I had to open my Roland D-50 recently to fix some keys and switches, a problem very common to Roland synthesizers of that period. I also replaced the Volume fader and the corresponding dust cover with a homemade piece of packaging styrofoam. The old dust cover had become dust and debris itself.
After having completed that and while reassembling the synth I noticed a problem with the memory. It was not possible to select any sound, the synth was somehow frozen and stuck to patch 11.
There is a built-in test mode that can help regarding diagnostics. It can be found in the D-50 Service Manual that can be obtained from the D-550 page at polynominal.com (D-50 Service Manual). To boot in test mode remove all attachments like cards and switches and do the following:
Roland D-50 Test Mode:
Press and hold "0" and "Decrement" while switching power on
The panel test resulted in all keys and switches being ok so I had to dig a bit deeper.
I have a memory expansion board installed. It's not the one from Musitronics (being still available) but probably a similar approach extending the original 64 patches to 256. There is a lack of information about this board, unfortunately, it might have been manufactured by PA Decoder, a company that vanished long ago. There is written "D-550 Super Speicher" (German) on the circuit board which means "D-550 super memory". Obviously this D-550 Expansion works in a D-50 also. To reach the 4 memory banks you have to press and hold "Internal" and press "Patch 1/2/3/4". If you just selected a new bank you have to press any patch button to choose a sound.
I suspected some of the expansion stuff to be faulty but before I started ohming and scoping I did a factory reset. This is described both in the Owner's Manual and the Service Manual:
Roland D-50 Factory Reset:
Press and hold "Data Transfer" and "0" and switch power on
There will be a "Complete." message in the display. However, resetting the synth this way does not touch the memory expansion. Any data stored there will remain the same as before.
Anyway, that fixed it. I do not know why the memory was stuck. Probably I touched some of the expansion board contacts by mistake while dis-/reassembling.
As the synth was disassembled I noticed that I never had a closer look at the PSU. It was set to be used with 220 Volts. There had been a shift in voltage in Germany and probably other european countries in the early 1990ies, from 220V to 230V in average. Fortunately, on many devices built for global export you can switch the voltage input. The D-50 transformer allows for 100/120/220/240 Volts input by wiring the corresponding pin so I simply moved the wire from 220 to 240V. I also noticed that two of the Elkos did not look good. This might have been the result of too much voltage for more than 25 years, so I replaced all of them.
The synth works again and despite of all the years still sounds excellent.