Oiling a Grundig TK20 and capacitor choices

[DMcMahon]

Checked C13 in my second TK 20, it also is the 3 legged type with same connections as before, this time it is a Frako.

David

[JohnM73]

Interesting. Grundig obviously considered that cap key to getting lowest noise from the outputs connected to it.

[JohnM73]

Mentioned earlier that I’d got a complete set of replacements for all the WIMA ‘toffee’ caps, using Panasonic ECQE(F) 630V polyester types (the longer legs on these types make things easier than using modern WIMA types with their short legs). Before I dive in and start replacing, are there any of the WIMA ‘toffees’ that would benefit from being Polypropylene types instead, for increased reliability and/or performance? Or would this not matter in a relatively ‘lo-fi’ TK20? Thanks.

[DMcMahon]

I think in this application there would be no increased reliability or performance with using any modern good quality Polyester, Polypropylene or Ceramic etc capacitors.

Evidently Polypropylene are constructed from Metalized Polyester film as their base medium.

I personally prefer Polypropylene but that is just a personal preference.

David

[JohnM73]

Thanks David. Think I’ll stick with what I already have then.

[JohnM73]

Making slow but steady progress. Have nearly finished replacing all the WIMA ‘toffee’ caps and a few electrolytics and the motor run cap. A few random pics below.

Any thoughts on what one should ideally use to lubricate the reel hubs? I was thinking of a drop of Singer sewing machine oil, but would a light grease be better long-term? Thanks.

[Maarten]

Polypropylene withstands peak voltages better and has lower losses than polyester. As a general rule, capacitors connected to inductances should be replaced by polypropylene capacitors.

Of course across the mains and mains to earth capacitors should be of a suitable safety type, X or Y respectively.

[JohnM73]

I hope I haven't spoiled this repair then. I used polypropylene for the motor run cap, but polyester 630V types to replace all the Wima toffee types. Hope they'll be OK. Would assume even though they're not polypros that they're still going to be far more reliable than the Wimas that were in there? if not I'd better replace them all now whilst it's still relatively 'easy' to do so.

[DMcMahon]

Yes your new Polyester caps will be much better than the original Wima capacitors, so no need to replace them with Polypropylene.

David

[JohnM73]

Cheers David, I’ll stick with those then. The thought of going in again to replace them a second time wasn’t appealing haha!

Am cleaning up the transport hubs. Is a drop of sewing machine oil appropriate when reassembling or might a light grease be better? Thanks.

[Maarten]

Most if not all capacitors except the motor run ones and maybe some around/across the mains transformer and/or output transformer won't be connected to inductors or other circuits that tend to generate peak voltages, so polyester as a general replacement will be fine.

[DMcMahon]

Originally Posted by JohnM73

Am cleaning up the transport hubs. Is a drop of sewing machine oil appropriate when reassembling or might a light grease be better? Thanks.

I would go with the oil.

David

[JohnM73]

Cheers David, and Maarten.

[JohnM73]

Does anybody know if neoprene rubber sheet might be a good replacement for the asbestos sheet which (presumably) protects the three capacitors in that area from excessive heat? I’ve used Kemet PEG124 for those three replacement caps which are long-life high-temp types, so hoping I can get away with some kind of rubber sheet behind them instead of asbestos.

[DMcMahon]

I do not know the original intended function of the asbestos type material white sheet but I would not think anything in that area gets particularly warm.

It possibly was just for electrical insulation of the electrolytic capacitors that are mounted directly above it, i.e. to keep the capacitor legs isolated from the metal mounting panel.

I would say rubber sheet should be fine. If the existing sheet is actually asbestos then as long as it is not disturbed then it will be safe to leave it as it is.

David

[JohnM73]

Cheers David. Will likely give the neoprene foam rubber a go then for a day or two before I put the TK20 back into its chassis. In the home stretch now.

[DMcMahon]

Originally Posted by DMcMahon

Have often seen the outer screening foil indication symbol on the Grundig (and maybe others) schematics, but normally when I have previously replaced said capacitors they are standard 2 leg capacitors. I wonder if this is a German trait because UK Grundig schematics often do not show the outer foil symbol.

Checking my first TK 20, it has the same 3 leg C13 as yours.

The screening foil connection leg is the one on the bottom left of the photo, the leg is on the outside edge of the capacitor as opposed to the normal 2 legs which are centralised.

The foil leg connects to the first terminal of the terminal strip where a black wire also connects up to earth/metalwork connection at valve base. The screen of the screened cable (that connects to the input of C13) also connects to the left hand terminal of the terminal strip.

My C13 measures 27nF & 22 Ohms ESR, so I will replace it with a standard Polypropylene, there is no direct connection to DC voltage so it may well be OK but as it is one of the easiest to replace might as well replace.

David

Working on my Grundig TK 40 I have just come across another one of those 3 legged capacitors where the outer foil connection 3rd leg connects down to ground. In this case it is not on the control grid of the EF 86 but on the control grid of the next valve after the EF 86, the first stage (pin 2) of the ECC 81.

In the photo the capacitor (C16 10nF) is directly under the 470K resistor (R19).

David

[JohnM73]

Interesting. I guess it was belt and braces hum reduction/eradication in sensitive areas.

I powered up my TK20 a week ago and fortunately it all worked first time. Was a joy to hear music coming out of it again for the first time since I bought it in 2011. Had one as a kid so feels very nostalgic too. Did have a bit of hum, but I didn’t have the soldered-in shielding plate on the volume pot, or the shielding assembly screwed back into place for the areas behind that, which likely explains it. Added copper tape and a wire to duplicate that original cap too. Added new foam behind the speaker, and neoprene sheet to replace the asbestos behind those three psu caps.

Yesterday night put the chassis back into the housing. Just need to fettle the left hub braking position a little which is proving to be a tad frustrating. Need to clean and demag the beads and pinch roller too, so by later today may well have her all working again.

John.

[JohnM73]

Is it normal for the TK20 series to have a little residual hum when the volume is turned higher? (It changes with volume level). I've replaced all the WIMA toffee caps, and the electrolytic, and was extremely careful with each one when replacing but just wondering if I did something wrong perhaps. I have yet to install the top plate - would that make the difference?

Also, I've noticed it gets loud very quickly i.e. I don't have to turn the volume control much. Normal or have I goofed somewhere? Surprised how good it sounds though. Just been playing a pre-recorded Beatles album (Rubber Soul) and it's very enjoyable to listen to on the Grundig. Very crisp/clear. That smell as the unit warms up bought back many happy memories as a kid entranced by the reels doing their thing too.

[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