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

« Last post by JPK on April 28, 2026, 09:07:39 AM »

7  0.05vdc
This should be 0.00VDC.
Maybe something prevents the output from changing.
Check that there is not a short circuit between TP4 (output of U1.2) and 0V or an other point around J1-pin17-18, C12, C9, R15.

72

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

« Last post by Ben on April 28, 2026, 12:33:29 AM »

I have 1.65vac at pin 5 of U1, voltage doesn't seem to get to pin 7, I have tested R15, R16, C9, C10, C11 multiple times and all appear good and the traces all seen ot have the correct continuity with the components.  I've also swapped the op amps and get the same result.

73

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

« Last post by Ben on April 27, 2026, 08:27:50 PM »

I get:
1  0vdc
2  0vdc
3  0vdc
4  -15vdc
5  0v
6  0v
7  0.05vdc
8  15.37vdc

74

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

« Last post by JPK on April 26, 2026, 08:57:29 AM »

What are the DC voltages on each pin of U1, against 0V?

75

SK49 Multi-patern Tube Microphone / Re: How long is the 7-pin XLR cable that comes with the SK49 kit?

« Last post by Alex Harmer on April 25, 2026, 08:59:29 PM »

I just made the test with five 6m cables connected together for a total of 30m.
The voltage accross R13 was still adequate at 3.61V and this include 4 additionnal male/female XLR junction.
So I think you are OK with the standard PSU and a 25m cable as long as it is a 7 wires good quality cable.

Thank you so much, that is incredibly helpful!

76

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

« Last post by Ben on April 25, 2026, 07:21:16 PM »

I still keep running into no signal at TP4 with and without the front panel and U2. I've verified every resistor and diode on the board. What should I try next? 
Thanks, 
Ben

77

SK49 Multi-patern Tube Microphone / Re: How long is the 7-pin XLR cable that comes with the SK49 kit?

« Last post by JPK on April 23, 2026, 11:55:51 AM »

I just made the test with five 6m cables connected together for a total of 30m.
The voltage accross R13 was still adequate at 3.61V and this include 4 additionnal male/female XLR junction.
So I think you are OK with the standard PSU and a 25m cable as long as it is a 7 wires good quality cable.

78

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

« Last post by JPK on April 23, 2026, 11:28:44 AM »

It should be cold to the touch.

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

« Last post by egelmett on April 21, 2026, 06:59:11 PM »

Check if any component is overheating, U2 in particular.

How does one know if U2 is overheating? Is this estimated by touch, or measured in some manner?

OttoWei's discription, and troubleshooting is the exact same issue as mine...I had to double-check it wasn't me who wrote the initial post! 

Ed

80

SK49 Multi-patern Tube Microphone / Re: How long is the 7-pin XLR cable that comes with the SK49 kit?

« Last post by Alex Harmer on April 21, 2026, 12:50:59 PM »

I think the best method would be to check the voltage across R13 in the PSU.
You want around 140mA in the tube heater so the voltage should be about 0.14 * 27 = 3.78V.
If the current falls under 133mA (3.6V accross R13) you can lower the value of R13 to increase the current.

Hi JPK,

I'm resurrecting this topic about the 25-metre long 7-pin cable for orchestral recording. 

Would you consider selling me two new power supplies with a specific value in R7 that would be appropriate for the voltage drop across this length of cable? 

The microphones are amazing - and rather than risk damaging the power supplies with some modifications that I don't understand, I'd prefer to have two dedicated power supplies for when I am using longer cables. 

What do you think? The microphones are amazing, and I'd like to use them for this task of longer cable runs. 

Cheers from New Zealand.