Variac or not?

Can you explain why lowering supply voltage (variac) would burn out resistor in series?

I found schematic of 510M and note that "Resistor R3 must be shorted with a jumper before autotransformer is used".
In normal operation there is a time constant of about 0.3s to activate relay that shorts this resistor, but perhaps at low supply voltages relay might not turn at all. This resistor is 10 ohm in 110V version of this amp. It is hard to imagine how this resistor can burn at idle power. Assuming about 50W of idle power supply current will be about 0.5A resulting in 2.5W dissipation on this 10W resistor, but I won’t argue with manufacturer - they know better, and shorting large resistor is easy to do.

adamant40

Variac or not?

I acquired a Marantz 510M amp that had not been turned on for about 10 years.

Yes a Variac is a must, a little at a time, in 20vac mains increments over an hour to full mains volts, so to "condition (form) all the electrolytic capacitors" again, BUT!!! at the same time with meters hooked up and a scope if you have one you must monitor parameters of the amp while going up in mains voltage EG: dc offset, bias, etc etc and whatever adjustments are in the service manual

Cheers George

"Can you explain why lowering supply voltage (variac) would burn out resistor in series?"           Just thinking there may be a corollary, here:          An AC motor, connected to too long of an insufficient gauge wire/cord and the attendant voltage drop, will usually overheat, do to a higher current draw.      Will the power transformer/filter caps try to draw higher current, at charge-up, at a much lower voltage than intended?      One would think a fuse would go first.