Well, I jumpered a wire across 3 and 4 and plugged it into my dim bulb tester. Flipped the switch, lo and behold....the device lit up! I almost fell off my seat. It was magical.
Nice! We're finally starting to making real progress.
1. How did you know from looking at that additional schematic that 3 and 4 were the required leads to be jumpered? 3 goes to winding 3 on the primary side and 4 goes to 5.
Well, you are on 240Vac mains, that implies the two primary windings must be in series for proper operation. So I simply traced the lines in the diagram and it checked out. It was convenient that the rotary switch diagram was already drawn with the 240Vac connection in place. The M pins on the switch are simply a jumper, so if the inner contacts fell out, pins 3 and 4 would be left disconnected and thus require a jumper. Or alternatively a jumper could be soldered at the transformer directly, to connect the two primary windings in series without a bunch of wires running around.
2. If there was an open circuit, why was I getting a voltage reading of around 80 Volts going into the transformer?
Well, the hot and neutral lines were connected to the ends of each primary winding. Hot switched by the relay and neutral always connected. So WRT earth ground, you may have measured some residual voltage, but there was no current passing through the transformer's primary winding due to the open circuit at the selector switch. This is why a differential measurement at each pair of pins on the primary side would have helped. You would have seen 240Vac between the ends of the two separate primary windings, but 0Vac between any two pins on each individual winding, as the two primaries were electrically disconnected.
3. Even when the device turns on, I'm still getting only 3V at D753 on the startup circuit - I'm confused as to why the device is able to start when the voltage is low!
The standby 5V rail only powers a small portion of the circuit, so its effect on the total operation of the unit is considerably less than that of the power transformer side. So it would make sense that it should still sort of work, even if the 5V rail has a fault somewhere (assuming everything else checks out okay).
Just to let you know what I've done - instead of just capping of the redundant wires, I've removed them entirely. Except a purple wire, that leads from a point marked '3' on the PCB to winding one on the primary side of the power supply - nothing works at all if that's removed. I've also left the grounding wire too.
That works too. I believe the purple wire goes to the relay on the PCB controlled by the standby circuit. So it would make sense that the amp wouldn't work without it.
The devices turns on, but only for about two seconds. It will then rapidly turn off. Sometimes, the display will flicker. Other times, it will 'hang' (i.e not respond to any input). I'm no means an expert, but just from tinkering around with computers, it *feels* like an intermittent voltage supply or a low voltage situation. I wonder if it could be related to some dodgy capacitors?
I suspect there is a short circuit loading the zener diode on the 5V stby circuit. We just need to look at the schematics and see what is powered by this rail and do a little poking around. Most electrolytics fail open circuit, so it could be a shorted transistor that’s causing the loading. If we measured high AC ripple, that would imply there are bad electrolytics. But the voltage is low, so we definitely have a loading issue.