I'm trying to understand the failure mode of this circuit

Thread Starter

s-petersen

Joined Aug 13, 2022
6
I have this jukebox amp to repair, and the failure is there is no volume control, it is stuck on full volume, I have one channel in the schematic posted, the other part is below and identical. I don't understand how the circuit works, the 10k pot is pictured in red, only 2 terminals are used.
I believe Z5 has failed, because it is the closest to the outside world, and static may have killed it. Z3 appears to be in the signal path.130w zoomrd.png
as a test the pot is removed and there should be no volume, and if the pot is shorted it supposed to be full volume.
 

Thread Starter

s-petersen

Joined Aug 13, 2022
6
I guess there is no edit function on the forum. So I was looking over the schematic, should I see a swing from +12.v to -12.5v when the pot is adjusted from 0 - 10k?
 

michael8

Joined Jan 11, 2015
333
The string of diodes cr11->cr14 are grounded for ac at both ends. The audio path connects to the midpoint
(between cr12 & cr13). So the diodes shunt the audio path.The ac resistance of the diodes depends on the
DC current through them with higher current being lower resistance. Z5 controls/provides the current.
Low current should be high audio, high current = low diode resistance shunting audio to ground -> low audio.
 
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