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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

92

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

93

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)?

94

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

95

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. 

96

CP5176 FET compressor / Re: CP5176 only compresses in 20:1 / SLAM mode

« Last post by JPK on December 15, 2025, 09:44:20 AM »

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EQP501 Passive Equalizer / Re: Pot issue

« Last post by JPK on December 15, 2025, 09:43:22 AM »

Does the problem persist when the front panel is removed? If not, it would mean that the rotary switch is not perfectly flat on the circuit board, which could be the cause of the problem.
Otherwise, maybe a damaged rotary switch? Contact us for a replacement.

98

CP5176 FET compressor / Re: CP5176 only compresses in 20:1 / SLAM mode

« Last post by Ptb on December 14, 2025, 07:12:20 PM »

Hi, and thank you for your help. I went through all the solder joints and cleaned up some of them. Everything is now working perfectly

99

EQP501 Passive Equalizer / Re: Pot issue

« Last post by Horicios on December 14, 2025, 11:46:52 AM »

Thanks, I understand, but that's not the problem. It's hard to explain...
When I move the AT pot, the kHz switch doesn't move from its position (20 kHz). It simply goes into bypass while at 20 kHz. It also does this at 15 kHz, but less frequently.
  It seems that when I move the AT pot, something shorts out or loses contact, on the switch causing it to go into bypass.
I could send you a short video

100

EQP501 Passive Equalizer / Re: Pot issue

« Last post by JPK on December 14, 2025, 09:00:53 AM »

Check that you haven't overtightened the screw on the red knob. This could cause the potentiometer adjustment to also rotate the rotary switch.