This was an interesting one, the main issue customer had was with added modification not working. This modification was done sometimes in the 70’s and it took me a while to figure out how the mod was done.
Basically the bottom octave of the pedal had a contacts duplicated to the lowest octave of the bottom manual and one of the BUS wires which used to be associated with one of the draw bars was instead taken externally via additional BASS coil and switch to the extra jack output.
When I managed to resolve this issue with non operational bass there was still fair amount of hum. This was partly because the modification created ground loop. I resolved this by earthing the additional output in the same place as the main out was. This removed some of the hum but there was still fair amount of it left. With ground loops eliminated this had to be caused by the aged electrolytic condensers in the PSU. Following a brief discussion and estimated Bill Of Materials and work involved decision was made to do full recap.
Modules had a mixture of original Mallory american condensers from 1950’s and old TESLA electrolytics from sometime in the 70s.
Since the dual/triple modular caps were not available during the communist era somebody who did the work in the 70’s installed replacement caps inside the chassis and sadly placed them right next to the power resistors. Such a high temperatures are a recipe for a quick death of electrolytic and that’s exactly what happened. One of them popped and leaked out and the other had also the safety valve popped.
I replaced all the electrolytics throughout and utilized modular caps made by JJ in Slovakia. These are great for typical PSUs of Valve amp designs.
Result is one happy customer and L100 sorted for another 40 years