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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 31st Jan 2020, 6:47 pm   #1
DMcMahon
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Default Grundig TK 5 Restoration

In late December I brought a fairly nice looking Grundig TK 5 which came with a very nice looking Grundig microphone (a GCM3 I think).

After a quick clean, replacing the 2 core mains cable with 3 core and unseizing the seized volume control, I tried recording with the microphone but the microphone appeared dead.

I found that the DC polarising voltage supply for the condenser mic was 0V (should be somewhere around 60 - 100 volts), traced to a shorted (fully shorted) Wilma paper capacitor C35 (0.05uF) that was shorting the R44/R45 potential divider down to earth and additionally C12 (5nF) at the microphone DIN socket was also shorting down the microphone signal.

While checking this out found another 3 paper Wilma capacitors that were bad (not fully shorted like the other 2 but still passing plenty of DC volts), so decided it will be best to replace all the paper Wilma caps (around 15), especially as most of them cosmetically looked bad condition

Started to have a fresh look at this today and found the main HT DC voltage supply is around 30 volts lower than when I previously checked around 3 weeks ago, so something has changed/deteriorated.

Maybe I disturbed something when I previously was checking some of the capacitors, as some of them are very difficult to access and you have to move other components/wires out of the way.

So before I start work replacing the capacitors I will first ascertain what has happened to the HT voltage, could be the mains transformer output (unlikely), the Selenium bridge rectifier (possible), the main reservoir capacitor (possible but I do not think so) or something on the load side dragging it down like one of the de-coupling electrolytics etc (quite possible).
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Old 28th Feb 2020, 11:29 am   #2
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Default Re: Grundig TK 5 Restoration

Measured the main HT voltage at the output of the Selenium bridge rectifier at 165 volts, around 70 volts lower than when I originally measured it a couple of months back.

Measured the AC mains ripple at reservoir capacitor C32 (40uF) at over 20 volts p-p, around 4 times higher when previously measured. Found that C32 -ve lead was broken off the capacitor

Also measured the -ve 24 volts supply at C36 (100uF) which was under -20 volts, previously it was -26 volts , its ripple level was much larger than before. Found that that the +ve lead was broken off C36.

Touching the broken leads back onto the capacitors restored good voltage levels and normal ripple levels.

When I originally looked at the TK 5, all 4 leads broke off these 2 capacitors during measurements and despite cleaning up the metal ends of the capacitors where the leads had broken off, had a hell of a job getting solder to take to them but eventually got the 4 leads soldered on but obviously now 2 of the repairs were not good enough.

So now will look to renew the capacitors, rather than trying to solder the leads back on.
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Old 29th Feb 2020, 11:04 pm   #3
DMcMahon
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Default Re: Grundig TK 5 Restoration

Replaced C32 & C36.
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Old 29th Feb 2020, 11:24 pm   #4
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Default Re: Grundig TK 5 Restoration

Those caps look like they could be fixed with a modified crimp connector.
I have also got a TK5 that I dragged out of a waste paper basket.
When I opened the lid the hinge screws pulled out.
I have not applied power to it yet due to signs that it has been damp.
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Old 1st Mar 2020, 9:21 am   #5
DMcMahon
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Default Re: Grundig TK 5 Restoration

Maybe could have been possible, I never considered doing that. In this case I think best to consign them to the R2R parts graveyard, when I subsequently tested their leakage the 40uF one for the main HT supply had high leakage and was passing quite a lot of DC volts.
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Old 1st Mar 2020, 1:27 pm   #6
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Default Re: Grundig TK 5 Restoration

Quote:
Originally Posted by DMcMahon View Post
... despite cleaning up the metal ends of the capacitors where the leads had broken off, had a hell of a job getting solder to take to them but eventually got the 4 leads soldered on but obviously now 2 of the repairs were not good enough.

I think the problem is that that capacitors are actually made of aluminium which is impossible to solder unless special solder is used (and I think the temperature has to be higher than usual too).
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Old 1st Mar 2020, 9:16 pm   #7
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Default Re: Grundig TK 5 Restoration

Yes makes a lot of sense.
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Old 3rd Mar 2020, 10:09 pm   #8
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Default Re: Grundig TK 5 Restoration

Tonight replaced all the old paper capacitors (12), from previous tests had found that several were badly leaking/passing DC so decided best to replace all. 10 were Wilma, 1 was a Hunts and the final one I think a Mullard Mustard.

The Mullard Mustard (C16 on the anode output of the first stage of the ECC81) is somewhat more modern and not a paper capacitor, it measured fine around 33,000pF but was the wrong value, should be 22nF (22,000pF), so looks like C16 has been replaced at some time. Probably would have been OK to leave C16 alone, as it is not a critical value capacitor but thought best to renew it.

While replacing the capacitors (some easier than others) found a mystery potentiometer, the one on the right of the 3 shown in attachment. None of the 3 different sets of schematics I have show this pot.

One end of the pot connects to the junction of R29 & R19, have not been able to trace where the other end of the pot connects to. In an original German TK 5 service manual I have, which has 2 different schematics, one of them (identified as "version with 5 switch levels" ?) does show an extra pot (R41) but connected between ground and R8 (with an extra switch contact in parallel with R41), no idea what either of these extra pots might do, a wild guess would be an equalisation adjustment.
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Old 4th Mar 2020, 12:24 am   #9
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Default Re: Grundig TK 5 Restoration

I would keep the Mullard capacitor while the rest are for the bin.
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Old 4th Mar 2020, 12:45 pm   #10
DMcMahon
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Default Re: Grundig TK 5 Restoration

Have done exactly that. Probably will never need a 33,000pF but you can guarantee if I got rid of it, then sooner or later I would need one of that value.
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Old 4th Mar 2020, 2:40 pm   #11
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Default Re: Grundig TK 5 Restoration

Then there is the 27,000pf in spinach green with a tighter tolerance.
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Old 4th Mar 2020, 3:44 pm   #12
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Default Re: Grundig TK 5 Restoration

After replacing the paper capacitors have checked all the DC voltage levels at the valves and elsewhere, all are good, including the condenser microphone polarising voltage supply at around 88 volts, this is the supply that first started this thread at post # 1.

Next to check out the tape transport operation.
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Old 5th Mar 2020, 12:57 pm   #13
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Default Re: Grundig TK 5 Restoration

Tape transport checks out well on 5 3/4" spools, 7" is to be checked later. RWD & FFD are surprisingly positive in operation end to end of the tape.

Tape counter not working as belt is missing, should be able to find something suitable later.

Play works fine mechanically, but first attempt at Playback of a tape (no tape came with the machine) only gave poor quality/very low volume sound. Could noticeably improve the quality and the volume by lifting the tape away from the first tape guide, this made the tape ride up higher on the PB/Record head.

Turning the tape over gave much better quality and volume but sound in reverse, so a 4 track recording, the machine is 2 track, by lifting the tape away from the first tape guide made little if any difference this time.

So next need to try and do some recordings and see how they play back.
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Old 5th Mar 2020, 4:37 pm   #14
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Default Re: Grundig TK 5 Restoration

I thought briefly that I had two TK5s.
I had a quick look because yours looks different.
The one I have got has a dark brown deck plastic and the other has the lighter deck plastic but is a TK15 and not a TK5.
Can you let me know the lengths of the belts?
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Old 5th Mar 2020, 5:45 pm   #15
John Caswell
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Default Re: Grundig TK 5 Restoration

DMcMahon wrote
"Turning the tape over gave much better quality and volume but sound in reverse,"
I know this sounds silly but in my servicing days it was common for people to record on the wrong side of the tape, having wound it on the spools the wrong way or kinking it twisted or splicing it wrongly.

John
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Old 5th Mar 2020, 6:01 pm   #16
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Default Re: Grundig TK 5 Restoration

The TK5 was made in two colours to my knowledge Brown and Blue.

John
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Old 5th Mar 2020, 6:07 pm   #17
DMcMahon
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Default Re: Grundig TK 5 Restoration

Yes I have seen at least 3 different colour schemes for the TK 5.

Without doing a bit of a strip down I cannot easily accurately get the lengths currently.

The German link below lists the drive belts for most of the Grundigs, it gives the ID (internal diameter) from which you can calculate the circumference/overall length, and the thickness is listed under Stärke.

http://www.taperecorders.nl/RIEMEN-nw.htm

Basic translations are :- Motor-rechte Kupplung = Right-hand clutch
Motor-Reibrad (schnelllauf) =engine friction wheel (high-speed)
Kupplung-Zählwerk = clutch counter

I also have a UK Grundig document which has some slightly different sizes:-

Tape counter belt 149/228 (1.5mm diameter)
Fast wind belt 82/229 (4mm diameter)
Clutch belt 82/228 (3 mm diameter)

The UK Grundig sizes I find most confusing, the first number I assume is the ID, if that is the case then the second number is too small to be the circumference ??
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Old 5th Mar 2020, 6:27 pm   #18
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Default Re: Grundig TK 5 Restoration

Did a very quick test recording using the lovely looking Grundig G.C.M.3 condenser microphone, it recorded OK but I did not do enough to quantify how good. Microphone seemed pretty sensitive and easy to get feedback howl if loudspeaker not switched off.

The microphone cable is damaged and the live wire is exposed, as there is over 80 volts DC on the live wire I thought best to replace the cable before doing any more.

I know it is only low current and high impedance but better to be safe than sorry.

Unfortunately where the cable is damaged is around half way along the cable so would then be too short to remake the existing cable, so will replace with similar cable.
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Old 5th Mar 2020, 6:41 pm   #19
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Default Re: Grundig TK 5 Restoration

I won't be revealing the belt goo on mine just yet as I have a nice late 1970s Aiwa cassette machine waiting to have its belt goo revealed first.
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Old 5th Mar 2020, 7:04 pm   #20
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Default Re: Grundig TK 5 Restoration

Quote:
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The TK5 was made in two colours to my knowledge Brown and Blue.

John
You may well be right John, I thought I had seen 3 colour variations.

David
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