I found that the power supply board has this large bridge rectifier. It looks to take the AC post-transformer and split it into + and -45V rails.
I expected to find some AC voltage on the two ~ terminals of the diode bridge but did not. So I'm thinking that the front panel is either not correctly signalling the power supply board, or the relay/signal circuit on the power supply board blew up.
There is this 5 pin terminal on the front panel that has power. It is directly connected to the power supply board. It's the only direct connection. If I turn the amp on from the front panel, the internal fan comes on. The fan is connected to a 2 pin terminal right next to the 5 pin terminal on the power supply board.
The front panel has connections to both the input board and the power supply board. So indirectly through the input board and through a dsp board, it has these additional connections to the power supply board, but only a single 5 pin cable that is directly going to the power supply board.
On the 5 pin terminal I found 12.0, 5.0, GND, 0.7, 10.3 volts. When I turn the front panel on, the 0.7 drops to 0.1 and the 10.3 drops to 9.3. The other 2 voltages stay the same. A 5v fan turns on as well. That led me to find the other 2 small diode bridges that filter 5v and 12v power, they seem to work with the power switch.
So I was thinking there are 3 transistors that turn on the 3 diode bridges? And one of them is bad? I'm thinking the 10.3 power rail should be dropping to 0? If I'm headed in the right direction, should I be able to visibly see a bad transistor? They all look fine.
But then I identified the two 3-pin terminals to and from the main xformer. There are only those 2 terminals (makes sense). The "input" terminal always has 120V, no matter whether the power is on or off. The output terminal always has 0V. If there is input voltage into this large toroidal transformer, shouldn't there always be output voltage? Now I'm wondering if I should try applying 90VAC to the output terminals, bypassing the xformer, thus identifying a bad xformer. I'm hard pressed to understand how it could have gone bad with no visible damage. It's wrapped in paper and saran wrap and looks perfect.
Another thing that would help is being able to release these very low profile connectors (last photo, J4). There are a number of them. It prevents me from removing the power supply board entirely. Is there a special tool? I think I'm lifting all the fingers but the connector is still not coming apart, so either I'm not getting them all or my makeshift way of doing it doesn't release them properly.