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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 4th Mar 2021, 3:09 pm   #1
ribbonmicsrus
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Default Grundig TK1 repair

Hi
Restoring a Grundig TK1, has anyone worked on one of these? The flywheel has or in my case had a rubber type ring glued to the flywheel that interfaces with the motor drive spindle.
I am thinking of using a rubber O ring 43 x 5mm that seems as far as I can measure for replacement, any comments or better ideas, I think ideally a square or rectangular cross section would be better but I cannot seem to locate any.

Andy
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Old 4th Mar 2021, 5:42 pm   #2
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Default Grundig TK1 Motor

I am checking the motor on this tape recorder and I think speed contoller is not working correctly. If I vary the voltage over the specified range in the manual i get a large variation of speed. I think the centrigual switch might be faulty.
Resistance across contacts yellow/white at rest show approximately 0.5 ohm. and if i open the contacts the resistance goes up to around 35 ohms, this to me seems strange as it should just be a switch, I should also say the centrifugal switch is disconnected from circuit to avoid misleadimg readings. Anyone else measured theirs?

Andy
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Old 4th Mar 2021, 10:58 pm   #3
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Default Re: Grundig TK1 Motor

Just quickly checked one of my TK1s.

At rest with both wires disconnected, I get a range of resistance readings from a few kOhms to as high as 50kOhm, readings somewhat all over the place, a bit like capacitor charging effect, seems very strange but maybe just a bad switch contact.

How do you open the contacts ? mine is not powered up.

When I can locate my second TK1, I will check that one out.

David
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Old 4th Mar 2021, 11:20 pm   #4
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Default Re: Grundig TK1 repair

On my first TK1 it looks like a tyre around the flywheel, I have not yet been able to get full access to it, to be able to view better.

It is brown and pretty rock hard.

David
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Old 4th Mar 2021, 11:28 pm   #5
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Default Re: Grundig TK1 Motor

Hi David

Yes, it sounds as if your contacts are possibly dodgy. I took the motor out of its clamp and in mine the brushes at one end and inertia switch at other have access holes covered by tape. I removed this and with watchmakers fine screwdriver just held the contacts apart whilst measuring with a DVM.
Looking at voltage across the 100uf capacitor with scope I only get an AC variation of 100mV so the transistor is permanently turned hard on by the low resistance of the switch.
For the life of me, cannot think why such a small variation, unles there is a resistor or winding in there that has gone low.

Andy
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Old 4th Mar 2021, 11:33 pm   #6
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Default Re: Grundig TK1 repair

Yes David
Mine same but worse than yours, it has gone totally segmented, I think my idea of the O ring replacement will work, but won't get some until I am happy with the motor. The flat belt on mine has totally disintegrated but luckily I have a supply of cassette belts of appropriate dimensions.
BTW , my TK1 is a late model with the inertia switch at back of motor.

Andy
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Old 5th Mar 2021, 10:50 am   #7
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Default Re: Grundig TK1 repair

Removed the flywheel drive plate off the top of the flywheel.

As you say the drive belt/ring is glued to the top outer circumference of the plate.

I guess it is hardened rubber, although have a vague memory of reading somewhere that it might be leather ? maybe because of its brown colour.

I will clean mine up and see if it can be softened at all with some Glycerine.

I would think that an O ring would do the job. My other belt is round cross section.

David
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Old 5th Mar 2021, 11:00 am   #8
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Default Re: Grundig TK1 repair

Removed my motor. After manually spinning the shaft the switch is now open circuit at rest.

Difficult to clearly see the switch contacts but now currently the contact arm is sprung downwards, I can lift the arm up against the spring but have not yet measured it to see if lifting the arm electrically closes the switch.

Are you certain that normal operation is for the switch to be closed at rest.

I know that with normal capacitor start motors that the centrifugal switch does open when the motor is up to speed, but this motor is not capacitor start and I do not yet really understand how the motor components regulate the speed.

David
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Old 5th Mar 2021, 11:06 am   #9
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Default Re: Grundig TK1 repair

Later in the day I will spin my motor up from a PSU to see if I can get the switch to mechanically/electrically close at some point.

David
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Old 5th Mar 2021, 1:18 pm   #10
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Default Re: Grundig TK1 repair

Ok David

When I removed the material of the flywheel it certainly felt like rubber.
I would have thought the switch would be closed to start with then open when the tension point set by the screw reaches the correct speed. My contacts definately are closed and held by spring tension and if I flick the shaft pulley quickly I get the higher resistance indicating the switch has opened. With the capacitor across the base / emitter creating a charge pump to give an average that keeps the motor stable.
However if I can't get the motor to work correctly or get another one, might have to work out some opto electronics to replace the switch!

Andy
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Old 5th Mar 2021, 5:48 pm   #11
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Default Re: Grundig TK1 repair

Andy,

I see the Grundig TK 1 Repair Manual says it is rubber.

David
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Old 5th Mar 2021, 6:02 pm   #12
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Default Re: Grundig TK1 repair

Just shows I never read manuals properly!
I am contemplating opening the motor up, could be a do or die moment!

Andy
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Old 5th Mar 2021, 7:19 pm   #13
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Default Re: Grundig TK1 repair

After some switch contact cleaner and exercising the motor from PSU, my switch is now working correctly very consistently, at rest switch is closed (a few Ohms max), at speed switch opens and a good open circuit reading, more details later.

David
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Old 5th Mar 2021, 7:53 pm   #14
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Default Re: Grundig TK1 repair

Quote:
Originally Posted by ribbonmicsrus View Post
I am contemplating opening the motor up, could be a do or die moment!

Andy
Brave man, does not look easy to take apart, good luck.

David
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Old 5th Mar 2021, 8:33 pm   #15
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Default Re: Grundig TK1 repair

David

That proves the way the switch works then. Unless I can get a donor machine it's got to be worth taking a risk, I just cannot imagine why I have such a low differential between open and closed switch.

Andy
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Old 5th Mar 2021, 9:13 pm   #16
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Default Re: Grundig TK1 repair

Yes difficult to undertand what is causing it.

With my motor connected to a DC PSU, the motor starts to spin up at around 0.5V, operationally current drain is around 50mA

Looks like the motor speed linearly increases with linear increase of motor supplied voltage.

At around 3V the switch goes open circuit (> 100's kOhms) and when reducing voltage down from > 3V the switch goes short circuit (a few Ohms max) at around 3V. Both switching points are very repeatable and consistent.

I canot say that I can visually see the contacts switch over, maybe could if had better eyesight.

David
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Old 5th Mar 2021, 10:51 pm   #17
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Default Re: Grundig TK1 repair

Interesting, obviously its spinning much faster unloaded since 3v would mean the regulation would not work at normal voltage of around 6v. My motor gives similar readings to yours so its most probably in reasonable condition from that point of view.

Tomorrow I shall look carefully at motor and try and work out way of dismantling it with a reasonable chance of reassembly !

Andy
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Old 6th Mar 2021, 12:14 pm   #18
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Default Re: Grundig TK1 repair

Yes at 3V (no mechanical load) it is spinning pretty fast.

David
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Old 6th Mar 2021, 3:08 pm   #19
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Default Re: Grundig TK1 repair

Will re-assemble my TK 1 while I can can still remember where all the various parts go.

Probably wil be quite a while before I start looking at it again properly as many more important items to progress. When I do restart it, I will start a new Thread and not hijack your thread.

David
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Old 6th Mar 2021, 6:29 pm   #20
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Default Re: Grundig TK1 repair

OK David

Still not touched motor yet!
Andy
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