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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 16th Feb 2020, 12:29 pm   #161
Leon Crampin
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Default Re: Grundig TK 819 Any Useful Info

Just to clarify the operation of the reel hubs; take up and back tension are dependent on the tape entry and exit radius. Grundig and many others use the weight of tape on the corresponding spools to vary the drive and driven torques.

On these machines, the upper hub has two concentric grooves, one to take a rubber ring which engages with the solenoid armature (large washer) for winding and braking, and the other to take a white felt ring. It's protrusion and condition are critical in achieving the right torques. The felt ring should be proud enough to ensure that the solenoid armature is fully free when the coil is not energised.

When adjusting the lower turntable height, there is a screwdriver slot in the centre shaft to facilitate rotation. Be aware that the shaft is hardened. The screwdriver blade should be slightly narrower than the slot length to avoid chipping the very hard shaft.

Leon.
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Old 16th Feb 2020, 12:42 pm   #162
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Default Re: Grundig TK 819 Any Useful Info

Quote:
Originally Posted by DMcMahon View Post
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.
The results of your tests will be very interesting. Pace Ted Kendall, it will be strange if the P/B heads give no output at all from tape if they aren’t o/c – even if hopelessly low-level and frequency-limited. When I first saw your photos in #113 I didn’t know what to make of them – I’m only familiar with 1970s Bogen heads, which had a 1 micron gap – which is hardly visible to the naked eye. I assumed that there must be something very different about the construction of the heads in your picture with what looked like huge gaps. If that really is the state of their gaps, then I can’t imagine how they got as bad as that – they would surely have been abandoned long before reaching that state.
My (perhaps simplistic) view is that, if your defluxer is able to induce hum in them, then there must be a complete circuit there of some sort. So, a bit of several mysteries so far.

I also have heard that the Bogen heads (your Uher)were notorious for going o/c (as were the Papst motors that often sat close to them – the Papst capstan motor on my Brennel failed at about 3-4 years from new).

I await the denouement with bated breath!
Mike

Last edited by Boulevardier; 16th Feb 2020 at 12:49 pm.
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Old 16th Feb 2020, 2:01 pm   #163
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Default Re: Grundig TK 819 Any Useful Info

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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
I should have said a digital MM (multimeter) not digital DMM !
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Old 17th Feb 2020, 12:46 am   #164
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Originally Posted by Ted Kendall View Post
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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.
Ted, I have studying lots of worn head images and have got myself confused.

When you get a chance could you please explain to me (sorry if I am being thick) what the large gap is in the photos. I have attached 2 fresh photos from the TK 819 and also 2 from my TK 5.

So for example in photo TK 819 # 1, is the wide gap the section from the left hand edge of the narrow left vertical red potting compound to the right hand edge of the narrow right vertical red potting compound (either side of the black narrow vertical line) or is it the width of the black narrow vertical line or something else ?

Than you in advance,

David
Attached Thumbnails
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Last edited by DMcMahon; 17th Feb 2020 at 12:54 am. Reason: Update
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Old 17th Feb 2020, 2:01 am   #165
Ted Kendall
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OK - the short answer is that the TK5 head looks fine and the 819 heads are shot.

The head structure is mostly of brass, with the nose of the polepieces inset - this is the bit in bright metal which you can see on the TK5 head. The whole is potted in the red compound. The polepieces carrying the gap itself are quite shallow to reduce losses, and on the 819 heads these have worn through to the point where the gap as such has gone, leaving the stubs of the polepieces, As a rule of thumb, if you can see the gap with the naked eye on a record or replay head, it is worn out.
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Old 17th Feb 2020, 2:29 am   #166
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The only practical explanation I can think of for the state of that TK819 head, is that someone has made a disastrous attempt to re-lap it...

Mike

Last edited by Boulevardier; 17th Feb 2020 at 2:37 am.
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Old 17th Feb 2020, 8:53 am   #167
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Yes it does happen. Someone once mailed me a 4 channel Teac head which he had tried to relap using a very coarse hand file. He had filed off all the remaining metal but not content with that, kept ploughing on until the "gap" could now be seen from 5 paces away... Extraordinary.

Those heads look strange in relation to the guides. They don't seem to line up. I also see burred screw heads and nuts, and a capstan shaft that looks like it may have a tape sized slot worn it in due to a lot of use. These machines may have not only had a hard life but also some rough repair work on them at some stage.
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Old 17th Feb 2020, 8:58 am   #168
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Especially in picture #2 above, it looks like there is a bit of shiny metal where the gap would be, looking rather like the surrounding brass. Is this in fact a spacer that's part of the internal construction of the heads?

The thing that looks strange with these TK819 heads is that, across the face of the head, the brass gives way to what looks like something red, i.e. the potting compound, followed by a strip of metal (?), followed by more red, followed by the brass. There's no remnants of the pole pieces whatsoever.

Given the uniformity of the center piece, I would second Boulevardier's opinion that someone has attempted to re-lap the heads without understanding what they were driving for. The fact that the head in #1 shows virtually no sign of wear substantiates this. On the other hand, the head in #2 does show significant signs of wear despite having the same overall appearance, perhaps it was re-lapped first, and the machine continued to be used with head #1 unchanged?
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Old 17th Feb 2020, 9:48 am   #169
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OK - the short answer is that the TK5 head looks fine and the 819 heads are shot.

The head structure is mostly of brass, with the nose of the polepieces inset - this is the bit in bright metal which you can see on the TK5 head. The whole is potted in the red compound. The polepieces carrying the gap itself are quite shallow to reduce losses, and on the 819 heads these have worn through to the point where the gap as such has gone, leaving the stubs of the polepieces, As a rule of thumb, if you can see the gap with the naked eye on a record or replay head, it is worn out.
Thank you Ted, at least the TK 5 head sounds like it might be usable. Even though the 819 heads are shot, out of pure interest I will have a look at any signals (as rubbish as they maybe) that might be produced out of the heads to see why no sound at all.

I do have one replacement head on its way (hopefully a lot better condition) and a possibility of another replacement head source.

What are the 2 narrow black vertical sections on the TK 5 head ?

If the heads were relapped it was not me

David
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Old 17th Feb 2020, 10:16 am   #170
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Especially in picture #2 above, it looks like there is a bit of shiny metal where the gap would be, looking rather like the surrounding brass. Is this in fact a spacer that's part of the internal construction of the heads?

The thing that looks strange with these TK819 heads is that, across the face of the head, the brass gives way to what looks like something red, i.e. the potting compound, followed by a strip of metal (?), followed by more red, followed by the brass. There's no remnants of the pole pieces whatsoever.
I think that's just what's left of the gap spacer. I'm not sure this is the result of an ill-advised re-lap, because there seems to me no wear outside the tape width. What could have happened is that somebody used ex-studio tape, which was commonly available in the sixties as cheap "white box" tape. Some of this was very abrasive, the surface roughness being intentional to aid neat spooling on studio machines. Twenty or so passes of something like BASF LR56 could account for the state of the heads and maybe explain the capstan wear, too.

The two "black" vertical pieces on the TK5 head are in fact the polepices at either side of the gap - mu-metal was and is expensive, so composite heads of this type were common in domestic machines. As yet, you have full track width on that head and the gap is intact.

As regards output on the worn heads, I should expect a degausser to prduce some signal, as there are still coils and polepieces, but if anything recognisable comes off a tape, I'd be surprised.

Last edited by Ted Kendall; 17th Feb 2020 at 10:28 am.
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Old 17th Feb 2020, 10:56 am   #171
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Originally Posted by Leon Crampin View Post
Just to clarify the operation of the reel hubs; take up and back tension are dependent on the tape entry and exit radius. Grundig and many others use the weight of tape on the corresponding spools to vary the drive and driven torques.

On these machines, the upper hub has two concentric grooves, one to take a rubber ring which engages with the solenoid armature (large washer) for winding and braking, and the other to take a white felt ring. It's protrusion and condition are critical in achieving the right torques. The felt ring should be proud enough to ensure that the solenoid armature is fully free when the coil is not energised.

When adjusting the lower turntable height, there is a screwdriver slot in the centre shaft to facilitate rotation. Be aware that the shaft is hardened. The screwdriver blade should be slightly narrower than the slot length to avoid chipping the very hard shaft.

Leon.
Thank you Leon for those extra helpful details. One thing I am still unsure about is how much protrusion there should be for the friction rubber pieces, obviously operational experiments will help to determine that.

But to my mind the protrusion cannot be much otherwise it will offset the felt friction operation, i.e. if the rubber protrudes above the height of the felt then the felt will not make contact with the lower hub.
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Old 17th Feb 2020, 11:00 am   #172
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Quote:
Originally Posted by DMcMahon View Post
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.
The results of your tests will be very interesting. Pace Ted Kendall, it will be strange if the P/B heads give no output at all from tape if they arenít o/c Ė even if hopelessly low-level and frequency-limited. When I first saw your photos in #113 I didnít know what to make of them Ė Iím only familiar with 1970s Bogen heads, which had a 1 micron gap Ė which is hardly visible to the naked eye. I assumed that there must be something very different about the construction of the heads in your picture with what looked like huge gaps. If that really is the state of their gaps, then I canít imagine how they got as bad as that Ė they would surely have been abandoned long before reaching that state.
My (perhaps simplistic) view is that, if your defluxer is able to induce hum in them, then there must be a complete circuit there of some sort. So, a bit of several mysteries so far.

I also have heard that the Bogen heads (your Uher)were notorious for going o/c (as were the Papst motors that often sat close to them Ė the Papst capstan motor on my Brennel failed at about 3-4 years from new).

I await the denouement with bated breath!
Mike
Yes Mike I agree with you, that unless the heads are open circuit (seems unlikely) that should get some sort of sound out of the amplifier, so subsequent tests will be very interesting (for me at least).

David
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Old 17th Feb 2020, 6:58 pm   #173
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Very slow going with the PB head diagnostics at the moment. Both heads have good continuity of 525 and 553 ohms respectively. Each hum cancelling coil is around 13 ohms, so the total series resistance of the heads and coils measures around 538 and 566 ohms respectively, measured at the heads assembly output connectors.

One of the head signals should connect through to capacitor C1 (via relay contact and switch contacts) which goes to the input of the EL 804 pre-amplifier valve (but EF 86 in my case is fitted).

So far I have been unable to measure the connection from the heads to C1 continuity wise, both with unit powered and unpowered. As always not easy physically identifying the correct cables, components and switch contacts. It may well be OK because the previous induced hum tests got through.

One thing that is confusing me a little, is that the schematic shows one side of the head signals being selected by relay contact KIII (I think selecting upper/lower head for Forward/Reverse Play) relay K is designated as "kopfrelais" (head relay) but relay contact KIV (poor drawing, it may not be IV but definitely is a K) is the motor run/start switching contact with C34/C35 snubbers across it.

This does not seem correct ? that Relay K switches both the motor and the heads signals.
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Old 17th Feb 2020, 7:13 pm   #174
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Made mistake above, relay CII is the motor run/start I think with snubbers C36/C37 but contact KIV is still motor power with C34/C35, which now I realise must be for motor reversal (change of direction), so this would tie up with relay K switching the motor and the heads, so no longer confused.
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Old 17th Feb 2020, 7:22 pm   #175
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KIV contacts are for the erase heads, KV (C34/C35) are the contacts for the motor direction so far as I can make out.

EDIT: Whoops...Post crossed.

Lawrence.
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Old 17th Feb 2020, 7:35 pm   #176
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Thank you Lawrence, yes can now see the KIV contacts for the erase heads, so what looks like KIV (on my drawing) for the motor reversal must be KV.

David
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Old 17th Feb 2020, 7:39 pm   #177
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Thank you Lawrence, yes can now see the KIV contacts for the erase heads, so what looks like KIV (on my drawing) for the motor reversal must be KV.

David
Yes, look at the TK820 schematic, it uses proper numbers for the contacts, they all tally up with the TK819 so far as the K relay contacts go.

Lawrence.
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Old 17th Feb 2020, 8:45 pm   #178
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" I agree with you, that unless the heads are open circuit (seems unlikely) that should get some sort of sound out of the amplifier, so subsequent tests will be very interesting (for me at least)"

When you get back to the heads issue, what might be a useful test (with a reel of blank or unwanted tape) would be to set the machine running on replay, and to bring your defluxer close to the head gap. Even with tape running over the head, the field of the defluxer should get through the gap to the gap and give you a hum. That running test should eliminate the possibility of head-switching issues killing the signal.

Mike
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Old 17th Feb 2020, 9:00 pm   #179
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Thank you Lawrence, yes can now see the KIV contacts for the erase heads, so what looks like KIV (on my drawing) for the motor reversal must be KV.

David
Yes, look at the TK820 schematic, it uses proper numbers for the contacts, they all tally up with the TK819 so far as the K relay contacts go.

Lawrence.
Once again thank you Lawrence for that tip. It is a shame that there does not appear to be a SAMS Photofact folder for the TK 819 like there is for the TK 830, as it might make it easier generally to find ones way around the schematic.

David
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Old 17th Feb 2020, 9:08 pm   #180
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" I agree with you, that unless the heads are open circuit (seems unlikely) that should get some sort of sound out of the amplifier, so subsequent tests will be very interesting (for me at least)"

When you get back to the heads issue, what might be a useful test (with a reel of blank or unwanted tape) would be to set the machine running on replay, and to bring your defluxer close to the head gap. Even with tape running over the head, the field of the defluxer should get through the gap to the gap and give you a hum. That running test should eliminate the possibility of head-switching issues killing the signal.

Mike
Mike from post 158 I did the head demagnetiser test of both heads individually in Forward and Reverse Play getting loud hum for the respective head, this was done without tape, I don't think doing it with tape would make any difference to the results.

David
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