Grundig TK120 volume won't turn down.

[Tractorfan]

Hi,
Yesterday I wanted to use my trusty 1967 Grundig TK120 to check some tapes only to find that the volume, when the control is at minimum, is fairly loud, but undistorted. Advancing the control only makes it louder.
The last time I used it, the volume would not go down to silence, but certainly a lot quieter than it does now. I've not taken any voltages yet as the print side of the PCB is a pig to get at.
Could it be a leaky capacitor problem? However, I think (from memory) that most of the caps are those mustard coloured ones that give little trouble.
I've cleaned the pot and the record/play switch.
Thanks in advance.
Cheers, Pete.

[Boater Sam]

Best bet is that the earthy end of the pot has gone open circuit or very high resistance on the rivet that fixes the carbon track end.

[Silicon]

I have recently taken apart a Morganite volume control with similar symptoms to yours.

The resistive coating under the metal wiper has been completely worn away at the low end of the carbon track.

[Tractorfan]

Hi,
The volume pot looks like a specially made item for this range of tape recorders, and not a standard one, so I hope I can do something with it. It's stored upstairs in our old house so, maybe, a touch of damp has got it. That would explain why it's worse now than it was earlier.
Thanks for your replies.

[Tractorfan]

Hi,
Well, well, well. Fancy that!
I had a rare hour to myself and decided to open up the Grundig and have a look at said volume control. I didn't fancy the hassle of unsoldering it, so metered it instead. It all seemed OK, so I was having a bit of a head scratch when I noticed the only Hunts electrolytic cap which is situated at the top of the PCB. For the want of anything better to try, I replaced it and,
a complete cure!!
It's 8uF 350v and is connected between the HT supply to the ECC83 and earth. Does it smooth or decouple? I don't know. But replacing it did the trick!
I presume that it was leaking and dragging the HT down. Alas, I didn't check the voltage before the replacement, but it's correct now.
Cheers, Pete.

[Tractorfan]

Hi,
If the capacitor was leaking and dragging the HT to the ECC83 down, as I assume, then why would that stop the volume going to zero when the volume control was turned right down?
I also noticed that the input audio was causing distortion, as if it was overloading the first stage, which I assume it would if the HT was low.
I'm not very good at valve theory, so would welcome your comments.
Cheers, Pete.

[Techman]

Unlike the 'paper' versions we all know and love (or rather NOT love), Hunts produced some of the best electrolytic capacitors ever made, so I think it's very unlikely to be the 8uf capacitor that was at fault. They're so good in fact, that I'm still fitting some 'new-old-stock' Hunts electrolytics that I've had for probably forty odd years and they're still perfect in every respect.

It's funny you should have this problem on a Grundig tape recorder, as I have one (a Grundig) with exactly the same fault waiting in the wings on the 'to-do' pile. The difference is that it's solid state and a stereo version and both volume sliders have the same problem and developed the fault at the same time. When I get round to investigating the fault I'm expecting to find a grounding issue on the board to chassis - probably an oxidised connection under a mounting or similar.

So to sum up - I think you've accidentally cured the fault during replacement of the said capacitor. The grounding point of the circuit board was probably associated with that particular capacitor mounting even.

[Tractorfan]

Hi,
That's interesting. I just lumped them together along with other Hunt's products and assumed it was age related.
What I'll do, in that case, is pop the old cap back in and see what happens. The soldering on the PCB appears OK, but it's difficult to get a good look-see because the PCB is very close to the case.
Cheers, Pete.

[Techman]

I was nearly going to say why not put the original back in but thought you probably wouldn't want all the bother of pulling it all apart again. As they say, originality is everything and something can only be original once - not that it's all that important when it comes to an old Grundig tape recorder, particularly as a few of the original paper type lower value capacitors will probably have been replaced anyway, as the types used in those Grundigs were just as bad over time as any Hunts.

The important thing that I should have said in the previous post and forgot, was that the 8uf electrolytic sounds from your description as though it's part of the HT decoupling (smoothing), so whether short, leaky or open, won't be causing the fault you describe. If this capacitor had any of the above mentioned faults I think you'd find the effects much more dramatic than just not being able to turn the volume all the way down.

Your post is giving me the nudge to get on and fix the one that I've got with a similar fault

[Tractorfan]

Hi,
How odd. I put the original Hunt's cap back in and the fault did not reappear. Over the period of about forty minutes the HT dipped from 227 volts down to about 208 volts, then came back up to about 215 volts, but that may be due to other factors.
With the old cap in place I could still faintly hear the playback with the volume control at minimum.
Hm.
Cheers, Pete.

[vidjoman]

I think that if the cap was open circuit, no capacitance, then if the volume control is between the 2 halves of the ECC83 then there will be path for the audio signal to go via both anode resistors and on to the next stage, albeit at a low amplitude, and look as though the volume control has not reduced the signal to zero.

[ms660]

Yes, the common decoupling resistor is 27k, enough signal voltage would be developed across that to show up at the grid of the next stage I would have thought.

Lawrence.

[Techman]

An interesting thought and certainly a possibility.

The capacitor is marked as 16uf on my diagram (as shown in the picture below of a portion of the circuit) and it's possible a bit of reforming has been going on as perhaps indicated by those variable HT readings. That final reading maybe indicates that the electronics need some servicing as regards to certain inter-stage paper coupling capacitors causing valves to pass too much current, so I wouldn't leave it running for hours on end until this has been investigated.

To prove a point, I would get your new replacement capacitor and connect it via a couple of croc clip leads across the original one while in circuit to see whether it makes a difference to the fault when connected and not connected. Obviously be careful when playing with HT and charged up electrolytic capacitors. Also, feel if the original electrolytic shows any signs of the case warming slightly indicating slight leakage.

Well I got down to it and sorted out that Grundig that I had waiting for repair with a similar fault symptom. It turned out to be pretty much as I expected, except that the slider volume controls are part of a plug in circuit board unit, and metering from the cold end of the tracks to deck was not showing the low resistance expected for a functioning unit. Long story short, some careful cleaning in various places restored normal operation. As can be seen in the second picture below showing that part of the circuit, each slider has two tone compensating taps off the tracks, so any major physical problem with that unit and it could be a real pest. While I was at it the mechanics needed sorting, so a few hours were spent in all with good results. I probably ought to start another thread dedicated to that tape recorder.

[Techman]

I've just re-read your first post and see that I'd forgotten you'd said that you thought that most of the smaller caps were perhaps the Mullard mustard type, if this is the case then they're probably alright, but worth checking the odd g1 voltage etc.

I suspect you're starting to wish you'd just stuck that other electrolytic capacitor in place and left it at that, but it is interesting to get to the bottom of exactly what was causing the fault and why - time allowing, of course.

I think that circuit clip I posted is a later revision of the original design. An interesting thing I've spotted since on an even later circuit, is a modification of that particular circuit, and that electrolytic value has been increased to 32uf, perhaps indicating that 8uf was originally very borderline for that job. It could be that they were finding that it didn't take much loss of value for there to start being issues, even within the percentage tolerances of some of the original new parts.