Grundig TK46 motor issue

[Jan Zodiac]

Hello.

I was recently given two tape recorders, a Grundig TK46 four track stereo, and a smaller Philips EL3542 four track mono. I only took them to save them from ending as electronic waste. Both are in great cosmetic condition.
I started them up on the variac (of course). The Philips plays flawlessly and runs very quiet. It will not fast forward/rewind, which is due to small rubber pads under each tape platter (is that the correct word?) having deteriorated. Once they have been replaced, it will work again.

I did not record on any of the machines. I am not sure I will keep them, just wanted to see if they are still "alive".

The Grundig also came back to life after slowly starting it up on the variac. It shows very little wear on the heads (three heads btw). It is a bit noisy from the idler wheel though, but apart from that it does fast forward/rewind.
I played pre recorded tapes on it, and I was impressed by how nice it sounds for a completely untouched recorder.
It has an issue though. When it has played around 30 mins. the motor suddenly loses power. The speed drops quite a bit and also fast forward/rewind gets very slow until the motor eventually stops and the stop button has to be pressed.
I checked everything that could slow it down, like dragging brakes etc. but nothing unusual going on there. It is for sure the motor that suddenly lacks torque. It does have a motor capacitor which I also checked, and it has the correct value. It does not get warm, which would indicate a defect.

Can it really be the motor itself that maybe has a bad winding that gives problems once it has warmed up?

Another thing, if anyone should be familiar with these Grundig machines, which standard does it use? CCIR or NAB? I did download the service manual, which shows the actual record and playback curves, but nothing is mentioned about which standard it follows. Maybe they used their own?

It could be fun to get this recorder fully working, and see how well it performs once everything has been adjusted to specs. I really don't think a lot has to be done, except of course for the motor problem.

Best regards,

Jan

[Analogue man]

I would still suspect the motor run capacitor or one of it's soldered connections (the later because it could be a dry joint which shows up when the recorder gets warm).

[DMcMahon]

Normally I would say very unlikely to be the actual motor at fault (not impossible of course) and also fairly unlikely to be the run capacitor.

If nothing obvious found I would recommend monitoring the voltage to the motor windings to see what happens to the voltages when the motor runs slow.

If your TK 46 is like my TK 40 unfortunately is is difficult to monitor the motor voltages, you have you get to the back of the mains voltage setting links board where the 3 motor wires are connected, the wiring to the back of this board is very delicate, particularly the mains transformer wires. You can monitor from the front of the board (without disturbing it) if you can work out where the motor wires connect to.

David

[Jan Zodiac]

I checked the connections and re-soldered it. It did not help, but maybe it could be internal. I think I will try with another capacitor next. Or just try to unsolder it, let the motor run, and then see if it has the same loss of power as when the problem appears.

Regards,

Jan

[Jan Zodiac]

Thanks David, I will check up on that also. I think it is easy to get to the connections on this one, so it should not be a problem. I can also check what happens across the capacitor when the motor starts to run slow. I only experienced a bad run capacitor a single time many years ago. It exploded, so no doubt about what to replace...

Regards,

Jan

[Jan Zodiac]

So today I removed the run capacitor and checked it. It does have some leakage that starts around 300V. I heated it a bit with a hairdryer, and after that, leakage starts at around 100V. I use an old Heathkit capacitor tester, which is also great when old electrolytics needs to be reformed.

That means it actually appears to be the run capacitor that causes the motor to lack power. I tried to run the recorder without the capacitor, but the motor didn't run at all. At least not with the belt on.

I will order a new capacitor and a new pinch roller, which is rock hard. I decided to restore it, since the heads has very low wear. I will test the valves (all original), which also gives a good indication on how much it has been used. Especially with the ELL80 output valve that runs very very hot.

Btw I was a bit impressed to see that no carbon composite resistors are used at all. Nice Rosenthal carbon film resistors throughout, as well as polyester film caps.

Regards,

Jan

[DMcMahon]

I saw on the TK 46 & TK 40 schematics that there are 2 motor options, yours with the 8uF must be the higher voltage one shown on the schematic as 300V, my TK 40 is the lower voltage one listed on the schematic as 125V (actually 110V shown on motor) using a 2uF run capacitor.

Very unlikely that you have any grid coupling capacitors that are passing DC voltage because they are Polyester but in light of the hot running ELL 80 I would double check the ELL 80 control grids for no +ve voltage (pins 2 & 6 i.e. capacitors C129 & C229).

I do not know for sure but I think all Grundigs (maybe apart from some very early ones) are CCIR.

David

[Jan Zodiac]

Hi David,

The run capacitor on mine is 2uF as well. I did notice two motor options from the schematic though. Maybe it is related to export models. I checked for grid voltage on the output valve and no voltage present. I will replace the cathode electrolytics though. If they short, the valve will suffer as well. So better replace them for safety. Especially since the ELL80 is not a common valve anymore. It runs very hot even under normal conditions, due to the very compact construction.

CCIR/IEC is probably correct, given it is a german recorder. I know Uher from that time used it also.

Jan

[DMcMahon]

Yes I got confused, mine has the 8uF run capacitor.

David

[Jan Zodiac]

Problem solved! A new motor run capacitor got rid of the issue.

[DMcMahon]

Good news, well done.

David