Hoping to repair this for a friend. The problem seems to be in one channel of the power amplifier, as it presents when the "separate" switch is engaged and a signal goes directly into the power amps.
Dim bulb is fine, main relay clicks "on", there is good audio through one channel and bad audio through the other. By bad I mean broken up, intermittent signal that pushes the cone of my bench speaker, irregularly. Looks like DC on the output, but not in a continuous way.
There are two sets of six pins easily accessible from the top (which will look like the bottom - this amp layout is upside down) , so I have included my voltage readings there, on the pics below. One pin's fluctuating DC looks suspiciously related to the bad audio in one channel.
I have a good schematic thanks to HiFi Engine. It's too big to upload here but it can be viewed here. I have not been able to secure a service manual so I don't have the DC offset or the bias instructions. If anyone can divine them from the schematic I would appreciate it. Two trim pots are easy to access but I don't see the TPs or the target voltages.
Any suggestions on where to look next to track down this problem, I would appreciate it. It looks like the power amp uses three ICs instead of discrete circuits.
Here are the pics:
.
Dim bulb is fine, main relay clicks "on", there is good audio through one channel and bad audio through the other. By bad I mean broken up, intermittent signal that pushes the cone of my bench speaker, irregularly. Looks like DC on the output, but not in a continuous way.
There are two sets of six pins easily accessible from the top (which will look like the bottom - this amp layout is upside down) , so I have included my voltage readings there, on the pics below. One pin's fluctuating DC looks suspiciously related to the bad audio in one channel.
I have a good schematic thanks to HiFi Engine. It's too big to upload here but it can be viewed here. I have not been able to secure a service manual so I don't have the DC offset or the bias instructions. If anyone can divine them from the schematic I would appreciate it. Two trim pots are easy to access but I don't see the TPs or the target voltages.
Any suggestions on where to look next to track down this problem, I would appreciate it. It looks like the power amp uses three ICs instead of discrete circuits.
Here are the pics:
.