1

MU524 Vari-Mu Tube Compressor / Re: Cant find problem in my build

« on: April 27, 2024, 11:56:40 AM »

The unrealistic output level of 22.67VAC could be caused by the DMM.

*I don't think that DMM is a problem at this time.

* In changed conditions meter shows -6dB

Have you tried to listen to the output. How does it sounds?

* I think compressor is not working properly. Here are links to the two videos i made. One is with voice and one is with sine wave.
https://failiem.lv/u/ymfec8jdsq

https://failiem.lv/u/kt74mcexdq

2

MU524 Vari-Mu Tube Compressor / Re: Cant find problem in my build

« on: April 23, 2024, 08:05:37 PM »

Yes, sine wave value is 1VAC when compressor is connected. P2= 2.505kohm (when U1 and U2 are removed)
R16=219.7ohm

3

MU524 Vari-Mu Tube Compressor / Re: Cant find problem in my build

« on: April 23, 2024, 08:25:14 AM »

Thanks,

Turns out my multimeter is not good enough to determine AC voltage when there is DC present at the same time. Got better multimeter and here are results:

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 to 0V = 32,23VDC  
TP2 to 0V = 30,1VDC
TP1 to 0V = 1.12VAC
TP2 to 0V = 1.38VAC
TP3 to 0V = 1.71VAC
TP4 to 0V = 1.71VAC

 
Output =3,41AC when pot P2 is at min (This one looks good now) 
and  22.67VAC when pot P2 is at max. 

Changed conditions:
Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max

Results:
TP3 to 0V = 0.26VAC 

TP4 to 0V = 3.2VAC
TP5 to 0V =  2.9VAC                           
TP6 to 0V = -3.51VDC            

Output: 6.39VAC

4

MU524 Vari-Mu Tube Compressor / Re: Cant find problem in my build

« on: April 20, 2024, 11:31:59 AM »

Sorry indeed needed to change battery.
So here are testing results:

V+ = 15.44V
V- = 15.31V
B+ = 46.3V
Tube = 6.13VDC

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 to 0V = 31.0VDC 
TP2 to 0V = 31.0VDC
TP1 to 0V = 67.3VAC
TP2 to 0V = 67.3VAC
TP3 to 0V = 2.1VAC
TP4 to 0V = 2.1VAC
TP1 to TP2 = 3.1VAC 
TP3 to TP4 = 0 VAC
TP3 to 0V = 0VDC
TP4 to 0V = 0VDC

Output = 4.7VAC when pot P2 is at min
and  25.7VAC when pot P2 is at max.



Without front PCB
TP1 to 0V= 31.0VDC
TP2 to 0V=31.0VDC
TP1 to 0V = 67.3VAC
TP2 to 0V = 67.3VAC
TP3 to 0V = 3.1VAC
TP4 to 0V = 3.1VAC
TP1 to TP2 = 4.7VAC
TP3 toTP4 = 0VAC



Changed conditions:
Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max

Results:
TP3 to 0V = 0VAC
TP4 to 0V = 4.8VAC
TP5 to 0V = 4.3VAC                         
TP6 to 0V = -6.05VDC                     
Output: 10.2VAC

5

MU524 Vari-Mu Tube Compressor / Cant find problem in my build

« on: April 19, 2024, 12:44:47 PM »

Please help:


V+ = 15.3V
V- = 15.2V
B+ = 46.3V
 
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 to 0V = 30,9VDC  
TP2 to 0V = 31.0VDC
TP1 to TP2 = 3.0VAC
TP3 to TP4 = 0 VAC    
TP3 to 0V = 0VDC
TP4 to 0V = 0VDC

Output = 4.6VAC when pot P2 is at min and  25.6VAC when pot P2 is at max.

Without front PCB
TP1 to 0V= 30.9VDC
TP2 to 0V=30.9VDC
TP1 to TP2 = 3.1VAC
TP3 toTP4 = 0VAC
 
Without IC's and front PCB 
TP1 toTP2  =4,6VAC
TP3 to TP4 =0 VAC
TP3 to 0V =3.1VAC
 
Changed conditions:
Jumper JMP1: on 1-2
Input signal: Sinus 1kHz, 0.3VAC
OUT pot @ max

Results:
TP3: 0.VAC (black probe on 0V)
TP4: 4.8VAC
TP5: 4.3VAC                            
TP6: -6.03VDC              
Output: 10.1VAC

The usual suspects look OK.
The input transformer label is facing to the back and the Torx screws are on the correct spot upwards. 
Values on  R1, R2 and P1 ,P2, P3 are correct. R1, R2, are not touching transformer pins. C12 doesn't touch the tube or transformer.
When I remove the input signal, the voltage drops to 0