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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 8th Nov 2022, 9:49 pm   #1
Charles Wallis
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Default Grundig TK 20 poor replay and not recording

I am looking for some advice please. I have a TK 20 that was found in an attic a couple of years ago, not used for decades. Though I now know better, I just switched it on and much to my surprise it played back family events and music of the mid sixties. The problem is that the replay quality is poor with the sound quite muffled. I have tried to record, and though the microphone seems to work and is able to be amplified via the loudspeaker, it does not write to the tape. There is also a lot of background hiss when in play mode, even if the tape is paused. The magic eye EM84 lights up nicely but barely moves when I press the mic buttom and shout at it.

I have cleaned the tape heads but it made no difference. I am not sure how much of the problem relates to the very old tape, but I suspect the machine itself. I am wondering if its possible to make it work properly but not sure how feasible it will be.
I have taken the chassis out and obtained the German schematic. All looks in order. The valves look fine for what that is worth and no obvious heat damage. I have checked HT voltages and with schematic values in brackets they read as follows: MainHT 255 (260)v, HT1 226 (240)v and HT2 218 (235)v. A little low but nothing way out. From this I assume the metal rectifier is OK. Smoothing capacitors C31, 21 and 19 at the points above are bolted onto an asbestos sheet which is a bit worrying, so given the choice I would rather leave them alone.

From my reading of this forum the HF oscillator often fails and this would explain the failure to erase. It is in a metal box but looks like I could get at it and do some recapping. I gather C3,4 and 5 are the usual suspects.

The audio output stage looks reasonably accessible and I could recap that.
The Pre-amp stage however looks quite tricky to get at. The components are on both sides of a circuit board in a metal shield bolted upright onto the chassis with EF86 and ECC 81.

So I guess my question is where to start given that the pre amp looks the most disruptive to work on. I have restored a few valve radios before but this would be my first tape recorder.
Any advice?
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Old 9th Nov 2022, 12:10 am   #2
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Default Re: Grundig TK 20 poor replay and not recording

With several fault symptoms it is not easy to know where to start.

For erase/record not working I recommend that you check that the main HT voltage is being switched through to the EL 95 oscillator valve via the series combination of the W2 contact of the Play/record relay and the B3 contact of the Selector switch when switched to record.

If that is OK and you have an oscilloscope then scope the oscillator circuit to see if it is oscillating, it will be a fairly high frequency (too high for many voltmeters to read) sinewave.

Many but not all of the TK 20 non electrolytic capacitors typically will be the brown or black Wilma paper capacitors, which are commonly called "Toffee sweets".

These (because of their age) will often have become electrically leaky and will pass a certain amount of DC voltage, which can lead to various problems, particularly when the capacitor is coupling an anode output from one valve to the control grid input of the next valve. In the case of the EL 95 output pentode valve this potentially can have serious consequences leading to damage to the output valve and the output transformer.

Unlike some people, I generally do not recommend wholesale recapping unless it is proven to be required. In the case of the TK 20 and similar vintage TKs, I do find that most of the paper capacitors do need to be replaced.

For the grid coupling capacitors it is easy (if can be done safely) to measure if they are leaky by seeing if there is any +ve voltage arriving at the control grids.

For the output valve there are actually 2 grid coupling capacitors in series, C22 & C26.

Some of the paper capacitors can be tricky to locate and replace.

Because of its relatively low value, C4 in the oscillator may not be a paper capacitor, if it is not then it probably will be OK.

David
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Old 10th Nov 2022, 8:54 pm   #3
Charles Wallis
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Default Re: Grundig TK 20 poor replay and not recording

Thanks David for your advice,
I dont have a scope but will check those voltages where possible. There do seem to be a few brown "toffee sweets" in the circuit. It may take a while but I will let you know how I get on.
Charles
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Old 10th Nov 2022, 9:10 pm   #4
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Default Re: Grundig TK 20 poor replay and not recording

I would first confirm the signal from the tape is getting fully into the tape head and then into the electronics. Possible causes if it's not: worn head, misaligned head, corroded or dirty head even though it was thought to be fully cleaned but not actually checked visually, faulty pressure pad or pressure tape on some Grundigs, etc. As much as they are electronic, tape machines are also very much mechanical and often require mechanical attention.

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Old 11th Nov 2022, 3:09 pm   #5
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Default Re: Grundig TK 20 poor replay and not recording

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Originally Posted by Charles Wallis View Post

From my reading of this forum the HF oscillator often fails and this would explain the failure to erase.
It would not explain the EM 84 display hardly changing when shouting into the Microphone in Record mode, are you adjusting the correct record level control ?.

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Old 11th Nov 2022, 3:18 pm   #6
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Default Re: Grundig TK 20 poor replay and not recording

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Originally Posted by Charles Wallis View Post
Thanks David for your advice,
I dont have a scope but will check those voltages where possible.
Charles
In the Service manual the record bias and erase are checked/adjusted by monitoring the voltage (V) drop across a resistor (R), the manual states a current (I) spec. The resistor is fitted in the chassis return side of the head for the test measurement (there is a nearby terminal strip where a link can be removed and a resistor fitted in its place).

But you still need a voltmeter that can measure the nominal 78kHz oscillator signal

David
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Old 12th Nov 2022, 2:54 pm   #7
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Default Re: Grundig TK 20 poor replay and not recording

Personally, when I see a 'toffee' Wima, it's a change on sight. I've never found a good one in any Grundig (or any other brand that used them) over a certain age. They really had a terrible reputation in the trade.

I even found one in a Revox once, the audio coupling cap....dead short and it hadn't done the output valve any good at all.
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Old 12th Nov 2022, 4:16 pm   #8
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Default Re: Grundig TK 20 poor replay and not recording

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Originally Posted by Charles Wallis View Post
I have taken the chassis out and obtained the German schematic.
From the Forum Instant Downloads there are 3 English Service sheets available for download, all including the schematic.

https://www.service-data.com/section.php/3453/1/tk20

The Manufacturer's Service Data one is the best, it also covers the TK 25.

The Radio & TV Servicing Data one has more pages and also covers the TK 24, TK 25 & TM 20 but its schematic quality is not that good.

David
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Old 24th Nov 2022, 10:38 pm   #9
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Default Re: Grundig TK 20 poor replay and not recording

By way of update and thanks for the advice above.
I have established that HT gets to the oscillator OK so set about some recapping.
I took the oscillator apart, which was actually quite easy. You just unbolt the metal box, twist away the retaining tabs and the circuit board pops out. It turned out I was not the first to go there. Clearly there have been problems before. As the first picture shows C3 has already been replaced with a grey RS cap (looks like circa 1970's). I replaced this and C4 and C5. (2nd pic). I noticed that the disc of 300 ohm variable resistor R3 was fractured, and a 100 ohm fixed resistor has been put across it. It connects the cathode of V3 to ground. I am not sure the adjustment is meant to be for, so I just left it alone.

Then I turned my attention to the output stage (third pic). It was a bit more fiddly to work on, but I replaced C26, 29, 34 and 35 (5th pic). Some of these are the notorious "toffee sweeties" Wima caps. After this I checked the V5 cathode voltage and it is zero or negative with respect to ground, which I think is correct.

I have tested it out, and it does seem to be better. There is still a lot of background hiss and noise, sometimes it crackles loudly, but the sound is much louder and it does now record when prevously it did not. Its a bit variable, but sometimes it is quite good reproduction and the magic eye does seem to work, but you have to be quite close to the mic. I do wonder if some of the switches and relays are dirty and that might explain the crackling.

That leaves the preamplifier stage with V1 and 2. It has several "toffee sweeties" and looks quite difficult to access. (4th pic with valves out). I have the caps ready , but wonder if it will benefit and dont want to damage it.

So does anyone have any advice on the noise etc and whether I should go ahead with the front end. I like a challenge!
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Old 25th Nov 2022, 12:36 am   #10
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Default Re: Grundig TK 20 poor replay and not recording

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Originally Posted by Charles Wallis View Post
I noticed that the disc of 300 ohm variable resistor R3 was fractured, and a 100 ohm fixed resistor has been put across it. It connects the cathode of V3 to ground. I am not sure the adjustment is meant to be for, so I just left it alone.
R3 is for adjusting the bias current to the set value detailed in the service manual, after first peaking (if required) the bias current with variable capacitor C2.

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Old 25th Nov 2022, 12:55 am   #11
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Default Re: Grundig TK 20 poor replay and not recording

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Originally Posted by Charles Wallis View Post

So does anyone have any advice on the noise etc and whether I should go ahead with the front end. I like a challenge!
For noise and other symptoms, it is always recommended to clean the contacts of the Record/Playback switch.

However on the TK 20 the record/playback switching is done by relay W.

On one of my TK 20s I opened up relay W with the intention of cleaning the relay contacts and loads of washers/spacers and contact arms all sprung out, I doubt I will ever be able to re-assemble it.

I would recommend to replace the front end capacitors, it is tedious I know.

David
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Old 25th Nov 2022, 5:00 pm   #12
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Default Re: Grundig TK 20 poor replay and not recording

Thanks David. I will do the caps, I dont mind it too much. I think I might skip the relays to avoid wrecking it, though I guess switch cleaner will do no harm?
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Old 26th Nov 2022, 11:12 am   #13
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Default Re: Grundig TK 20 poor replay and not recording

Yes if you can get access to spray some contact cleaner in, then exercise the relay many times.

I was planning to spray contact cleaner in after removing the relay cover as a first measure and if that did not help then try using fine abrasive on the relay contacts particularly any that looked burnt/tarnished etc.

I have done this before without problem on similar TKs and do not know why on the TK 20 I had the disaster of the many insulated spacers and washers and contact arms all springing apart and bits everywhere.

David
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Old 26th Nov 2022, 11:29 am   #14
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Default Re: Grundig TK 20 poor replay and not recording

The best way to clean the contacts is NOT with abrasive means but to use a piece of thin card (business card or similar) cut into strips and soaked in switch cleaner then worked back and forth between the contacts gently pressing them together. Most contacts have a thin silver coating and an abrasive will remove that and the contact will need re-cleaning after a short while.
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Old 28th Nov 2022, 9:55 pm   #15
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Default Re: Grundig TK 20 poor replay and not recording

My Grundig TK 20 is better, though the sound is still muffled after recapping the oscillator and output stage.
I have managed to get into the preamp and take some readings with DMM while running.

The actual voltages in volts (schematic) are as follows with respect to ground:

V1 EF 86 Grid -0.71 Anode 84 (56)

V2a ECC 81 cathode 2.5 (1.9), grid 2.45 and anode 28 (110)
V2b Cathode 2.9 (2), grid 2.9 and anode 64 (140)

So my conclusion is that the V2 triodes have very low anode voltages and positive grids.

My plan is to replace grid coupler caps C17, C18, C20, C22.
Does that seem a sensible approach?
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Old 29th Nov 2022, 12:34 am   #16
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Default Re: Grundig TK 20 poor replay and not recording

Yes it does but a few extra comments.

The Control Grid voltage at ECC 81 pin 7 will vary depending upon volume setting.

The +ve control grid voltages do show that the respective grid coupling capacitors are somewhat leaky, however replacing them may not help with the low anode voltages or the muffled sound.

Are your HT voltages at top end of the anode load resistors reasonably close to the values shown in the schematic, almost certainly will be somewhat lower due to aged bridge rectifier being less efficient.

EF 86 R21 HT = 190V
ECC 81 R26 HT = 235V
ECC 81 R30 HT = 240V

Quite common for anode load resistors to go high resistance, so check yours, if outside their max % tolerance, consider replacing them.

On the EF 86 check the Screen Grid voltage (44V on schematic), if very low check Screen Grid components R22 & C16, C16 most likely will be a leaky Wima.

Was not C22 changed when you did the output stage ?

David
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Old 29th Nov 2022, 1:47 pm   #17
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Default Re: Grundig TK 20 poor replay and not recording

Thanks David,
I have checked those voltages again.

EF 86 R21 HT = 190V Actual 185
ECC 81 R26 HT = 235V Actual 221
ECC 81 R30 HT = 240V Actual 230

So HT not far off.
EF86 anode now 70v, screen 36, again not far off though anode still bit high.
ECC81a anode now 50v, ECC81b anode still 64 v. All very low.
R26 200K and R30 50K HT feeds are within tolerance and HT is as above, so I cant see a fault there.
The set is stone cold this morning in a garage so might explain slight differences from yesterdays readings which were after a long run.
I could not get at C22, but now I can.
Can you explain the low triode anode voltages?
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Old 29th Nov 2022, 4:07 pm   #18
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Default Re: Grundig TK 20 poor replay and not recording

No not easily if R26 & R30 resistances are OK.

David
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Old 17th Dec 2022, 10:18 pm   #19
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Default Re: Grundig TK 20 poor replay and not recording

Problem solved!
I have now replaced all the grid couplers in the pre amp stage and it is working well. Specifically C13, C16, C17, C18, C20 and C22. These were mostly Wima capacitors and were quite a struggle to get at (pic 2), in particular C20 which is buried on a back row of a tag strip under the circuit board. The whole thing is very cramped to work on.
Of note the ECC81 a and b anode voltages that were far too low (28 and 64v) are now close to what they should be (96 and 130v) and the grids read zero to just neg rather than positive. The removed Wima caps all measured way out of tolerance and a few megohms of leakage with a DMM. I noticed that the HT feed resistor R26 was partially melted into one of the caps it was next to. So my conclusion is that the leaky grid couplers led to positive grid voltages, high current draw hence voltage drop and heat from the HT feed resistor (which measured fine). I presume this had severely affected performance and had caused the poor reproduction and recording.

Putting it all back together (pic 1) it has good clear reproduction and is able to make good quality recordings, free from any hiss or muffling. The magic eye now works well. A success story I think. Thanks to all who gave advice on the forum.
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Old 18th Dec 2022, 12:21 am   #20
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Default Re: Grundig TK 20 poor replay and not recording

Well done, a very good result.

A nice looking TK 20, the blue lower front panel looks more attractive than the green ones.

David
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