1

MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on Today at 09:43:04 AM »

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.

2

CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by vwyf11 on April 16, 2026, 04:52:21 PM »

Ok thanks for clearing all that up!  Definitely gives me a better understanding

3

MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 16, 2026, 04:19:05 PM »

I have 1.22vac between TP1 and TP4 and 0.058vac between TP0v and TP4 with U4 removed

4

MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 16, 2026, 10:59:14 AM »

TP4 = .054vac
TP1 to TP3 all good except TP4?

Do you say that TP4 is back to 1.22VAC when U4 is removed? 

5

CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by JPK on April 16, 2026, 10:14:20 AM »

(+probe) on collector (-probe) on base gives (+27.93V)
It shows that Q7 is dead.

So measuring from cathode of d20 to tp3 should give the same measurement as tp2 to tp3.  Is this correct?
No. Measuring from cathode of d20 to tp2 should give the same measurement as tp3 to tp2.

When I measure from the anode of d20 to tp2 I do see voltage but not from the cathode of d20 to tp2.
It just shows that there is no current flowing through Q7. The voltage you see is the voltage drop in the Zener, normally a little above 4V.

6

MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 16, 2026, 06:42:36 AM »

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

7

EQP501 Passive Equalizer / Re: Signal gain increase when engaged

« Last post by jziemba001 on April 16, 2026, 01:01:30 AM »

Ok great, thanks for the info! If the level jump isn’t due to a build error or problem, I’ll see how it goes in operation and now I know how to proceed if I want to balance it out. 

I’ll check out the switch and indeed if it needs replacing change it out 

8

CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by vwyf11 on April 15, 2026, 07:01:55 PM »

Yeah sorry i understand the cathode of d20 is tp3. So measuring from cathode of d20 to tp3 should give the same measurement as tp2 to tp3.  Is this correct?

When I measure from the anode of d20 to tp2 I do see voltage but not from the cathode of d20 to tp2.  I’m sometimes confused by zener diodes in how they operate but this doesn’t seem correct.  As if d20 is blocking from Anode to cathode.  

Sorry, so (+probe) on collector (-probe) on base gives (+27.93V)

9

CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by JPK on April 15, 2026, 05:00:54 PM »

TP3 is at the cathode of D20, not the anode.

Collector -base 27.93V
What do you mean?
- Collector and base @ 27.93V or
- Collector to base is minus 27.93V

I know Q7 was cold where normally it’s pretty warm. 
 It confirms that is is dead or disconnected (check the solder joints).

Is this the correct replacement for Q7?
Yes, correct.

10

MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 15, 2026, 04:51:24 PM »

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.