Oiling a Grundig TK20 and capacitor choices

[JohnM73]

Originally Posted by DMcMahon

Have yet to find time to get my two TK 20s playing, so cannot comment specifically about the TK 20.

I would say generally it is not abnormal to get some hum at the higher volume settings, particularly on old valve machines, but it should not be really noticeable at normal music play levels.

I doubt you have goofed.

David

Cheers David. I’m so close to the finish line it’s tempting to just get the top plate on and call it a day to be honest, but I’ll get one of my two spare ‘backup’ (if spares ever needed) TK20s out the loft and check the shielded head wiring between the two to make sure everything is routed correctly, and I’m not picking up any induced hum from AC areas. Didn’t recall my previous TK20 getting loud quite so quickly with the volume control only turned up slightly, which might explain the hum behind the music increasing too. Maybe I’ve done something that somehow increased the gain.

John.

[JohnM73]

Just noticed the following in a service manual. So will try giving R49 a slight tweak on replay. I’m assuming it’s the adjustable resistor housed inside the oscillator can, but will trace the wiring from the mains transformer secondary first.

[DMcMahon]

No its the humdinger potentiometer.

David

[JohnM73]

Ah, so that (rheostat?) pot to the left of the left hub then? That’ll be handy as I can leave the chassis in the case.

[DMcMahon]

Yes the wire wound pot near the supply spool carrier.

David

[JohnM73]

Thanks David, that’s great! Will mark its current position and see if a slight adjustment might reduce the slight hum I’m getting.

[JohnM73]

Well not sure what’s going on with my recapped TK20. Adjusting that humdinger doesn’t seem to do much/anything. But of more concern is I also noticed during a silent bit of tape, if I turn the volume above 2, I get a sort of slow vibrating noise from the speaker. Is this motorboating? Will see of I can get a video up of it somewhere, and put a link here.

[DMcMahon]

I generally find that humdinger adjustments seldom do much to any residual hum both on listening tests and also when scope monitored.

David

[JohnM73]

I’ve put it back to where it was originally, which was all the way to the left.

Video of the possible ‘motorboating’ below. Wondering if a squirt of contact cleaner into the ‘W’ relay might help here:

https://********/p/2naXyJv

Edit: Just seen this thread. Will definitely give that relay a try:

https://www.vintage-radio.net/forum/...ad.php?t=66731

[DMcMahon]

Yes certainly a good example of motorboating.

Depending upon how bad the relay contacts are, they may need physical cleaning as opposed to contact spray.


David

[JohnM73]

Cheers David. Will see if I can give the contacts a thorough clean. Looks difficult to get in there though unless the clear plastic cover can be easily removed? Failing that will spray contact cleaner onto strips of paper and run that over the ends of each contact point.

[DMcMahon]

What ever you do, do not remove the 2 top long screws like I have just done, thinking they hold the cover on. They do not hold the cover, I think the cover can be prised off over the screw heads.

With the screws removed the switch contact arms and numerous insulating and metal pieces all fall out in a heap, I very much doubt I will be able to work out how to re-assemble correctly

David

[JohnM73]

Sorry to hear that David. I have two ‘parts spares’ TK20s in the loft. If you need a complete relay again I can desolder one and send it you for free.

[DMcMahon]

Thank you very much for the offer John, I will keep that in mind.

I have a second TK 20, both are in poor condition and I was intending to probably end up with one working machine, so I should be OK.

When I get more time I will have a go at trying to reassemble the relay.

David

[JohnM73]

No problem - the offer’s there if the relay proves to be tricky to put back together.

Just had a quick look at mine. Looks like there’s a metal tab at the front which needs to be bent down, and (on my unit at least) theres a large piece of grey electrical tape on the bottom, and a smaller piece of black tape along the top of the transparent relay housing.

Hopefully some lighter fluid will aid removal of both bits of tape, so the housing should then pull off from the front, allowing me to clean those contacts thoroughly.

[JohnM73]

Cleaning of the relay went without a hitch (famous last words!), and I'm no longer getting the motor boating when the volume is raised.

Still getting the (admittedly) fairly minor hum when the unit it turned on, which is a bit disappointing after the hassle that replacing all the caps turned out to be. Still concerned that the volume appears to be far too loud at the first 'dot' setting on the volume wheel too.

Im going to take off the shield again behind the EF86 and ECC81 (*) area, just to make sure I didn't nudge any wires to where they shouldn't be when replacing the WIMA toffee caps. It is pretty cramped in that area.

Wondering if any of the valves might be the cause?

Failing the above I think I'll have to get somebody to look at it at some point as it's sadly getting beyond my ability (and simple test equipment) to solve alas.


(*) Would accidental transposing of those two values cause the volume to be too loud?

[JohnM73]

Getting a light crackling sound coming from the speaker now as well as the constant low level hum. I must have done something wrong somewhere, unless a dodgy valve might cause these symptoms. Cleaned all the valve socket pins etc.

Recap of all the symptoms:

1) Volume level is already too loud before the dial is even on ‘1’.
2) There’s a low level but still rather audible hum from the speaker. Constant volume level. More of a ‘purr’ than a hard edged hum.
3) Starting to get crackling, like static, through the speaker.

Deck has been recapped - All Wima ‘toffee’ caps replaced. All electrolytics replaced. Motor run cap replaced.

[DMcMahon]

Originally Posted by JohnM73

Deck has been recapped - All Wima ‘toffee’ caps replaced. All electrolytics replaced. Motor run cap replaced.

Does this include the cathode bypass electrolytics ?

David

[JohnM73]

Originally Posted by DMcMahon

Does this include the cathode bypass electrolytics ?

David

Yes indeed. From memory I think there’s just two 10uF ones. Will double-check the wiring and polarity on those, just to rule them out.

[JohnM73]

Still having issues with steady low level hum from switch on, regardless of volume pot level. Wondering if one of the new smoothing caps is bad, though seems unlikely as they’re a good brand (Kemet, formerly Rifa). Have rechecked the wiring too.

Crackling has gone once I gave the relays a further clean, so one issue down.

I’m going to take one of my other TK20s down from the loft and carefully compare the wiring between each to make sure it’s nothing obvious I’ve done after replacing all the WIMA caps, and all electrolytics. Might be my imagination but I’m sure I can smell a plasticky type aroma once it’s been on a few minutes, though no signs of any obvious burning/melting anywhere. Might check inside the oscillator box however.

Otherwise, I’m stumped, and will have to seek out somebody in due course who can service this and travel to Basingstoke (I don’t drive alas due to illness).