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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 15th May 2021, 8:15 am   #1
RobertFM
Triode
 
Join Date: Feb 2016
Location: Fife, Scotland, UK.
Posts: 18
Default Roberts R500

I could do with some help with this.

I am very much a beginner at repair, although I have been a casual collector for 30 years. I have fixed a few transistor sets and a couple of valve sets over the last year. Most of my knowledge (such as it is) is from trawling through the forum here, and watching the usual suspects on YouTube.

I got given this 2 or 3 years ago and have just got round to looking at it over the last few weeks. I've seen a few dirty Roberts but this might be the worst. I had previously had a R500 and got it working by replacing the AF series transistors, so was hopeful I could tackle this. I have the traders sheet 1635.

When switched on there is static/ white noise but no discernable signal on any band. The noise level is unaffected by the volume control. I've replaced TR1 TR2 and TR3 with gt322bs. I've replaced the usual electrolytics. There is no sign that the oscillator is working - I've checked with another radio. The quiescent current is slightly high at 18mA (the trader sheet suggests it should be 13mA). The rest of the readings suggested on the trader sheet are fairly close with the exception of the reading on R3 which is 2.1v as opposed to 1.1v. The 2 diodes seem to check out OK. My cheap signal tracer doesn't pick anything up on it. Touching the volume pot with a screwdriver doesn't elicit anything. I've cleaned and replaced the switches. Nothing I have done has made any meaningful changes as far as I can ascertain to the way the radio presents.

I think I've reached the end of my limited knowledge.

Any ideas ?

Robert
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Old 19th May 2021, 6:46 am   #2
Scotorvm
Pentode
 
Join Date: Jul 2020
Location: Falkirk, Stirlingshire, UK.
Posts: 147
Default Re: Roberts R500

Hi Robert

I have an R500 in the exact same condition.
Got this one to remove the knobs for another set, but figured I may as well try and get working and use non spec knobs on this set, just to play in the workshop.

Have zapped all the transistors, replaced all the caps, now replaced all the transistors. TR1 and 2 for GT233B and TR3 for Af125. Same as you though only noise is a crackle when switched on or turning the volume up. Have thoroughly cleaned the tuning cap and switches. Hopefully there is something else we can try.

Cheers
Lee
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Old 24th May 2021, 5:49 pm   #3
RobertFM
Triode
 
Join Date: Feb 2016
Location: Fife, Scotland, UK.
Posts: 18
Default Re: Roberts R500

Hi Lee
Thanks for the response. I'm no further forward - I've changed 2 resistors (R12 and 13) which were way over tolerance, but the rest seem OK. I don't like being beaten, but there are other radios to play with while I try and work this out, so I'll put to the side for a week or so.
Robert
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Old 25th May 2021, 4:53 pm   #4
agardiner
Hexode
 
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 340
Default Re: Roberts R500

Hi,

So, if you are not getting anything when touching the volume pot, then the first port of call should be the audio amp. Have you checked the biasing from the centre of the two batteries? It should be going to R15. After that, I would check TR4 and TR5; either of these could cause the problem and also increase the quiescent current.

I would want to be able to hear the amp working correctly before moving to the oscillator section.
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