I acquired a P-55 from ebay that wasn’t working,
Problem: No power to P-55
There was a dead short across the power pin (after blowing the transformer a few time before putting a meter across it) These Roland devices have reversed Negative Collar power inputs so its easy to blow if you just plug what ever supply you have laying around.
L78MR05 Linear Regulator with reset generator, removed and tested , I acquired some “new” ones from a spurious supplier and i wasn’t keen on using it, luckily the original one tested fine once removed from the board.
The 78MR05 is a 7805 with a circuit to hold a reset line active high/low until the power line is stable that is fed to the uC.
Input diode D1 turned out to be blown causing the short circuit, D1 is the first device inline to the power input. This is probably due to blown from a mispolarity.
I replaced D1 with a 1N4007 (700V RMS reverse) total over kill but I need to use a lesser diode like a 1N4001 (35V RMS reverse) this is all I had on hand at the time I will have to replace at a later date with a more appropriate diode. The original D1 looks like some sort of zener, there were no identifying marks.
How does it sound?
the Patches that the P-55 supports sound so much better that the equivalent patch on the SC-55
The SC-55 relationship to the P-55
When the P-55 is connected to the SC-55 via the MIDI THRU port and the appropriate settings are input to mute the SC-55 Patches, the P-55 takes over the piano patches of the SC-55 giving a higher quality result.