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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 13th Feb 2020, 6:34 pm   #141
DMcMahon
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Default Re: Grundig TK 819 Any Useful Info

Tested the same tape on another machine, no recordings on it (at least in areas tried).

Tried a known good tape with recordings on the 819, still no sound from it. Swapped over the drive belt from the Take Up spool to the Supply spool and tested reverse playback, still no sound.

Heads have been cleaned several times and the tape appears to be making good contact with the heads.

Found out which pins on plugs that plug into the rear of the heads assembly were the playback head signals, with the plugs removed and touching the relevant pins get loud hum signal when play is selected. So this looks maybe ominous for the playback heads but will do more tests to try and prove conclusively one way or the other.

Measured the clutch solenoid voltages at the solenoid wires with FWD & RWD selected, RWD = +23 volts and FFD = +28 volts, so voltages probably OK but maybe a little strange the difference between the 2 solenoids (will check that out further later).

The solenoids may be energising but definitely are not locking the upper hub and there is no audible click. As thought before looks like the metal disk/washer is stuck in the decomposed rubber in both reel tables.

So will remove the tables, strip down and clean.
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Old 13th Feb 2020, 6:43 pm   #142
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Default Re: Grundig TK 819 Any Useful Info

"So this looks maybe ominous for the playback heads but will do more tests to try and prove conclusively one way or the other."

Another quick test would be to bring something with a 50 c/s magnetic field very close to the playback head while in playback mode (e.g. a small mains transformer or a tape-head demagnetiser). This should produce a discernible hum on the speaker if the head is working at all.

Mike
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Old 13th Feb 2020, 6:47 pm   #143
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Default Re: Grundig TK 819 Any Useful Info

Excellent idea Mike, I will locate my head demagnetiser.

David
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Old 13th Feb 2020, 7:25 pm   #144
Leon Crampin
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Default Re: Grundig TK 819 Any Useful Info

The difference in the 25V supply between tracks 1 and 2 is normal. There is a giant relay near the ECC81 which reverses the motor and also carries multiple springsets to effect the track changeover. It's energised on track 2 and the coil current lowers the supply voltage a bit. Your voltage readings look to be about right.

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Old 13th Feb 2020, 9:04 pm   #145
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Default Re: Grundig TK 819 Any Useful Info

A bit of a disaster on the clutch front. I split the tape transport chassis away from the electrical chassis to get better access to remove the 2 reel tables.

When I went to remove the Take Up reel table discovered there were no wires coming out of its solenoid coil !!!! So when I previously measured the solenoid supply as 28 volts at the 2 little terminals on the chassis where the solenoid wires connect to (which I assumed they were connected) there was no solenoid connected, so this may explain the higher voltage (i.e. no solenoid load).

It certainly explains why the solenoid was not working

Very surprised I had not noticed the lack of wires before ! I suppose because before I never was looking/focused in this particular area.

Looks extremely unlikely will be able get into the coil to re-instate the 2 wire connections. Seems very strange they are missing, maybe they just broke away ?

Even though the solenoid will not work without a connected supply, FFD was still working at least to a fashion, I guess it may well slip under certain tape load conditions.

Have not yet stripped the Supply Reel Table, this one does have wires.
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Old 14th Feb 2020, 12:08 am   #146
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Default Re: Grundig TK 819 Any Useful Info

Stripped and cleaned the Supply Reel Table, including getting the decomposed rubber out of the groove, this table was pretty bad with the decomposed rubber gunking up everything, the clutching disk was certainly stuck down, hopefully now it will work when re-assembled. The upper spindle clutch felt is more contaminated when compared to the Take Up one.

Measured the solenoid coil as 254 ohms, so should work when powered up. Will re-assemble everything tomorrow and re-test. No time yet to do any more work on the lack of playback sound.
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Old 15th Feb 2020, 10:38 am   #147
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Default Re: Grundig TK 819 Any Useful Info

Quote:
Originally Posted by Ted Kendall View Post
Sorry to be the bearer of sad tidings, but those heads look worn out - when I reluctantly retired my TK8 n years ago, the R/P head looked like that...
Probably impossible to make any judgement as photo not close enough but any thoughts on the heads condition in the attached TK 830 photo.
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Old 15th Feb 2020, 10:49 am   #148
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Default Re: Grundig TK 819 Any Useful Info

After re-assembling everything, the Supply reel table clutch is now working well, can hear the fairly loud click as the energised solenoid lifts the clutching disk (the disk now being no longer stuck in the decomposed rubber post cleaning) and can feel that the upper hub is being locked to the lower hub.

The lock is quite strong but if I pull the upper hub fairly hard upwards I can overcome the lock, i.e, can lift the upper hub but I think this is probably normal.

A slight problem at the moment with the Take Up reel table (the one with no wires to the solenoid coil) in that after securing it back to the chassis it is too tight to turn manually, trying to get the first/upper nut position correct. Need to fix this before I can test the tape transport operation again post fixing the Supply table clutch fix.

Just found my head demagnetiser so later will be able to try it to see if the playback heads detect it.
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Old 15th Feb 2020, 11:02 am   #149
Ted Kendall
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Default Re: Grundig TK 819 Any Useful Info

Quote:
Originally Posted by DMcMahon View Post
Quote:
Originally Posted by Ted Kendall View Post
Sorry to be the bearer of sad tidings, but those heads look worn out - when I reluctantly retired my TK8 n years ago, the R/P head looked like that...
Probably impossible to make any judgement as photo not close enough but any thoughts on the heads condition in the attached TK 830 photo.
I thought I saw a close-up of your 830 heads earlier in the thread and they didn't look too bad at all. The giveaway is when you can see the colour of the potting compound in the gap area (red in the case of the 819). If my reading of that machine is correct, the heads are unlikely to give much output at all, as the gap is now very large.
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Old 15th Feb 2020, 11:13 am   #150
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Default Re: Grundig TK 819 Any Useful Info

Thank you Ted, I will have to have a good visual study of my TK 819 based upon your inputs. The TK 830 image I posted for this question I think will be a different machine from the 830 in an earlier thread (I will check it though).

I have just found out that the TK 5 may use the same Play/Record head so I must have a look at my TK 5 head to see what the wear looks like.

David
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Old 15th Feb 2020, 3:08 pm   #151
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Default Re: Grundig TK 819 Any Useful Info

As far as I know, most of this generation used the same head (TK5,8,9,12, 819/20/30). The contemporary Stenorette head is externally the same, but I'm not sure whether the inductance is the same - it's been a few decades since I fiddled with this. The other problem with substitution is that the wear pattern on the head will be different - the pressure roller gives a pattern which tapers off at the edge of the contact patch, which a pad doesn't, and the wear on the bottom track head is offset. In extremis, you could re-lap...
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Old 15th Feb 2020, 11:18 pm   #152
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Default Re: Grundig TK 819 Any Useful Info

The Stenorette records on the lower half of the tape, so it will have the same wear area as the reverse heads on the 819.

Even if the gap has started to widen, I would think there would be some form of signal from the tape, even if it completely lacks treble and is overall weaker than expected.
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Old 15th Feb 2020, 11:48 pm   #153
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Originally Posted by ricard View Post

Even if the gap has started to widen, I would think there would be some form of signal from the tape, even if it completely lacks treble and is overall weaker than expected.
That seems right to me. I think the tape-head demagnetiser test would indicate whether the head is working (however badly). If that gives no signal, then the path through the head and back to the input of the playback amplifier (which seems to be working) needs to be tested for DC continuity. David mentioned some hum-cancelling coils, which he said were in series with the head(s), and there is presumably some head switching somewhere to switch between the two heads for autoreverse (if I understand the system). So there is presumably more between head and amp than just a piece of co-ax cable (I don't have a circuit diagram). And I don't think we know whether he has tried both R/P heads yet.

Mike

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Old 16th Feb 2020, 1:03 am   #154
Ted Kendall
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Even if the gap has started to widen, I would think there would be some form of signal from the tape, even if it completely lacks treble and is overall weaker than expected.
Started to widen? We used to joke about heads being worn down to the windings, but these appear to run that pretty close. The gaps are about 1mm, where they were originally 10uM or so. A degausser should give some output, agreed, but the extinction frequency for that gap length must be in the tens of Hz at best.
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Old 16th Feb 2020, 1:11 am   #155
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Quote:
Started to widen? We used to joke about heads being worn down to the windings, but these appear to run that pretty close. The gaps are about 1mm, where they were originally 10uM or so. A degausser should give some output, agreed, but the extinction frequency for that gap length must be in the tens of Hz at best.
It makes you wonder how/why anyone would have run the machine until it reached that state. You'd think it would have been given up as a bad job long before it got to that.

Mike

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Old 16th Feb 2020, 9:21 am   #156
Ted Kendall
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Fair point. My TK8 still had a little track width left on the pole-pieces, so there was still some top but the output was low.
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Old 16th Feb 2020, 10:35 am   #157
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Default Re: Grundig TK 819 Any Useful Info

After finding a suitable sized drive belt for the Take up table, re-tested the tape transport (post cleaning the reel table hubs). Everything basically still works, but as before the spools are still slow to come to a stop and sometimes accelerate initially when stop is pressed.

Forward Play & FFD are worse than before in that they sometimes struggle due to the reel table upper hubs slipping on the lower hubs.

I assume because all the previous decomposed friction rubber material has now been remove during the cleaning, so less friction between upper and lower hubs. Also of course the Take Up reel table solenoid does not work (no wires) so no hard locking of those hubs in FFD.

So would need to experiment with adding some suitable friction material like drive belt etc into the grooves in the upper hubs.

But at this time am not going to spend time doing this as it might be a waste of time if the playback heads turn out to be totally useless.
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Old 16th Feb 2020, 11:04 am   #158
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Default Re: Grundig TK 819 Any Useful Info

Quote:
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Quote:
Originally Posted by ricard View Post

Even if the gap has started to widen, I would think there would be some form of signal from the tape, even if it completely lacks treble and is overall weaker than expected.
That seems right to me. I think the tape-head demagnetiser test would indicate whether the head is working (however badly). If that gives no signal, then the path through the head and back to the input of the playback amplifier (which seems to be working) needs to be tested for DC continuity. David mentioned some hum-cancelling coils, which he said were in series with the head(s), and there is presumably some head switching somewhere to switch between the two heads for autoreverse (if I understand the system). So there is presumably more between head and amp than just a piece of co-ax cable (I don't have a circuit diagram). And I don't think we know whether he has tried both R/P heads yet.

Mike
Hi Mike,

In an earlier post I did reference that there was no sound or tape hiss in Forward and Reverse Play. The finger test on live sides of the playback heads wiring resulted in hum and only hum for the correct head depending upon whether Forward or reverse play selected, which to some extent partially checks the wiring/switching etc.

Last night did the head demagnetiser test, once again got loud hum when the probe was close to the respective head with Forward or Reverse Play selected.

For completeness I will do a full continuity resistance check of the all the associated wiring including the hum cancelling coils and the track switching.

After that will scope probe the head signals going to the electronics to see if any actual sound signal and if so, then see why it is not getting trough to the output stage, when tape is played.

Of course the playback heads could be open circuited. In that case I might still expect to get a hum signal when head live wiring touched and when demagnetiser used. So also may check the continuity of the playback heads.

I know there is a lot of press about potentially damaging heads when checking their resistance/continuity.

For diagnostic purposes I have often for a long time checked head resistances with no incidents (until last year), choosing a resistance range on DMM that gives the lowest voltage output, so generally to be honest have been historically somewhat sceptical of possible head damage.

Last year one Uher I purchased arrived with a dead right channel, after checking all the wiring connections and electronics all OK, I measured the Playback head which was open circuit. I also measured the left channel playback head which was OK of course as the left channel worked OK.

But not long afterwards the left channel died and the PB head was open circuit. Whether this was a co-incidence that the head not long afterwards died or as a direct result of checking its continuity I do not know.

The Uher heads are German Bogen which evidently can go open circuit.

So as a result of the Uher experience I am a little bit more cautious about measuring heads.

David
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Old 16th Feb 2020, 11:25 am   #159
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Default Re: Grundig TK 819 Any Useful Info

There is a popular misconception about burning out tape heads by testing them for continuity. On the face of it, this is absurd: look what the bias oscillator is delivering into the windings during recording!

But there is a grain of truth in there somewhere. You do need to be a little bit careful with tape heads, but for a different reason.

If you test a tape head for continuity with an analogue multimeter, you can leave some magnetism in the core. You will then need to demagnetise it before using it again, otherwise your favourite tapes will get quieter and quieter with every listen as they are being partially erased. In the days when replacement tape heads were still readily available, this would have been worse (especially as it's hard to notice until too late).

A modern digital multimeter will use a much lower test current than an analogue one, which will not be sufficient to reach the edge of the hysteresis loop and so not have any permanent effect.

If you test a tape head for continuity with an AC bridge, there won't be any remaining magnetism anyway.

If you demagnetise your tape heads on reassembly, you will be fine.
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Old 16th Feb 2020, 12:08 pm   #160
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Default Re: Grundig TK 819 Any Useful Info

Thank you Julie, that is very good to know and comforting, I always thought it very unlikely for the reason you say about the large bias on the record/erase heads.

I would always use a digital DMM.

David
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