21

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.

22

EQP501 Passive Equalizer / Re: Signal gain increase when engaged

« Last post by JPK on April 15, 2026, 04:36:20 PM »

Is this less < 1 dBu increase in level I’m measuring at 1 kHz when engaging the unit considered normal behavior?  Or should I investigate anything?
The increase is caused by the tolerance on the potentiometers value. All your measures are within specs.
If you need an absolute no gain change, you need to temporarily replace R13 (15k) by a potentiometer, adjust the pot for 0 gain change, measure the pot resistance and replace it by a fixed resistor of equal value.

I do hear a small tick when it passes through the “off" positions of the KCS switch.
Normal. It is caused by the relay switching with an audio signal not null.

if I put pressure on the switch towards the right, the unit disengaged intermittently.
Did you solder the rotary switch absolutely flat on the PCB? Normally, this should not happen because the potentiometer shaft inside the hollow switch prevents it from bending.
If it happens with very little pressure, it will be necessary to replace the switch.

23

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

« Last post by vwyf11 on April 15, 2026, 05:13:18 AM »

Q7 collector - emitter 28.05V
Collector -base 27.93V

I was looking at where the test points are for tp2 and tp3 and tried testing from tp2 to the anode of d20 and I do get around 3V at that point.  Testing in this way still gives no change in voltage when turning tr2.  If I test from tp2 to the cathode of d20 I get 0V.

24

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

« Last post by vwyf11 on April 14, 2026, 07:37:21 PM »

I’m gonna double check tonight.  I know Q7 was cold where normally it’s pretty warm.  

Is this the correct replacement for Q7?

https://www.mouser.com/ProductDetail/863-2N6488G

25

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

« Last post by JPK on April 14, 2026, 05:14:09 PM »

E: 28.1V
F:0.001V

This can only happen if Q7 is dead or disconnected.
Can you measure the DC voltages directly on the pins of Q7?

26

CP5176 FET compressor / Re: 2nd CP 5176 Failed at TP3 Troubleshooting

« Last post by RFR on April 13, 2026, 11:59:37 PM »

Hi, topic is resolved.

After checking and checking and not finding any issues, I decided to go through the set-up procedure again.
I'm not sure what I did wrong on my first try but there is something in the setup-guide what leads to do interprate to do something wrong.

I believe it's the route cause is the P5 initial position during set-up at step 6.

Anyway, there was nothing wrong
TP3 and JMP1 values match now.

27

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

« Last post by vwyf11 on April 13, 2026, 08:30:35 PM »

As a last ditch effort I would guess, but would like to avoid sending it overseas if it’s possible.  

The voltage at the transformer was always the correct 24 volt reading before, now suddenly it’s 28.  I was atleast getting voltage between tp2 and tp3 now I’m suddenly seeing nothing.  Seems pretty drastic and like there should be something blatantly obvious.  I’ve looked at the schematic but it’s a little over my head with the amount of paths.  Is there a good straight shot to test voltage where the issue may be originating?  I obviously know I’m getting the correct voltage to the unit but having a tough time pinpointing the dc voltage path order.

28

CP5176 FET compressor / Re: 2nd CP 5176 Failed at TP3 Troubleshooting

« Last post by RFR on April 13, 2026, 07:57:28 PM »

Adding measuments from Trouble Shooting Guide

2.) 
V+: +15,9  VDC
V-: -15,93 VDC    
L1:  55    mVDC

No Signal
A: 12,8   VDC
B:  0,587 VDC
C: 13,41  VDC
D: -0,135 VDC
E: 15,14  VDC
F:  0,784 VDC
G: -15,16 VDC

3.) 
TP1: 538   mVAC
TP2:  7,26 VAC
L1:  10,9  VAC 1:2-1:20
     13,3  VAC @Slam

4.) 
TP3
1:2  = 0,178 VAC
1:4  = 0,353 VAC
1:8  = 0,570 VAC
1:12 = 0,950 VAC
1:20  = 1,62  VAC
Slam  = 7,18  VAC

TP4
1:2  =   0   VDC
1:4  =  -0,5 VDC
1:8  = -1,34 VDC
1:12 = -2,46 VDC
1:20 = -5,50 VDC
Slam = -0,64 VDC

JMP1 Pin 1
1:2  =  0,073 VDC
1:4  = -0,010 VDC
1:8  = -0,012 VDC
1:12 = -0,015 VDC
1:20 = -0,018 VDC
Slam =  8,860 VDC


TP6

-3,61 VDC

TP5
0,32 VDC

29

EQP501 Passive Equalizer / Re: Signal gain increase when engaged

« Last post by jziemba001 on April 13, 2026, 07:25:03 PM »

Hi,

I’m having a similar result, though maybe not as extreme.

When I send the unit a 0dBu (.775V) 1 kHz signal, I get the following reading from the output pins:
Bypass: .769V
Engaged: .852V

So if my math is correct, that’s roughly a ~ .89 dBu increase when the unit is engaged.

Following this thread, I’ve checked the following:

R5, R6, R7, R8, R12, R13, R25 all correct
R10, R24 correct

3V 1kHz signal
TP1: 2VAC (1.963)
TP3: 0.17VAC (.168)
TP2: 1.7VAC (1.731)

With the module unplugged, all pots at 0, 

- what resistance do you measure between TP1 and TP3 (?10k)? 
(9.87R)
- what resistance do you measure between TP1 and 0V (?11k)? 
(10.82K)
- what resistance do you measure between TP3 and 0V (?1k)? 
(.943K)

P3:
Can you measure it, between the 2 side pins, in circuit, after disconnecting the front PCB from the main PCB and with Low frequency ATTEN pot on 10 ? (9.87R)

I also checked all solder joints on all the potentiometers and they look good.

Is this less < 1 dBu increase in level I’m measuring at 1 kHz when engaging the unit considered normal behavior?  Or should I investigate anything?

Otherwise all functions seem to be operating normally, both with test audio signal generation (sine and white noise) and also musical material passing through it…and the unit sounds quite fantastic!

The small level bump is barely noticeable if at all when engaging and disengaging the unit with music playing through it.  Though I do here a small tick when it passes through the “off positions of the KCS switch, but I think this is normal as it is with most 500 series units since it uses an active relay?  The clicks are not present when switching without audio passing through.

I can send mdat files or screenshots of sweep measurements I took in REW with the unit in bypass and also engaged with all knobs set fully counter-clockwise if that's helpful.

Please let me know what you think!

Cheers,
Joe

30

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

« Last post by Ben on April 13, 2026, 06:58:31 PM »

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