Hey guys. So I took this amp cheap, for fixing, but I'm kinda stuck. The symptoms are : both channels sound incredibly low and distorted ( kinda like headphone level when the volume is turned 90% ). One of the channels is getting DC on the output which varies from 1V to 6V, the other is fine sub-50mV. The output transistors and drivers seem fine at first glance.
The problem is this thing has an optical bias, so it uses some thermistors on the heatsink to send a signal ( which is 6V atm ) to an IC per side , that sends a signal to an optocoupler per side ( both channel optocouplers receive 960mV across the LEDs, I think if one of the ICs feeding them were broken I would get different results ? Also 1 channel would be fine ). Now here's the catch. If I press on the right channel optocoupler ( the one with the Vcc on speaker ), the voltage varies greatly, randomly, and when exceeding 2V obviously triggers the protection. This does not happen on the other side , however what happens there is when I measure the output of the optocoupler I get a volume increase and also buzzing on the speaker, just by probing.
Does anyone have any idea what this sorcery is ? Should I mention that the A and B speaker channels are reversed ? When I connect A, only B keeps playing and vice-versa. The amp does not look serviced at all and all connections seem correct so IDK what that is either... I know this is a lot to take in but honestly I have no idea what do do next.
The problem is this thing has an optical bias, so it uses some thermistors on the heatsink to send a signal ( which is 6V atm ) to an IC per side , that sends a signal to an optocoupler per side ( both channel optocouplers receive 960mV across the LEDs, I think if one of the ICs feeding them were broken I would get different results ? Also 1 channel would be fine ). Now here's the catch. If I press on the right channel optocoupler ( the one with the Vcc on speaker ), the voltage varies greatly, randomly, and when exceeding 2V obviously triggers the protection. This does not happen on the other side , however what happens there is when I measure the output of the optocoupler I get a volume increase and also buzzing on the speaker, just by probing.
Does anyone have any idea what this sorcery is ? Should I mention that the A and B speaker channels are reversed ? When I connect A, only B keeps playing and vice-versa. The amp does not look serviced at all and all connections seem correct so IDK what that is either... I know this is a lot to take in but honestly I have no idea what do do next.
I would check for dry joints in this very old amplifier.
Follow the signal using your service manual for guidance and all will become clear.
Follow the signal using your service manual for guidance and all will become clear.