Peavey Dark Matter DM112: powers on but no sound, mute relay never clicks (C131)
Symptoms
The DM112 powers up normally (front LED on, controls respond), but there’s no sound from any input. The mute relay RL7 on the input board never clicks in after power-up. Normally you hear it a few seconds after switching on.
Diagnosis
Tools needed: multimeter (DC volts and diode mode), soldering iron with a fine tip (or hot air) plus flux and desoldering wick for the SMD capacitor, and a replacement 1 µF 50 V SMD electrolytic. An ESR or capacitance meter helps confirm the bad capacitor out of circuit, and an oscilloscope shows the ripple on Q7’s collector directly, but neither is essential. The DM112 is mains-powered, so use an isolation transformer or a current-limited supply when measuring with it switched on.
The DM112 keeps its output muted until an AC-detect circuit on the input board sees the transformer’s 18 VAC.
AC detect: the AC is rectified by D13/D14, divided down by R177/R185 and smoothed by C131 (1 µF). Through R4 and zener ZD1 it then turns on Q7.
Delay and relay: Q7 switches off Q8, which lets C133 (47 µF) charge slowly through R194 from +15 V. When C133 is high enough, zener ZD2 conducts and the Q9/Q10 Darlington pulls in the mute relay RL7, about 5–15 seconds after power-up.
On this unit the diodes, zeners and resistors all tested OK in circuit, and the +15 V supply was present. Powered measurements to ground:
Rectified AC (D13/D14 cathodes): 16 V
Divider output (C131 +): 7.8 V
Q7 collector: 0.19 V. Q7 is on, as far as a meter can tell.
C133 +: stuck around 4.9 V, never rising far enough for ZD2 to conduct
Relay driver (Q10 collector): about 5 V, so the relay is never energised
The clue: with C133 lifted, the relay buzzed loudly instead of clicking in. That showed the relay stage worked, and that the drive reaching it was pulsing at mains-ripple frequency rather than steady DC.
C131 had lost its capacitance. Without it, the divider output carries large ripple, so Q7 drops out at every trough of the waveform. Each time it does, Q8 briefly turns on and drains C133, so C133 never charges past the threshold and the relay never engages. A multimeter averages this out, so the readings look almost normal; an oscilloscope on Q7’s collector would show the pulses.
The fix
Replace C131, a 1 µF 50 V SMD aluminium electrolytic, with + to the divider/R4 side and the dark bar (−) to ground. After the repair, RL7 clicks in cleanly a few seconds after power-up, with no buzzing, and audio returns.
While you’re in there, consider replacing C133 (47 µF 25 V) too. It’s the same type of capacitor on a circuit that’s powered all the time.
See where the parts sit on the input board: https://circuit-fixers.com/boards/map/193-peavey-dark-matter-dm112-input-board?highlight=C131,C133,Q7,Q8,RL7
Parts
