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Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: OttoWei on January 02, 2026, 11:13:52 PM

Title: MU524 is noisy
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 8).
• 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

Title: Re: MU524 is noisy
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.