Sound Skulptor forums
Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: Ben on March 26, 2026, 06:40:57 AM
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I am only getting 0.152v ac on the output when engaged. I believe the transformer is oriented correctly and solder joints appear clean. I am getting 0.14v ac @ U2 pin 4 and U2 has power. I hear the relay click when taking out of bypass. I also tried cleaning the tube pins. Please advise, Thank you
+15.6 v
-15.56v
B+ 45v
tube heater 6.15v and glowing
(https://i.imgur.com/1aFJtfQ.jpeg)
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Remove U4.
What values (DC and AC) do you get on TP1 to TP4 in the initial conditions here (https://www.soundskulptor.com/forum/index.php?topic=347.msg1907#msg1907).
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0.05vac and 29.69vdc @ TP1
0.077vac and .258vdc @ TP4
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I reheated all solder joints and rechecked for bridging or shorts, no change
Thanks,
Ben
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Can you please also report the values on TP2 and TP3?
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Sure can,
TP2= 0.297 VAC and 0.057 VDC
TP3= 0.077 VAC and 0 VDC
Pot 1, R1 and R2 test ok as well
Thanks,
Ben
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TP2= 0.057 VDC
Here is the issue. You should get around 30VDC.
You must check the solder joints on R7, on the tube socket, on the connector that links the tube PCB to the main PCB.
Check that the 48V gets to both sides of R7 (you can check without the tube inserted).
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Apologies, my DMM lead must have had a bad connection testing TP2. I actually have 29.79vdc and 0.145vac @ TP2. TP3 is the same
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With a 1VAC signal at the input, IN pot on 10, power off, how many millivolts AC do you get on the input transformer between pins 2 and 5?
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I get 0mvAC across T1 pins 2-5
I get 67mvAC across T1 pins 7-10
power off 1vAC on input.
T1
pin 2-4 37.1 ohms
pin 2-5 37.3 ohms
pin 7-9 99.9 ohms
pin 8-10 100.6 ohms
input pin 2-3 14.14k ohms (this looks off)
Thank you
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I get 67mvAC across T1 pins 7-10
It should be 650mV!
Check the value and solder joints of R1 and R2.
What resistor do your measure between the 2 left pins of R1 & R2? It should be 200 ohms, the transformer input DC resistance.
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Now I get 0vac at the output
i get 200 ohms between r1 and r2 on the legs closest to the face plate, after finding an issue with pin 10 on the T1 pcb.
R1=2.191k
R2=2.188k
I get 657mv across pins 7-10 of T1 and 269mv across pins 2-5 with power off and 1vac input
TP1 = 29.92vdc
TP2 = 30vdc
TP1 = 1.23vac
TP2 = 1.16vac
TP3 = 1.63vac
TP4 = .054vac
Input 4.57k ohms pin 2-3
29.97vdc and 1.22vac TP1 to TP4 with U4 removed
Thank you for your help!
Ben
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TP4 = .054vac
TP1 to TP3 all good except TP4?
Do you say that TP4 is back to 1.22VAC when U4 is removed?
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I have 1.22vac between TP1 and TP4 and 0.058vac between TP0v and TP4 with U4 removed
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So the issue is in the final gain stage around U1.2.
Check the value, direction and solder joints of U1, R14 to R16, R18, C9, C10, C11.
Do you get a signal on TP4 when the front PCB is completely removed, with power on?
Also try with U2 removed.
All voltages should be measured against 0V.
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I get no signal at TP4 with the front pcb removed, power on. With and without U2. Solder joints and component values seem to be good.
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I'm stumped, Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4
I still have 30vdc at TP1 and 2 as well.
Just to confirm, what value should I be getting on P1? the impedance stays at 0 when turning the pot on the right 2 pins when looking at the pot from the rear, It varies between 0 and 10k on the left 2, I get 10k on th outer 2 pins
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Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4
You can debug without the front panel PCB connected. This will help to narrow down the search area.
Just to confirm
Normal. 2 pins are connected together. 10kohms max between the 2 other pins.
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I still keep running into no signal at TP4 with and without the front panel and U2. I've verified every resistor and diode on the board. What should I try next?
Thanks,
Ben
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What are the DC voltages on each pin of U1, against 0V?
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I get:
1 0vdc
2 0vdc
3 0vdc
4 -15vdc
5 0v
6 0v
7 0.05vdc
8 15.37vdc
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I have 1.65vac at pin 5 of U1, voltage doesn't seem to get to pin 7, I have tested R15, R16, C9, C10, C11 multiple times and all appear good and the traces all seen ot have the correct continuity with the components. I've also swapped the op amps and get the same result.
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7 0.05vdc
This should be 0.00VDC.
Maybe something prevents the output from changing.
Check that there is not a short circuit between TP4 (output of U1.2) and 0V or an other point around J1-pin17-18, C12, C9, R15.
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I found it at last! there was a tiny solder bridge between pins 3 and 4 on U2. Thank you so much for your help and patience?
Cheers!
-Ben
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(https://fonts.gstatic.com/s/e/notoemoji/17.0/1f44d/72.png)