Right below the sensor pad.
On the volume board.

Martin

Thank you again Martin.

Today I again measured  the diodes and resistors on the volume board, and find no faults.

Did the same for the remote control section, and couldn´t find anything wrong there either.

I know the transistors on the volume board are ok, because I changed them. The only things I didn´t check there are those five small red capacitors.

Is there a possibility to disconnect the remote section from the volume board?

Hans.

I checked and rechecked the components(the resistors, diodes and  transistors, not the capacitors) , but cannot find anything wrong.

I went back to the volume board and started measuring voltage. The incoming voltage is 15V and steady.

But if I measure other parts I see that while the volume is increasing, the voltage drops from -12V till -5,5V and stays there.

I measured  this f.i. at IC1 and at C4 (both on the volume  board).

Does anyone know why the voltage is dropping?

Hans.

I've not looked at the schematic and I'm out and about at the moment so not in a position to do so right now so just thinking aloud at this point.

Could this be a rectifier fault? It might account for the halving of the DC voltage and the resultant ripple might trigger the counter to run away.

Ideally I'd want to be examining the -12V rail with a scope at this stage and working back towards the power supply, but in the absence of access to an oscilloscope you could try measuring the voltage in the same place as before using the AC voltage range on your meter, both whilst operating normally and whilst the fault is occurring. Then let us know what readings you see.

Martin.

Hello Martin, thanks for your reply.

DC-volts measure:

The first 15-20 seconds I can vary the volume and voltage stays at -12V while doing that.

Then the voltage starts to go down at a steady and even pace, in the same time volume goes up.

Voltage stays at about -5,5V

AC-volts measure:

The voltage is hard to read from the beginning, very jumpy digits, average estimates around 2,4V

Then it starts to go down, still very hard to read, till it stays at about 1,6V, digits still very nervous and jumpy.

Rectifier is replaced together with the caps,  maybe wrong connected?

Hans.

Ok, what I was hoping to measure using the ACV range was ripple, but the results look rather inconclusive. Without knowing how your meter measures ACV I don't think we can draw any conclusions from those measured values.

It's difficult to know how to guide you further without access to a scope.

Unfortunately, changing components by guesswork is often as likely to introduce new faults as it is to resolve them. If a component has failed it may have done so because of another failed component or a short somewhere and this can mean that the replacement fails just as quickly, resulting in you believing that you have ruled something out when in fact you may not have done so.

Martin.

Many DMMs cannot measure ripple (AC riding on a DC voltage) unless you add a series cap to isolate the DC, so this might be just measurement confusion.

Ok Martin and Mika,

I understand that I have to find the source of this voltage drop. I try to do just that, but know next to nothing of electronics.

That brings me back to the question: what can cause this voltage drop?

Martin you named the rectifier already, but are there more options? The system works fine for about 15 seconds and then the problem starts.

Keep on thinking all the time that there is a component that heats up and becomes more resistive. Or is this total nonsense?

Hans.

Hi Hans,

There are any number of potential causes and it is impossible for anyone here to do more than suggest areas to investigate.

Unfortunately without access to more test gear (and the experience / expertise to use it) I think you've probably reached the limit of what you can do yourself.

I would suggesting checking again (with a magnifying glass) for broken PCB traces, bridged solder joints, and that each of the capacitors you replaced before the current fault occurred are installed in the correct orientation, but beyond that it is probably time to hand the unit over to someone with the experience and tools needed to trace the fault.

Kind Regards,

Martin.

I agree with the above Martin.
Find someone geographically near to you to have a look at it.
I tend to like challenging, tricky and "mysterious" faults like this and I am getting curious, - if you are anywhere near me....

Martin