1

MU524 Vari-Mu Tube Compressor / Re: Output makeup gain not functioning

« on: February 03, 2025, 02:38:09 PM »

Removed U1.
Confirmed back of P2 stamped 2K2 LOG C.
Solder joints at P2, R16, C10, C11 all look good.
Resistance between P2 outside pins measures 39Meg.
Turned CW, measurement between outside pins and center pin 2k4 / >10Meg
Turned CCW, measurement between outside pins and center in 0 / >4Meg.
Desoldered P2.
Measured outside pins and center pin. One side is reading open/infinite. Other side varies 0 to 2k4.
Bad P2?

2

MU524 Vari-Mu Tube Compressor / Output makeup gain not functioning

« on: February 03, 2025, 04:43:47 AM »

I am having trouble at step 6 of the setup.
Turning the output pot does not change the gain at all.
I am using the XT500 to power the MU524 for setup.
IN pot @ max
OUT pot @ min
REC pot @ max

All Push buttons @ UP
Jumper JMP1: removed
Tube side pins reading at 6.17V after startup. Tube glows.
+16 Pin reading at 15.44V.
-16 Pin reading at -15.43V.
B+ reading at 45.4V.
Meter lights up when bypass switch to IN
Turn bypass switch to center (off).
Sending 1VAC to Input to pins 8 and 10 on XT500.
Reading 0.97V at pins 2 and 4 on XT500.
Turn bypass switch to IN.
Output rises to 3.21VAC.
Turn up output potentiometer.
No change in output, remains at 3.21VAC.
--
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 = 29.9VDC, 1.18VAC
TP2 = 29.9VDC, 1.29VAC
TP3 = 1.58VAC
TP4 = 1.6VAC
Output (pins 2 and 4) = 3.2VAC
--
Changed conditions:
Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max (note that there was no change in signal when turning output pot)

TP3: 0.26 VAC
TP4: 0.26 VAC <-- appears that the trouble is around U1.2
TP5: 2.65 VAC <-- a bit lower than expected 2.9 VAC
TP6: -2.22VDC <-- also off from target -3.5VDC.
Output: 0.5VAC <-- very low output
Meter: shows -3dB
--
I have checked the components around U1.2 (R15, R16, R17, C10, C11).
Potentiometer values are correct. The input pot appears to work.
Since TP4 and TP6 are both near U1, I tried swapping two of the OPA2134s but got the same results.