16th Feb 2020, 12:29 pm | #161 |
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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. |
16th Feb 2020, 12:42 pm | #162 | |
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Re: Grundig TK 819 Any Useful Info
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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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16th Feb 2020, 2:01 pm | #163 |
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Re: Grundig TK 819 Any Useful Info
I should have said a digital MM (multimeter) not digital DMM !
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17th Feb 2020, 12:46 am | #164 | ||
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Re: Grundig TK 819 Any Useful Info
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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 Last edited by DMcMahon; 17th Feb 2020 at 12:54 am. Reason: Update |
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17th Feb 2020, 2:01 am | #165 |
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Re: Grundig TK 819 Any Useful Info
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. |
17th Feb 2020, 2:29 am | #166 |
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Re: Grundig TK 819 Any Useful Info
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. |
17th Feb 2020, 8:53 am | #167 |
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Re: Grundig TK 819 Any Useful Info
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. |
17th Feb 2020, 8:58 am | #168 |
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Re: Grundig TK 819 Any Useful Info
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? |
17th Feb 2020, 9:48 am | #169 | |
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Re: Grundig TK 819 Any Useful Info
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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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17th Feb 2020, 10:16 am | #170 | |
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Re: Grundig TK 819 Any Useful Info
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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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17th Feb 2020, 10:56 am | #171 | |
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Re: Grundig TK 819 Any Useful Info
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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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17th Feb 2020, 11:00 am | #172 | ||
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Re: Grundig TK 819 Any Useful Info
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David |
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17th Feb 2020, 6:58 pm | #173 |
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Re: Grundig TK 819 Any Useful Info
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. |
17th Feb 2020, 7:13 pm | #174 |
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Re: Grundig TK 819 Any Useful Info
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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17th Feb 2020, 7:22 pm | #175 |
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Re: Grundig TK 819 Any Useful Info
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. |
17th Feb 2020, 7:35 pm | #176 |
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Re: Grundig TK 819 Any Useful Info
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 |
17th Feb 2020, 7:39 pm | #177 | |
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Re: Grundig TK 819 Any Useful Info
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Lawrence. |
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17th Feb 2020, 8:45 pm | #178 |
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Re: Grundig TK 819 Any Useful Info
" 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 |
17th Feb 2020, 9:00 pm | #179 | ||
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Re: Grundig TK 819 Any Useful Info
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David |
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17th Feb 2020, 9:08 pm | #180 | |
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Re: Grundig TK 819 Any Useful Info
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David |
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