Example service report

Diagnosis:
Runaway capstan. No ears or feet. Has been opened before, transport is not tightly screwed in place. Missing cable tie on the speed control board bundle. Digital display is dark. Case seems a bit warped.

Capstan board:
Replaced ceramic C2, electrolytical C5, C7, C9, C13, C15, and 9.1V Zener. Still same symptoms. Investigation turned out corroded ground trace next to pin 1 of IC1. Created a solder bridge.
Runaway resolved, but speed is uneven with “hiccups”, at a rate of about 5 per second.
Resolved by replacing C16 with a 1uF ceramic.
Lifted and re-soldered IC1 to reduce chance of further corrosion around pads.
Renewed solder joints on R12, C11, C10, R11, C6, R4 and R5.
Washed off flux residue.
Capstan and reel motors are tight.
Checked tape path with a mirror cassette, confirmed alignment of R/P and erase heads.
Calibrated speed in fixed, variable and external modes using calibration tape.
Removed layer of hardened rubber from pinch roller with 1000-grit sandpaper.
Cleaned tape path with isopropyl alcohol.
Power supply board:
Dark display due to two line cracks in the power supply PCB. Resolved by doubling the traces with PTFE wire. Further cracking is possible.
Checking and calibration:
Verified function with a remote control and footswitch.
Adjusted meters to 0dB/315mV input at 1kHz
Adjusted dbx timing
Adjusted playback level to +3dB at 200nWb/m
Adjusted playback equalizer
Adjusted bias to level frequency response within 2dB
Adjusted recording levels at 0dB/1kHz with DBX out
Adjusted recording levels at 0dB/1kHz with DBX in
Adjusted crosstalk to minimum
Adjusted Sync level