Help getting Motor to Run on TK20
 

Split from This Thread https://www.vintage-radio.net/forum/...41#post1373041

Hi, could I please ask for some help in trying to get the motor to run on my Grundig TK20? I picked up this Grundig last Sunday from our local CBF and am in the process of restoring it. Before attempting anything electronically and having cleaned the machine and so on, not knowing the history of the machine I thought I would get the motor going warming it up slowly on a variac whilst disconnecting the 100mA HT fuse. Unfortunately the motor isn’t turning. I checked the motor capacitor which is measuring 2.6uf instead of 1.2uf and temporarily replaced with 2 motor caps in parallel that make up 1.2uf however nothing is happening. I confirm that there is an HT supply on one side of the 100mA fuse but voltage across the motor cap is only measuring in mA so I’m suspecting it’s something to do with contact A2 which I guess is the auto-stop relay? I don’t have a SM of the machine and I’m rather stuck as I don’t know this machine very well. I restored my TK5 last year and can see that this will be just as challenging to get to those Wima black toffee caps. I don’t know how to get the AC voltage across the motor to let it run gently and warm it up. I’m probably missing something obvious.
Many thanks.

Re: Help getting Motor to Run on TK20
 

The motor will always be powered when the TK 20 is switched on. The only time the motor will not be powered is when the metal/conductive strip (if present) on the tape leader bridges the Auto Stop contacts and energises the coil of the Auto Stop relay A which opens contact A2.

Contact A2 connects the motor to the 220VAC tap on the primary of the mains transformer (when A not energised). The input side of the C32 Motor Run capacitor connects to one side of the on/off switch (live or neutral depending upon how the mains plug is wired)

The fact that you have HT voltage at the 100mA HT fuse means that the 400mA mains fuse must be OK so you should be getting 240 volts to the mains transformer primary (assuming the mains voltage selection switch is all OK), so there should be 220V getting to the input side of A2.

So unless you have some sort of bad connection on the relevant connections it looks like your Auto Stop relay may be energising.

With the HT fuse not fitted then the relay cannot energise, so try the motor and check for AC volts. If still no voltage then maybe the A2 contact has gone open circuit/stuck open, try linking it out in this case.

David

Re: Help getting Motor to Run on TK20
 

Hi David, thanks so much for your help on the matter. Could I ask you please where the relay is situated in the machine please? Also do you suggest that I give a direct 220v supply to the motor bypassing connections in the machine? I intend to use a variac for this experiment. Thanks again,
Carmine

Re: Help getting Motor to Run on TK20
 

Had to find my TK 20 to double check location of the relay.

In the 2 attached photos, it is the relay to the left of the motor, directly behind the right hand valve (ECC81). I see that my TK 20 is different to the schematic I was checking, as it does not have the mains voltage selection switch, so mine could well be a 220 volt model.

My TK 20 powered from Variac starts to spin the motor at around 130 volts.

You could power the motor direct from your Variac but keep the run capacitor connected.

Note that the output side of the run capacitor that connects to the other motor winding (auxiliary ?) will be at a higher voltage than the mains supply, for me at 240V it is 272V.

David

Re: Help getting Motor to Run on TK20
 

Thanks so much David. That’s really helpful. I have located the relay on my machine and will try out the motor with a variac as suggested and see what happens. It will probably be done at the weekend due to limited free time and that should then at least rule out the motor. Since I’m not getting any voltage reading across the capacitor it has got to be this relay that’s giving the problem. How to solve the issue is the headache I guess as it’s not so easily accessible.

Cheers.
C

Re: Help getting Motor to Run on TK20
 

Don't be downhearted yet, jascha. I've found that relays are usually resilient. When they've failed it's usually because of a loose connection, or oxidised contacts caused by many actuations. With my 1970 car and audio equipment of a similar vintage, I've solved complete power failures by cleaning the contacts and returning them to normal operation without having to get any peculiar parts.

Re: Help getting Motor to Run on TK20
 

Carmine in your first post you said "voltage across the motor cap is only measuring in mA".

Not sure if you actually meant mA (i.e. current) I do not think you would measure much if anything current wise across the run capacitor. You will measure voltage across the run capacitor but not full motor voltage.

Relay A would only normally energise (and open normally closed contact A2) if the Auto Stop contacts are bridged, which is unlikely to be the case here. There is a parallel circuit across the Auto Stop contacts of contacts A1 and B2, not sure what their functionality is but consider they should not cause relay A to energise under normal conditions.

When you power up the unit (with HT fuse fitted) you will be able to physically see and hear if relay A is actually energising, if it is you could temporarily disable it (to be able to test the motor) by disconnecting one end of 50kOhm resistor R45.

If relay A is not energising/been disabled then contact A2 should be closed, switching one side of the mains through to the motor. If A2 is stuck open or very badly oxidised (as Uncle Bulgaria suggests) then that possibly could explain the lack of motor voltage.

David

Re: Help getting Motor to Run on TK20
 

Hi guys,

Thanks for your help once again. I look forward to tracing this fault once I have some free time at some point this week and will update on the forum. First of all apologies for my mistake in writing mA instead of mV... that’s what happens when losing focus whilst writing. First thing to check is if the resistances between the 3 motor cables (disconnected from the cap) measure the same to rule out any open windings in the motor. My next step will be to apply a voltage to the motor with a variac and let it running at just above the minimum voltage it requires to turn.

If the issue is with the relay, I would not mind cleaning the contacts if only I can find a way how to disconnect it and take it out of the machine. These tape recorders clearly were not built for such servicing considering to get to some wins caps is an absolute nightmare. The other option I’m considering is to apply HT and insert back the 100mA fuse, again using my variac however only after I replace the selenium rectifier by restuffing it with 4 1N4007 diodes and also reforming the HT filter cap. Then I would have exercised some safety before applying a highish voltage.

Thanks once again

Re: Help getting Motor to Run on TK20
 

I see that the German TK 20 schematic on radiomuseum.org shows the 220V version with no mains voltage selection switch. This shows the output of the mains fuse connecting to the top end of the mains transformer primary (grey wire) and from there up to the A2 contact. The lower end of the primary winding (black wire) is shown connected up to the run capacitor.

All this is exactly how my TK 20 is wired.

David

Re: Help getting Motor to Run on TK20
 

Yes the TK 20 and similar TKs are not built for easy access to many of the components.

My TK 20 still has all its horrible black Wima paper capacitors, many as you say are really difficult to access. I think I checked the most important one, the grid coupling cap to the output pentode.

I have stayed with the Selenium bridge rectifier as it works fine, of course for long term reliability good to replace with Silicon diodes but I personally would not bother restuffing the old rectifier because one normally will never see it.

Certainly would be a lot easier to remove the relay for contact cleaning but does look a nightmare to access/remove, I would say possible to clean the contacts in situ and I would not contemplate removing the relay unless proven that the contacts are the problem.

The 50k R45 by the way is the large red resistor in my previous photos on the top right side of the relay.

David

Re: Help getting Motor to Run on TK20
 

My motor windings measure around 1,050 Ohms each.

Dave

Re: Help getting Motor to Run on TK20
 

I have also measured the resistances on my motor and they measure the same and am pleased to report that after changing the motor cap (original one is leaky), I have connected the motor directly to a variac and an isolation transformer and it’s happily turning gently. So at least the motor is good. Of course the next step would be to figure out the issue with this auto stop relay as it’s probably stuck to open. I think the transformer is hood as I measured HT voltage however the motor does not get a feed from the transformer.
I took some photos of my machine in action as it stands and showing the mod I had to make to connect the two caps I hade in stock connected in parallel to give me 1.2uf.
Thanks for all your help. Much appreciated:).

Re: Help getting Motor to Run on TK20
 

Good progress.

My motor is 7882-007 which must be a nominal 220V motor. The A2 contact switches the top end of the transformer (which the 240 mains connects to via mains fuse) through to the motor. My transformer is BV 9005 -507.

From your photos it looks like your is universal mains so I would have thought that your transformer would have the 220V tapping that is switched through to the motor.

David

Re: Help getting Motor to Run on TK20
 

Relay contact A1 is the holding contact for the A relay, switch contact B2 completes the path to ground in any function except Stop.

Lawrence.

Re: Help getting Motor to Run on TK20
 

My motor is also the same model number however my transformer is 9906-514. I shall start fault finding and trying to find what’s going on with this relay but first will test voltages around the transformer now that I know the motor is working fine when connected directly to my variac.

C

Re: Help getting Motor to Run on TK20
 

Thank you Lawrence, that makes sense. So when the tape conductive leader bridges the Auto Stop contacts relay A energises, closing contact A1 which keeps relay A energised/held until the function switch is switched to Stop.

I could not get my Auto Stop to work the other day, relay would not energise, manually pushing the armature plate down (to simulate the relay energising) to open contact A2 did stop the motor. Need to check this out.

David

Re: Help getting Motor to Run on TK20
 

Guys I finally had success and I am not sure what the issue what. I replaced the main fuse to start with as it was corroded but that was not the issue. I also had some suspicion on connections of the fuse holder and the voltage tappings screw, however the problem seemed to be trying to operate the machine upright. By trial and error I helped the motor to start at around 140v and it kind of kicked itself into motion, and it seems to be that flywheel was causing friction against the head plate as it sounded like a grinding machine. Pushing down the capstan shaft cleared the issue of that horrendous noise and now the motor is running gently for few hours on the variac. Thanks for your help once again. C

Re: Help getting Motor to Run on TK20
 

Yes have noticed before that some TKs that normally operate in the horizontal position do not like operating vertically, as you say if operated vertically the flywheel tends to move to the heads mounting plate. There is normally quite a bit of vertical play/end float on the flywheel shaft with the bottom of the shaft siting on a ball bearing in the lower bearing housing.

David

Re: Help getting Motor to Run on TK20
 

The schematic that shows the universal mains variant with 220V transformer tap off to A2, identifies the transformer as 9005-514, do not know what the difference is with your 9906-514.

David

Re: Help getting Motor to Run on TK20
 

I shall check again if I got the number right again as I remember I was not sure if it was a 5 or a 6 (4th digit). The print is not very sharp, looks a bit smudged. Most probably I got the number wrong.
On a very different subject, has anyone got any tips how to restore the front of the cabinet please? A number of Grundigs and Telefunkensuse the same type of cabinet covering which is quite strong however it’s so fragile at the area covering the speaker. Mine in fact has suffered damage and would like to know if anybody has attempted cabinet repairs or replaced the mesh-like covering with an identical material please. Cheers.