1

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: June 23, 2025, 09:12:06 PM »

Salut JP!

In a last ditch effort, after some digging through the forum, I tried cleaning the valve pins and socket and it all now works and I was able to calibrate the unit within spec, other than the GR meter which I maxed the trimmer out on just under 6 db!

I'm a million miles ahead of where I was, but if you have any suggestions as to how to fix that last bit, I'll be forever grateful!

Merci!

Guy

2

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: June 22, 2025, 07:26:20 PM »

Hi JP. I've had a little more time to devote to this and am starting going through the setup again, before sending off to you.

It has now become clear that an issue must exist elsewhere, as when I reach step 6 > 6 (Check the output voltage has risen to 3-4 V AC) the output does NOT rise and remains at 1.1 V. If I turn the output pot all the way up, it reaches 13.44 V AC.

I have carefully been through the board to check all component values and orientations are correct. Is there an area you can suggest I should look?

Many thanks again.

3

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: May 28, 2025, 09:05:37 PM »

Hi JP,

Yes it's range is 0 - 1.3 k in circuit.

On step 7 I can only get down to .68 V with the trimmer at max travel


On step 8 adjusting Tr1 only goes down to 3.53 V at the maximum travel.
Turning TR1 Back to the max position as per instructions, At this point TP6 = -1.73 V if black probe is on 0 TP and U4 Pin 6 is -1.73 V if black probe in on 0 Tp.
Help!
Many thanks
Guy

4

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: May 19, 2025, 10:34:24 PM »

Hi JP,
Either I'm extremely unlucky or very stupid.
With the new Tube I was just about able to get the voltage down to 0 between tp 1 and 2 in step 7
BUT in Step 8, TR 1 doesn't seem to affect anything.
Similarly, adjusting TR5 further down, has no affect.

Could it be I've had two tubes each with one half bad? In any case should I send the unit in at this point to prevent you being further out of pocket?

I followed the rest of the instructions to the end, and whilst I wasn't able to calibrate the unit within spec, it works, in so far as it passes audio and compresses, albeit with an increased noise floor as you turn the attack - which would lead me to think that the tube halves are not balanced.

Any help gratefully received.

Many thanks.
Guy

5

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: May 07, 2025, 07:17:30 PM »

Thanks JP.

Unit is passing audio and output is up to 14.1V when out Is Maxed.

I've checked continuity between legs of tube and board to be sure, so I Think you're right!
I'll be in touch Via form/Email. Thanks again.


Guy

6

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« on: May 05, 2025, 06:52:23 PM »

Thanks for getting back so fast JP.

I've re-flowed all of those and am getting the same readings with U4 pulled out of circuit and still not able to adjust balance to 0.


TP1 = 34.8 VDC (black probe on 0V)
TP2 = 40.7 VDC (black probe on 0V)

TP1 = TP2 = 0.82 VAC
TP3 = TP4 = 0 VAC


Picture attached in case I've done something stupid.
Many thanks again

7

MU524 Vari-Mu Tube Compressor / MU 524 Stuck on Step 7

« on: May 03, 2025, 04:35:31 PM »

Hi there, I've just finished putting this together, and I can't get the balance to read 0 Volts between tp1 and tp2. It only goes down to 5.9 volts before the trim pot hits the end of its travel. I've stopped at this point before breaking anything!

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 = 34.8 VDC (black probe on 0V)
TP2 = 40.7 VDC (black probe on 0V)

TP1 = TP2 = 0.82 VAC
TP3 = TP4 = 0 VAC
Output: 1.12 VAC (between XLR pins 2&3)