Hi, I am new to this forum.  I have a BM3000 with a history and some unresolved issues.

It seems that this unit was for the European market 220V and arrived in Australia where it was used on 240V (I can see where the original mains cable has been cut and a new lead fitted).  When I got it, one channel was out, and hum on the other. Tuner was working.

I initially replaced all electrolytics and the power amp module. Everything worked, but residual hum which increased with volume.  Voltages off the rectifiers were high (+/- 34V instead of +/-30V to the power amp module).  That's when I found the mains wiring error and restored the 240V taps on the transformer.

The hum problem improved, but is still audible as volume increases.

Then attached my old Beogram 4002 and the phono preamp was working BUT with some background 100 Hz in the left side.  on checking, this 100hz was on the line out , and further checking showed it to be at the output from the RIAA preamp - disconnected C423 and the 100hz was on the collector of TR405, about 20mV pp.  The other right hand preamp has no measurable signal.  This is with noting connected to the preamp inputs.

I have checked the DC voltages around the preamp, they are as specified.

I checked all components in the left preamp, they are all ok.  I swapped the transistors between preamps, no change.

Checked the power supplies, ripple on 5V, 16V, 33V all less than 5mV.  100hZ ripple on 24V is 400-500mV but it only supplies the volume display LEDs.

Data Link line goes to the DIN socket input for RIAA preamp, isolating it makes no difference.

Since both preamps share the same power supply tracks on the PCB, it's hard to see how the 100hz could be from the power supply.  But 100Hz it is.

There is still some residual hum (again 100Hz ) which increases with the volume setting.  

I need some help - has anyone seen this behaviour before?