This Marshall JTM45 was blowing the main fuse in standby mode, which pointed to a problem with the heater circuit. Disconnecting the secondaries didn’t fix the problem so it had to be a faulty power transformer. Before connecting the new transformer, I tested all of the valves and found a short in one of the pre-amp 12ax7s. There are no fuses on the heater circuits in this amp, so it’s likely that the 12ax7 short caused an over-current that damaged the LT windings of the transformer.

New transformer fitted, service complete, and JTM45 sounding as good as ever.