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Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: JPK on February 20, 2023, 11:55:28 AM

Title: In case your MU524 doesn't work as expected
Post by: JPK on February 20, 2023, 11:55:28 AM

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