Regentone ARG71 Trimmer Capacitors

[Dave1360]

Hello,
I'm restoring a Regentone radiogram which I believe to be an early ARG71 and have the two pages of service data from here. As this is lacking in any alignment detail, I'm using that for the A155 whose circuit is similar give or take the missing SW band and magic eye.

There are the usual two trimmer caps for MW - OSC and RF, but neither has any effect on frequency or signal level. Touching the adjusting screws with a metallic screwdriver does slightly alter the frequency, which to me makes it sound like the earth isn't connected on either. I've sprayed them both with contact cleaner with no effect.

Having moved on to the VHF alignment, there's a similar trimmer between earth and the transformer feeding the mixer grid. The alignment instructions say to adjust this to give minimum voltage on the front end anode - not sure exactly what this is supposed to do, but again there's absolutely no change to anything when adjusted.

Hopefully I've uploaded photos of the trimmers. Does anybody recognise the type and have any suggestions as to what the problem is and more to the point, how to fix it? All three only have one terminal visible, I'm assuming the earth comes through from underneath, but I can't get to the underside as they're wrapped around the switch block. Whilst it works with a good signal on all bands, the alignment is a little out and ideally I'd like to get this closer to spec.

Thanks in advance,
Dave

[cathoderay57]

Hi Dave, I've encountered that type of trimmer before. I think they simply relied on a metallic contact deposited onto the porcelain dielectric, but with ageing the metal coating just disintegrates when you turn the screw. I'm afraid replacement of the trimmers is likely to be the best, probably the only, solution. Postage stamp type preferably or maybe beehive trimmer should do the job, if you know the value. Jerry

[Dave1360]

Thanks Jerry, that makes sense. I'll have to weigh up the pros and cons as it's looking like major surgery to rip out the trimmers to replace and it's pretty tight to in there to add anything on top. If I can get reasonable reception on the few remaining AM stations then I'll probably put up with the less than perfect alignment. I'd prefer to put in the effort for FM but that's probably even more risky.

Cheers, Dave

[cathoderay57]

Hi Dave, there's a possible bodge that you might like to try. Assuming it's most likely that the failed trimmers have lost capacity, you could try slaving in a low value fixed silvered mica or ceramic capacitor in parallel with the failed trimmers. Try a selection of values from 2pF to 10pF and see if the signal strength improves. It looks like at least one of the trimmers in your screen grab is at HT potential, so be careful and use capacitors of an appropriate voltage rating. Jerry

[Dave1360]

Good plan, Jerry. I should be able to find a nearby earth for all three caps. I've just checked the full circuit and nothing is at any potential - that transformer from the grid goes up to a 50pF capacitor elsewhere.

I'm working on this in the room over the garage so it's going to have to get back down to a sensible temperature first!

[MotorBikeLes]

The adjusting screw in the middle is "connected" to the top plate by silvering. Sometimes turning the screw leaves the top plate stationary.
If the top plate turns with the screw, you could try first cleaning with IPA, then painting the screw with conductive silver paint.
Les.

[See_Mos]

I was considering replacing some of those ceramic ones using the 6MM ones available on eBay. Just search for 'trimmer capacitor'