Revox Amplifier Repairs

Revox B750 Stereo Amplifier

This wasn't operating the speaker relays at switch on, turned out the tantalum capacitor in the 20V regulator circuit had gone short circuit.

Revox A50 Stereo Amplifier

Revox A50 Stereo Amplifier Revox A50 Stereo Amplifier Revox A50 Stereo Amplifier

This wasn't powering up, I went straight to the capacitors on the power amp boards. Most were unserviceable 250µF, replaced them all and powered it up. It didn't sound too good; crackles and pops, the 3 preamp boards needed the connectors cleaning. The capacitors were ok ish so left them in for now, (all the tantalum's measured ok too surprisingly). The mains was wired directly to the switch so I fitted a 2 pin socket as originally fitted. I have loads in the workshop from fitting IEC sockets to various stuff with the 2 pin versions. I set the output to 0V and the quiescent to 20mA and it actually sounded reasonable for a 1968 amplifier.

Revox B251

Revox B251 Rectifiers

This was dead and in need of repair, one of the secondary rectifiers was s\c and had taken out one of the chopper transistors. The chopper\inverter runs all the time there is mains input. The PSU runs at about 22k and high frequency rectifiers are used (RKBPC602), later the mains filter capacitor expired with a bang, when all put back together sounded fine, then the real trouble started sorting all the pre-set volume levels out!