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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 12th Nov 2023, 12:29 pm   #1
DMcMahon
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Default Grundig TK 20 Capstan Motor Problem

Yesterday started checking out this late 1950's single speed, 2 track mono valve reel to reel, which I had purchased a little while ago, it being my third TK 20.

Overall in very good cosmetic condition. Renewed the 2 internal fuses (HT and mains) as they were heavily corroded, also cleaned up the corrosion on the fuse holders sprung clips.

Powered up slowly all OK. Got reasonable sound on playback of a tape but found that fast rewind intermittently struggled with spools slowing right down and sometimes stopping completely. Initially I thought this was due to hardened rubber on the Supply spool holder table causing the metal motor pulley not to drive the spool holder properly, thought this because have encountered very similar across many of the TK1X/TK2Xmachines.

But when I removed the top lid I discovered that the problem appears to be the motor, the motor itself is slowing down and sometimes stopping, it appears to be time related, i.e. it all works OK say for 15 minutes but then starts playing up.

Measured that the motor main phase was getting its 220V supply from the mains transformer 220V tap via the A2 contact of the Auto Stop relay.

Have never known these motors to have any problems. Removed the end cap of the motor (which has ball bearing underneath) and cleaned and re-oiled, the motor manually rotates freely.

Now suspect the motor capacitor C32, a stud mounted metal can 1.2uF, this provides the second phase to the split phase induction motor, although have never known the capacitor to have any problems before. Capacitance wise it is OK, but does measure somewhat high at around 1.75uF, its insulation resistance at 250V is very low at around 0.14MOhm

Hopefully may have a good donor in my #2 TK 20 machine which I have designated as a spares machine, just now need to find the machine.

David
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Old 13th Nov 2023, 12:47 am   #2
Michael Maurice
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Default Re: Grundig TK 20 Capstan Motor Problem

I would change the capacitor. I've similar ones go in Akai's
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Old 13th Nov 2023, 12:14 pm   #3
DMcMahon
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Default Re: Grundig TK 20 Capstan Motor Problem

Found my spares TK 20 machine and removed the run capacitor, it measured no better at 1.7uF and only 0.1MOhm

Fitted it and fast rewind still bad, struggling when the Supply spool heavily loaded and stopping completely before tape fully rewound.

With no tape loaded fast rewind keeps rotating the Supply spool table but relatively easy to stop it by grabbing hold of the motor pulley, I am sure it should have more torque than that.

Have ordered a couple of 1.2uF run capacitors, went for the plastic CBB61 rectangular type. Could not find a stud mounted 1.2uF, could have got a 1.5uF but more expensive.

David
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Old 13th Nov 2023, 3:40 pm   #4
ricard
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Default Re: Grundig TK 20 Capstan Motor Problem

Before the 1.2 µF arrives, don't you have other capacitors that you could wire up in parallel just to verify that it is the capacitor that is at fault? I agree with Michael that that would be the most likely culprit given the rest of the facts.

Leaky caps tend to measure high on capacitance meters, so who knows what the actual value is of the original component at this point.
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Old 13th Nov 2023, 7:48 pm   #5
DMcMahon
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Default Re: Grundig TK 20 Capstan Motor Problem

Thank you Ricard for your suggestion, yes I had thought of that but strugging to find suitable voltage/capacitance capacitors. They need to be pretty high DC voltage wise to cope with the continous 220VAC

The only spare motor run capacitors I have are 4.5uF, (brought for my Revox A77) I have 4 which in series would get pretty close but cannot locate them.

David
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Old 14th Nov 2023, 10:54 pm   #6
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Default Re: Grundig TK 20 Capstan Motor Problem

During the fast wind testing, the machine at times was quite noisy with rumbling type sounds that appeared to be coming from the flywheel area.

I wondered if the ball bearing that sits at the bottom of the flywheel/capstan shaft had come adrift as I have experierenced similar before. Lifting up the flywheel to check the amount of vertical lift/play didn't appear to indicate that the ballbearing was not in place.

Decided to remove the flywheel to double check and also check for presence of grease in the capstan lower bearing assembly that the flywheel shaft sits in, the ballbearing was in place.

There was no sign of any grease just quite a bit of dry dark fine dust like material with some coarse gritty material. Have experierenced similar before, assume the dust like material is dried up grease, but unsure what the gritty component is.

Tried to strip down the lower bearing assembly but could not budge/loosen the centre screw fixing, so just cleaned and regreased the assembly as was.

Also took the opportunity to strip, clean and regrease the main function selector switch/cam assembly and the reel table control arms etc.

After re-assembling everything fast wind is definately smoother and quieter but as expected no change to the fast rewind problem.

From a finger touch test of the motor body, while the motor is rotating its body gets fairly warm, but when the motor stops/stalls during rewind the motor body gets fairly hot quite quickly.

David
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Old 15th Nov 2023, 11:25 am   #7
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Default Re: Grundig TK 20 Capstan Motor Problem

Found some new 470nF (0.47uF) 630VDC Polypropylene capacitors, 3 in parallel measure 1.41uF and > 2,000MOhms.

Fitted them temporarily in place of the run capacitor, fast wind performance noticeably improved, but disappointingly the motor slows down and stops when virtually fully rewound. Can't see it being any different with the new replacement run capacitor so needs some thinking about, may have to try another motor.

David
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Old 15th Nov 2023, 12:07 pm   #8
vidjoman
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Default Re: Grundig TK 20 Capstan Motor Problem

Could that be caused by friction on worn tape guides? Even slightly worn the area of metal contacting the tape will greatly increase and as the tape increases in diameter wound on to the take up spool the tape from the feeding spool will be going round a very tight path probably approaching 90 degrees. The increased drag will seriously affect the ability of the driven spool.
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Old 15th Nov 2023, 2:10 pm   #9
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Default Re: Grundig TK 20 Capstan Motor Problem

Yes a good point, I have encountered similar before which I had forgotten about.

I will check the guides for wear and rotate if required to unworn section.

Even with the temporary capacitors fitted which has improved the rewind, if I remove the tape it is still possibe to stop the motor rotating by grabbing hold of the motor pulley, it takes more force than before (with original run capacitpr) but to me it still seems like the motor should have more torque and be more difficult to stop it rotating/spinning. Must try the same on other similar machines.

David
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Old 15th Nov 2023, 6:50 pm   #10
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Default Re: Grundig TK 20 Capstan Motor Problem

Previously forgot to menton:-

All previous tests have been done with 5 3/4" spools fully loaded with tape on the Supply spool, have tried several different tapes with same results. No sign of the tapes being sticky or exhibiting SSS symptoms, no tape deposits on heads and tape guides

Sometimes when rewind stops working, the motor is still rotating but then not driving the Supply spool table.

Quickly tried a 4 1/4" spool fully loaded with tape on the Supply side and a 5" spool for Take Up and this performs flawlessly, fast winding end to end in both directions.

David
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Old 15th Nov 2023, 7:29 pm   #11
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Default Re: Grundig TK 20 Capstan Motor Problem

Examined the Supply side tape guide, there was a wear flat where the tape contacts, it was not that obvious to see. Rotated the guide so that the tape no longer made contact with the wear flat.

Now 5 3/4" tape is rewinding very well.

Now need to refit the original run capacitor to see if rewind is still good.

Many thanks to vidjoman for suggesting the worn tape guide possibility, I really should have thought of this myself, I have encounted identical before on more than one occasion, oh for a better memory

David
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Old 15th Nov 2023, 9:21 pm   #12
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Default Re: Grundig TK 20 Capstan Motor Problem

I try to remember to rotate worn tape guides to present an unworn surface to the tape as just part of the service. Even better of course are machines with rotating guides where wear is so much less. Lou Ottens designed the humble Compact Cassette with two small rotating tape guides in the cassette shell itself.
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Old 27th Nov 2023, 11:25 am   #13
DMcMahon
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Default Re: Grundig TK 20 Capstan Motor Problem

With original run capacitor refitted, Rewind still struggled on 5 3/4" (OK at 5") when the Supply spool was heavily loaded and the motor would stop before tape fuly rewound. Tried repositioning the right hand tape guide which was slightly worn but no change.

Refitted the temporary replacement capacitors and rewind then back to good operation, so will replace the original run capacitor with new 1.2uF that arrived in the post some time ago.

David
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