft and into the mounting bush. I replaced the control with a new replacement and then powered up the set with one of my bench supplies. VHF seemed to respond but there was no MW/LW reception whatsoever. OK I know what you are thinking, it's a Lokfit transistor in the IF stages somewhere.

Well you would be totally wrong, the fault was not in the IF stages and it was not the detector diode or it's associated components. This really had me thinking, as there is not much else involved. I had checked the wavechange push button switches for continuity and all was well with them.

As a last ditch effort I decided to try injecting a 1Khz audio tone at the volume control. This was loud in VHF mode and although still there in MW/LW mode a little quieter. Back tracking to the wavechange selector switch contacts in MW/LW mode produced silence, no 1Khz audio tone at all! Ah it's a broken track, I hear you say. Well not quite, there was complete continuity from the AM detector all the way through to the switchbank and then to the volume control. Confused? well I certainly was. I found the problem eventually. There was a broken track but it was one of the power switch contacts and had been linked by a previous repairer. The actual fault was because this previous repairer had managed to short the detector output which goes to the VHF switch to the power supply switch contact next to it!

After that the set had to have the IFTs realigned as everything was out of alignment. This was when I ran into another problem. Whilst comparing the MW/LW/VHF bandspreads against another radio, I found that the VHF was somewhat out. So I adjusted the padder capacitor on the main variable tuning capacitor and it prompty disintegrated! As you can probably imagine the air turned various shades of blue.

Given the chances of obtaining a replacement tuning capacitor being between nil and don't be silly, I attempted a repair. And I am pleased to say that my replacement padder capacitor worked a treat. Not a job for the faint hearted though. I firstly removed the original remains of the padder and replaced it with a low profile ceramic pre set capacitor. Both this and the above short can be seen in the 2 attached pics.
Dave
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Old 17th Dec 2020, 8:44 am   #2
Beardyman
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Default Re: Troublesome Roberts RT22

Well done on a tricky fix!
Although I've not repaired any transistor sets (so far!) I do know just how fiddly that must have been to do, all our kit at work is mostly SMD & that can try the patience somewhat.
Excellent job!
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Old 17th Dec 2020, 12:12 pm   #3
G6ONEDave
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Default Re: Troublesome Roberts RT22

Thank you for your kind words Beardyman. The set could probably do with a little more tweaking of the padder capacitors but having had one of them fail, I have no intention of disturbing the other 3. I don't want to have to replace any more of them, as it was bad enough doing this VHF one.

I did not fully state where the short on the pcb is in the picture for those who are not eagle eyed. If you look at the blue wire, which is the previous repair for a lifted track. At the top end of it where it joins onto a switch contact, you will see a bare wire joining to it from the right. That bare wire is the end of a link wire that goes to the AM detector stage and as such should not be going to the power switch.
Dave
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Old 17th Dec 2020, 8:35 pm   #4
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Default Re: Troublesome Roberts RT22

I see what you mean now, it begs the question as to why it was done?
Was it a mistake or deliberate?
Regardless, you found it, understood what was going on & put it back as it should be.
A bonus of that was you now have a fully functioning useful radio.
Well done, have a beer!
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