70's Grundig Radio - Volume Issue Repair Help

[thomieboy]

Hi everyone, I am currently trying to repair a Grundig radio from the 70's. I found a schematic for this radio here: https://www.radiomuseum.org/r/grundig_rtv_500.html

ISSUES:
- It still seems to work, but the volume slider is extremely sensitive. Putting it at 10% of the sliding range is already too loud.
- I also notice that only the left speaker seems to work, but pressing the 'mono' button makes it play through both, so I won't put too much time in this issue. (But would be nice if this is fixed as well)

INFO
Attached you can find the volume control & balance part.
- Top is left channel, bottom is right channel
- R44 & R47 is the balance sliding potentiometer (orange)
- R55 & R56 is the volume sliding potentiometer (green)



OBSERVATIONS/QUESTIONS
It seems that the volume potentiometer has 5 connections for each channel. I do not understand how these are connected inside, I would think that any potentiometer has only three connections (left, right, wiper). Anyone knows where the other connections are for?

When I measure resistance between ground and wiper output of the volume potentiometer (blue arrow), I notice that it rather quickly jumps from 0 to 68K in the first 10% and stays there for other 90% of the sliding range. Does anyone know what could be causing this? I would expect a nice linear increase over the whole range. I do not see any obvious sign of broken caps or resistors.

Let me know if there are things that I should measure etc. I very much appreciate any help! 

[mikerj]

The extra two taps on the potentiometer are to add a form of tone control, usually providing bass boost at low frequencies (e.g. "loudness").

Sudden jumps on volume are most often due to wear on the carbon tracks, and this will usually be at the low volume end of the slider.  With most of these older potentiometers it's not hard to disassemble them for cleaning and inspection.  If you find a badly worn track one trick I've done a few times in the past is to bend the wipers to use a fresh part of the carbon track.  Injecting some potentiometer cleaner (e.g. Deoxit) and working the pot up and down would probably be a good start, the wiper contacts can get oxidised over time and make poor contact with the track.

Have you measured end to end on the potentiometer, and if so do you get something close to the 500k it's specified at?