Mark IV lead channel switching problem..

[aiyiadam]

The Mark IV on my bench can't switch to the lead channel. Both the footswitch and the mode select switches don't work. Switching from the rhythm 2 to the lead channel, the lead LED blinks very briefly, I hear the channel very shortly and then almost absolute silence.. By switching, the voltage on the positive vactrol ( LDR ) rail drop suddenly from ~3,8V to 2,2V.

Any idea, why this is happening? Can a vactrol short the voltage rail?

 

[bgh]

Will the "Mode select" switch on the back panel switch channels effectively?

 

[aiyiadam]

Will the "Mode select" switch on the back panel switch channels effectively?

The original switch was actually defective and I have soldered a perfect lorlin rotary switch instead.. It still doesn't switch to the lead channel. This problem doesn't seem to have anything to do with this switch though. The footswitch and the external switching jacks also don't work.

Let me also add an update..

The voltages of this amp seem to be generally a bit low. At 110V mains I have 5 and 10V at the heaters. The rhythm channels seem to function but the green and orange LEDs are a bit dim. The lead channel switching is possibly demanding more current/voltage and it just doesn't switch.. However, 10V (F voltage) should be actually enough. I have also bridged the 6R7 resistor between the 10KuF caps to increase the voltage to 12V and the lead channel still didn't work. Something in the switching circuit might be pulling too much current..

I asked the owner about the previous repair jobs. He said that the previous amp tech has changed the LDR 27, because it was pulling the voltage of the lead channel switching down.. It seems to be a similar problem..

Don't know.. Maybe I should check a few resistors and try to find one with the extraordinary voltage drop..

This is a 25 year old amp. It is also time to change the filter caps I guess..