Hello good people.
I'm trying to repair an amplifier that exhibits exactly the same dc offset on both channels, 5.3v measured before the relays which are obviously open. I already measured output transistors and drivers for short circuit with the DMM but they all appear to be normal with equal behavior. Could it be the protection circuit triggering its own fault?
I see no sign of obvious burnt components.
Schematics:
http://sportsbil.com/yamaha/tricomp_scans/AX-400.pdf
Thanks.
I'm trying to repair an amplifier that exhibits exactly the same dc offset on both channels, 5.3v measured before the relays which are obviously open. I already measured output transistors and drivers for short circuit with the DMM but they all appear to be normal with equal behavior. Could it be the protection circuit triggering its own fault?
I see no sign of obvious burnt components.
Schematics:
http://sportsbil.com/yamaha/tricomp_scans/AX-400.pdf
Thanks.
Last edited:
It is unusual to see the same DC offest on both channels.
I see this Yamaha amp has a shared -ve bias line through R232 for both channels.
I would check voltages around it. It could upset both channels.
I see this Yamaha amp has a shared -ve bias line through R232 for both channels.
I would check voltages around it. It could upset both channels.
Thank you very much for your help.
Supply lines are all good, -55.5 and +55.5 and +16 and -16 like they should.
Voltage trough emiter resistors went down 7 mv on q125 and q127 and went up 5 mv on q126 and q128 but i might have touched idling current pots on the process of troubleshooting, and they are very very twitchy...
Supply lines are all good, -55.5 and +55.5 and +16 and -16 like they should.
Voltage trough emiter resistors went down 7 mv on q125 and q127 and went up 5 mv on q126 and q128 but i might have touched idling current pots on the process of troubleshooting, and they are very very twitchy...
Ok. Just to say i solved the problem.
The amp is old, from 1987. A few pcb tracks have some brown spots of oxidation, and apparently one tiny GND track stopped conducting because of one of these brown spots that corroded to the point of breaking the circuit.
I spent some time with a bright light, a magnifier and a DMM and spotted it. Corrected it with an insulated wire shunt and the Yamaha is 100% back to life.
Will now address other points of corrosion in the pcb and do some preventive maintenance.
thanks.
The amp is old, from 1987. A few pcb tracks have some brown spots of oxidation, and apparently one tiny GND track stopped conducting because of one of these brown spots that corroded to the point of breaking the circuit.
I spent some time with a bright light, a magnifier and a DMM and spotted it. Corrected it with an insulated wire shunt and the Yamaha is 100% back to life.
Will now address other points of corrosion in the pcb and do some preventive maintenance.
thanks.
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