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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 14th Jan 2020, 10:26 pm   #1
DMcMahon
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Default Grundig TK 23 Distorted Sound

Just having a quick look at another of my many ongoing R2R projects, another Grundig TK 23 (I have several of them).

Got it around Christmas time and basically most of it works well, still to check recording properly.

Playback works very well but after around 15 minutes of good running the sound gets severely distorted.

Plan to scope output signal at the EL 95 output pentode to see what is happening to the sound waveform and trace the problem to the source, expect that a component may be breaking down. Will check out also the HT supply voltages etc.
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Old 15th Jan 2020, 8:07 pm   #2
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Default Re: Grundig TK 23 Distorted Sound

Just typical it has run today for hours without fault, over Christmas it consistently would fault after around 10-15 minutes running from a cold start.

HT voltage lowish but not abnormal, ripple levels OK. Not easy to monitor when playing tape as the TK 23 is not too keen to work when vertical and for good access to the component side of the board, the vertical position is best.

Will keep checking.
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Old 18th Jan 2020, 11:41 am   #3
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Default Re: Grundig TK 23 Distorted Sound

With more playing sound is now starting to distort/break up, just need to get the unit in a good position where the tape can be played and easy to get to/monitor the relevant components.
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Old 18th Jan 2020, 2:24 pm   #4
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Default Re: Grundig TK 23 Distorted Sound

Now manged to get the TK 23 propped up at angle so that can play tape and monitor the electronics (its looks a bit precarious but is OK).

Am playing a 2 track 3" tape that came with the TK 819, good quality 60's pop music recordings (The Beatles on the scope waveform).

Started monitoring the anode (pin 5) of the output pentode valve EL 95 and see that it has a large AC mains ripple signal present with the audio signal superimposed/modulated onto it, the volume control adjusts the amplitude of the ripple as well as the audio signal.

If the volume control is turned up quite high (past 6) you can hear hum with tape paused and with tape playing at this volume level the sound becomes distorted, but at lower volume levels the sound sounds OK.

Note the above is before the previously described sound distortion kicks in, i.e. is in the good running condition.

So need to work my back through the circuitry to see where the ripple is getting in (presumably before the volume control).

The mains ripple on the HT supply (s) is still good/normal.
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Old 19th Jan 2020, 1:07 am   #5
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Default Re: Grundig TK 23 Distorted Sound

Checking back through the circuitry, the mains ripple problem appears to be originating from the dual ECC 81 valve, with the second stage outputting the ripple with little ripple arriving at the input to the first stage.

Went round and round in circles without being able to home in to what is causing it specifically.

Decided to monitor another TK 23 for comparison, but initially very confusingly this appeared to give similar, but worse results. But after lots of checks have decided that the symptom on the second TK 23 is a red herring, as it is only occurring in Play mode (the bad TK 23 exhibits the ripple when not in Play mode) and the frequency of the noise signal is around 1,250Hz i.e. not mains ripple.

I know that these TK23 boards are sensitive when hinged down without their metal screening plate being fitted, so I think the 1,250 signal is a pickup oscillation issue and not an actual fault.

So tomorrow need to have a fresh look.
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Old 19th Jan 2020, 11:54 am   #6
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Default Re: Grundig TK 23 Distorted Sound

Normally the ripple frequency at the reservoir is 100Hz for a full wave rectifier.

Lawrence.
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Old 19th Jan 2020, 12:42 pm   #7
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Default Re: Grundig TK 23 Distorted Sound

Thank you Lawrence. Yes the mains ripple frequency is the standard full wave rectifier 100Hz (10mS) on both units, both at the reservoir capacitors and elsewhere.

The 1,250 Hz on the good unit was something going into oscillation, will the board hinged back up and screening panel fitted the oscillation symptom went away (thank goodness).

After more diagnostics on the bad unit, renewed 3 capacitors, one at a time, C10 (0.1uF), C13 (0.047uF) and electrolytic C11 (10uF), each one reduced the mains ripple signal seen when volume control wound up, by approximately a third, i.e. they had a cumulative effect.

With all replaced the ripple problem is fixed. There is some standing small level ripple at the EL 95 anode output (but less than the other good TK 23). This remaining ripple is largely removed by combination of the inductor assembly that the EL 95 output feeds into and the output transformer.

C10 and C13 are in series and feed the output of the second stage of the ECC 81 through to the input (first grid) of the EL 95 (via a couple of resistors and a switch contact). C11 is a decoupler on the cathode of the second stage ECC 81.

Even though replacing the capacitors has fixed the problem, I do not really 100% understand what was going on.

On capacitance measurement they all appeared OK, though C11 was quite high at 15uF.

Interestingly some work had been done around C10 and C13 by someone previously, because one end (leg) of each capacitor had been cut through and soldered back together. These capacitors did not look like modern replacements, so could be the originals.

Of course even though this has fixed the symptom of mains ripple on the audio signal when volume control wound up (and subsequent hum), I do not know if the original symptom of sound distortion/breakup (after some normal running period) has also been fixed or whether that is a second problem, only time will tell.
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Old 19th Jan 2020, 1:31 pm   #8
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Default Re: Grundig TK 23 Distorted Sound

It might have been due to a leaking coupling capacitor putting +ve volts on the grid of the output valve.

I don't know if this will help but don't forget that only a fraction of any standing HT ripple on the anode of the output valve will find it's way to the loudspeaker, below is a link to a video that shows what goes on, skip to 1:09:34 to see the 'scope waveforms then keep watching to see the explanation:

https://www.youtube.com/watch?v=niBG...7AAU0&index=10

Lawrence.
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Old 19th Jan 2020, 2:45 pm   #9
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Default Re: Grundig TK 23 Distorted Sound

Thank you Lawrence, that video sounds good, I will check it out later today.

I did measure the coupling capacitors to see if they were passing any DC but nothing obvious, but totally agree with you.

The unit (on right hand side of the photo) is now back on soak test to see if the sound distortion/breakup symptom returns.

It is probably just my imagination, but the sound quality seems better post capacitor replacement, not that the quality was bad before (apart from when the distortion/break up kicked in).

David
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Old 19th Jan 2020, 3:11 pm   #10
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Default Re: Grundig TK 23 Distorted Sound

Do you have the full service manual? I have one(issue 1,July 1962) you can have for £2.00 plus postage.Les
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Old 19th Jan 2020, 5:56 pm   #11
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Default Re: Grundig TK 23 Distorted Sound

Hi Les,

If you mean the German one then yes I do have it thank you, if you have an English one then I would be very interested in it.

David
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Old 20th Jan 2020, 10:24 am   #12
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Default Re: Grundig TK 23 Distorted Sound

Left the 2 units alone on overnight soak test and this morning they had twins
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Old 21st Jan 2020, 6:35 pm   #13
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Default Re: Grundig TK 23 Distorted Sound

Quote:
Originally Posted by TowerRadio View Post
Do you have the full service manual? I have one(issue 1,July 1962) you can have for £2.00 plus postage.Les
Many thanks to TowerRadio for the excellent Service Manual, won't have to struggle anymore with trying to translate the German manual.

A real bonus was that it also included a Grundig Technical Bulletin with various useful info particularly the TK Drive Belt arrangements.
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Old 22nd Jan 2020, 5:13 pm   #14
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Default Re: Grundig TK 23 Distorted Sound

With repeated playing of music tapes, no more obvious sign of sound distortion/breakup, touch wood the original problem is fixed.
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Old 22nd Jan 2020, 5:16 pm   #15
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Default Re: Grundig TK 23 Distorted Sound

Quote:
Originally Posted by ms660 View Post
It might have been due to a leaking coupling capacitor putting +ve volts on the grid of the output valve.

I don't know if this will help but don't forget that only a fraction of any standing HT ripple on the anode of the output valve will find it's way to the loudspeaker, below is a link to a video that shows what goes on, skip to 1:09:34 to see the 'scope waveforms then keep watching to see the explanation:

https://www.youtube.com/watch?v=niBG...7AAU0&index=10

Lawrence.

Very good link !
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