Check the usual suspects:- Forgotten solder joint,
- Bad solder joint (reflow suspicious joints),
- Reversed components (electrolytics, diodes, transistors, IC's). In case of IC's, probably destroyed, better replace,
- Flipped IC's or transistors,
- Trim all the component leads and test pins as short as possible. Nothing must touch the chassis plate.
- Make sure that resistors R1, R3 do not touch the transformer daughterboard pins.
- Make sure that capacitor C12 do not touch the transformer daughterboard pins. It is very close and if the pins can pierce the insulation the circuit will oscillate.
Voltages checks: +/-20%Initial conditions:IN pot @ max
OUT pot @ min
REC pot @ max
Bypass toggle @IN
All Push buttons @UP
Input signal: Sinus 1kHz, 1VAC
Jumper JMP1: removed
Results:TP1 = TP2 = 30VDC (black probe on 0V)
TP1 = TP2 = 1.25VAC
TP3 = TP4 = 1.8VAC
Output: 3.5VAC (between XLR pins 2&3)
Changed conditions:Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max
Results:TP3: 0.26VAC
(black probe on 0V)TP4: 3.1VAC
TP5: 2.9VAC
TP6: -3.5VDC
Output: 6.1VAC (between XLR pins 2&3)
Meter: -6dB
