Pressing the power button gets relay click and power LED flash (on volume
control) but all shuts down immediately.

Looked for infamous "pink cap" on "sub power supply" (there is one) and
replaced but no change in symptoms.

Jumpered the relay contacts and using a Variac ran voltage up full (while
watching main transformer secondary voltages): all seems normal but still no
signs of life. Transformer voltages are as expected (2 pairs of windings;
each pair share a common center tap, total: 4 windings, all voltages increase
linearly with increase of primary V). Still no display or control
functionality at all.

Pressing the power button (with jumpered relay and full mains voltage
applied) gets same symptoms.

So far I think I can rule out a shorted output device.

How best to proceed with the troubleshooting?

Ideas?

I have the service manual.

Thanks.