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Old 11th Dec 2020, 7:52 pm   #1
G6Tanuki
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Default R24 capacitor problem fixed.

In the dim and distant past when car-boot-sales were still a thing [last year!] I bought a R24 for a pittance. The initial hum-when-on-mains-power issue was resolved by replacing the big grey smoothing-capacitor (these 'see' voltage all the time the radio's fed with mains-power, irrespective of whether it's actually turned on using the on/off/volume control, so they do tend to dry out a bit).

Problem was, the tuning was crackly on FM. I put this down to a failing Polyvaricon - which being a component that can't be serviced, led me to put the radio to one side and forget about it.

Dug it out again today for another play - the crackliness/scratchiness was now noticeable on MW/LW too. Which made me a bit suspicious.

short story - opened up the radio and realised that the capacitor case moved slightly when you tuned it.

A-Ha!

Fought with the tuning-string/pulley and was able to tighten the two screws holding the Polyvaricon to the PCB, and re-flow the soldered joints to its moving/fixed plates.

Result: no more crackling! I also took the opportunity to 'tweak' the coverage a bit - originally it covers 88-104MHz but now it does 93-108.5 which is more useful. OK, the dial calibration's no longer sensible but I can put up with that, and it's an easily-reversible tweak should I ever want to listen to Radios 2 or 3 in future.

People denigrate the later small Roberts radios but I rather like them.
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Old 11th Dec 2020, 11:58 pm   #2
pip5678
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Default Re: R24 capacitor problem fixed.

Quote:
Originally Posted by G6Tanuki View Post
I also took the opportunity to 'tweak' the coverage a bit - originally it covers 88-104MHz but now it does 93-108.5 which is more useful. OK, the dial calibration's no longer sensible but I can put up with that, and it's an easily-reversible tweak should I ever want to listen to Radios 2 or 3 in future.
I have one of these and wondered about doing the same thing, I take it this was an adjustment of the FM oscillator? Thanks
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Old 12th Dec 2020, 9:29 pm   #3
G6Tanuki
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Default Re: R24 capacitor problem fixed.

Yes, you just need to tweak the two FM trimmers [antenna and oscillator] on the Polyvaricon; set the tuning to around 103MHz and open the oscillator-tuning trimmer fully, then when you're receiving a station, peak it with the antenna-tuning trimmer.

it can be kinda handy to have a scanner or a frequency-counter on hand to see what frequency you're tweaking the LO to.

Tweaked, I can now happily listen to the Swindon relay of "Greatest Hits Radio" on 107.9MHz
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Old 13th Dec 2020, 11:05 am   #4
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Default Re: R24 capacitor problem fixed.

Thanks G6Tanuki much appreciated, I take it the FM oscillator trimmer is c313 and antenna trimmer is c306 off the diagram?
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Old 13th Dec 2020, 2:58 pm   #5
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Default Re: R24 capacitor problem fixed.

Yes - they're the ones - on the polyvaricon itself.

I used a scalpel-blade to put tiny scratch-marks on the plastic next to the original trimmer screw-slot positions before doing any tweaking so - if I got it wrong - I could return things to original with the minimum fuss.
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Old 13th Dec 2020, 4:17 pm   #6
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Default Re: R24 capacitor problem fixed.

Brilliant, many thanks again.
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