All outputs muted on XP-80

[Patrick Rosendaal]

Hi,

My trusty XP-80 (bought in '97) finally developed a problem: after starting up all audio outputs are muted. Even the click out is silent. Tested with various headphones and amplifiers, hit the keys and played the demo song, performed factory resets, changed backup battery, ran self tests, all in various orders, nothing helped.

I found a scan of the maintenance manual online. While I do have a technical background, these computer generated PCBs with surface mounted components can be very hard to follow! However, I found what looks like a muting circuit that is active during power on, and should be deactived by the DA conversion chip after a while. One of the resistors in this circuit (R139) that is quite easy to access (next to the headphones jack) still seems to provide 2 (on power up) and later 2.8 volts to the pull-down transistor (Q14) next to it. According to the schematics that voltage is shared with all other pull-down transistors, which might explain to complete lack of audio output.

Does anyone know why the muting circuit might stay active?

Thanks,
Patrick

[AncientGeek]

I presume that you've checked all the supply voltages? As a start, I'd be inclined to look at the circuitry schematic at the bottom right of page 28 of the PDF service manual (assuming we are looking at the same download). In particular I'd suspect C117. It would be worth inspecting this electrolytic for leakage and possible damage to the adjacent tracks.