1

MU524 Vari-Mu Tube Compressor / Re: Initial AC voltage jump

« Last post by CGMix on January 05, 2026, 06:27:18 PM »

Well damn, that was it. Transformer was upside down…

2

MU524 Vari-Mu Tube Compressor / Re: Initial AC voltage jump

« Last post by JPK on January 05, 2026, 10:06:16 AM »

Is the label on the transformer facing towards the back of the module?
Check the input transformer coils resistance (in circuit with potentiometer IN on 10):
pin2-pin4 : 37ohms
pin3-pin5 : 37ohms
pin7-pin9 : 98ohms
pin8-pin10: 98ohms
pin7-pin10: 194ohms

3

MU524 Vari-Mu Tube Compressor / Re: MU524 is noisy

« Last post by JPK on January 05, 2026, 09:46:42 AM »

Do you hear any crackling noise with the tube removed?
If not, I would try to replace the tube.

4

MU524 Vari-Mu Tube Compressor / Re: Initial AC voltage jump

« Last post by CGMix on January 03, 2026, 04:29:42 PM »

for reference

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MU524 Vari-Mu Tube Compressor / Re: Initial AC voltage jump

« Last post by CGMix on January 03, 2026, 03:47:31 PM »

JP! Thanks for the reply. At TP3/4 I’m getting 3.9vac. On my other working compressor it sits at 1.8, I believe. I pulled the transformer last night, thinking I may have pierced R1, but I had not. Also, I’ll have to check again, but I think I was measuring about half a volt on each grid of the tube. And then it jumps up to about the 4V pretty quickly thereafter. I believe the Transformers is correct because it’s installed in the same manner as my other

6

MU524 Vari-Mu Tube Compressor / MU524 is noisy

« Last post by OttoWei on January 02, 2026, 11:13:52 PM »

Hi,

I have had this issue since I built the compressor, but I set it aside for a while. I’ve now taken the time to do a proper fault-finding session and would appreciate your guidance.

On startup the compressor is quiet, but after a few minutes it begins to crackle and gradually becomes more and more noisy. The crackling noise is only affected by the OUT pot. None of the switches affect it at all, and changing JMP1 has no effect either.

Here is what I have checked and observed so far:

• I have tried the unit in different lunchboxes.
• I have gone through the setup procedure again and all values seem correct, except for the following:
  
  
  • In step 6 (“Check that output voltage has risen to 3–4 VAC”), I measure 2.6 VAC with the OUT pot set fully counter-clockwise.
  • TR5 (ATN2) does not produce a clicking sound at the end position, although the trimmer itself functions correctly (step .
• I have re-soldered most of the joints at this point.
• Tapping on components has not revealed any intermittents or changes in the noise.

Please let me know if there are any specific areas or measurements you would recommend checking next.

Best regards,
Otto

7

LA502 Optical Compressor / Re: Hissing LA502

« Last post by JPK on December 31, 2025, 09:52:50 AM »

Very common components:
https://www.conrad.fr/fr/p/yageo-mf0207f22kh-mf0207fte52-22k-resistance-a-couche-metallique-22-k-sortie-axiale-0207-0-6-w-1-1-pc-s-1417591.html
https://www.conrad.fr/fr/p/tru-components-tc-k47pf5-condensateur-ceramique-tht-47-pf-100-v-5-1-pc-s-1589408.html

8

MU524 Vari-Mu Tube Compressor / Re: Initial AC voltage jump

« Last post by JPK on December 31, 2025, 09:35:25 AM »

Check that you have not reversed transformer T1. The label must be facing the back of the module.
What are the AC levels on TP3 and TP4, relative to 0V (1.75V)?

9

LA502 Optical Compressor / Re: Hissing LA502

« Last post by igor.moreno on December 28, 2025, 04:37:34 PM »

Hi JPK,

Do you have the components in stock to do the mod in the LA502 ? (47pF capacitor and 22kOhm resistor)
If it's possible, i'll be glad to order them.

Thanks,

Igor

10

MU524 Vari-Mu Tube Compressor / Initial AC voltage jump

« Last post by CGMix on December 27, 2025, 04:43:53 PM »

After sending a 1V, 1K tone of not the compressor, my voltage jumps to a little over 7V instead of 3-4V. This does affect the rest of the calibration. Where should I look? All other voltages look to be where they should.