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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 19, 2024, 01:36:32 AM »

I reinstalled Q2 and thought I may have damaged the component because I wasn't getting any output in step 6 so I decided to check and seat connection points and then started getting signal from the output. It was still only reading 1.2VAC but as I was seating connections I pressed on the tube I noticed the voltage jumping up to the target range. I cleaned and scuffed the pins on the tube and squeezed the contacts in the tube socket to be tighter. After doing this the voltage was reading 4.5VAC at the output in step 6. I kept going through the steps and successfully got through all the calibration steps and have a working compressor. Thank you for your patience and guidance troubleshooting this project.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 16, 2024, 07:29:38 PM »

I will replace Q2 & retest it next chance I get, but in my initial testing & every time with all components installed I was getting the lighted meter when I took it out of bypass. Hopefully it all tests fine now.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 15, 2024, 11:35:44 AM »

Without Q2 in the circuit and the meter light working properly when inserted everything measures as it should until I reach step 6.
With the compressor inserted the voltage between pins 2 & 3 of the output is .710 VAC with the output pot at 0 when turned all the way up to 10 the voltage goes up to 8.20 VAC.  The 1K test tone was measured at .998VAC and then plugged into the input of the rack.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 12, 2024, 03:44:44 PM »

I tested this point in a previous step for continuity and there was but I went to retest it again this time through the pin on the back of J3b I wasn't getting anything. I took it off and tested J3a and got continuity then reinstalled J3b and was getting continuity as well. I tested the VDC there too it was .002 VDC. Also now the meter lights up.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 10, 2024, 08:09:12 PM »

With the compressor in I get 15.27 VDC between both ends of R12 and the 0V test point. If I switch it into bypass it goes to -15.39 VDC. The light was coming on when testing with all the components installed. I currently only have Q2 out of the circuit.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 09, 2024, 07:31:20 AM »

With Q2 Removed the DC voltages go from 26.27 with TR2 at it's highest to 9.07 at it's lowest. Also the light on the meter doesn't light up when taken out of bypass.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: January 04, 2024, 11:10:41 AM »

I removed Q2 & tested the transformer. This is still without U5

Without Q2 in the circuit
- Resistance between pin7 and pin10:  188.2 ohms
- Resistance between pin2 and pin4:  34.8 ohms
- Resistance between pin3 and pin5:  34.4 ohms

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 28, 2023, 03:46:20 PM »

I should have written it down, but I don't remember 100% if I took the measurements of Q4 and U5 with or without U4 in the circuit. I'm pretty sure I replaced U4 in the circuit when I tested Q4 and U5 out of the circuit. I think I also re-soldered Q4 before testing U5 out of the circuit.

I don't have a useable component for U5 at the moment which is TL431A I don't know if these are pretested before being added to a kit or difficult/rare to obtain. If it's necessary for further troubleshooting and doesn't require expertise or special equipment to test for specification I can try to get one here in the US to continue figuring this out. Either way please let me know how we should proceed and thanks again for your time. Sorry about all the extra posts. I kept getting a message I exceeded the maximum length of a post.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 28, 2023, 03:43:55 PM »

Without U5 in the circuit
- Resistance between pin7 and pin10:  193.4 ohms
- Resistance between pin2 and pin4:  35.8 ohms
- Resistance between pin3 and pin5:  35.3 ohms

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 28, 2023, 03:43:21 PM »

Without Q4 in the circuit
- Resistance between pin7 and pin10:  193.3 ohms
- Resistance between pin2 and pin4:  35.8 ohms
- Resistance between pin3 and pin5:  35.3 ohms

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 28, 2023, 03:42:28 PM »

Without U4 in the circuit
- Resistance between pin7 and pin10:  190.1 ohms
- Resistance between pin2 and pin4:  35.2 ohms
- Resistance between pin3 and pin5:  34.7 ohms

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 28, 2023, 03:38:03 PM »

This is not normal. Check that V+ comes to pin8 of U4 and pin2 (down-left) of J3a.
I checked continuity between V+ and pin 8 of U4 and pin 2 of J3a and there is continuity at those points.

One half of the tube is not conducting.
Check the tube connections.
I checked the continuity between every socket in the tube socket through the pins that are soldered to the main board. I got continuity through to the main circuit board and each top and bottom row of the pin connectors. There are 16 pin connectors and 9 points in the tube socket so one tube socket isn't connected to the main board (I assume this is by design), the top and bottom row of each pin connector are connected in my tube socket pcb. Meaning there are 2 rows of 4 on each side of the tube socket if rows are labeled a and b then pin 1a is connected to pin 1b and pin 2a connected to pin 2b and so on. I didn't find continuity between pin connections in any other configuration, ie pin 1a to pin 2a or pin 2b and so on.

Check the input transformer (in circuit):
- Resistance between pin7 and pin10: 286 ohms,
-Resistance between pin 7and pin10:  185.4 ohms

- Resistance between pin2 and pin4: 36 ohms,
- Resistance between pin2 and pin4:  35 ohms


- Resistance between pin3 and pin5: 36 ohms,
- Resistance between pin3 and pin5:  34.5 ohms
I tested the resistance with and without the tube and the measurements were the same.

Try removing Q5 and check again.
I think this may have been a typo I couldn't find a Q5. I did check with U4 removed and since it was over Christmas I wouldn't expect to get a quick response so I also tried removing Q4 and U5 from the circuit to retest the transformer. I was able to remove Q4 but I broke a leg off U5 while trying to desolder it from the circuit. The results of my measurements are as follows:

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 23, 2023, 10:25:09 AM »

Sorry for the slow reply I haven't had a lot of time to work on this project recently.


I measure 6.15 DCV between the side pins of tube holder.
OK but do you see the slow increase from 1 to 2VDC then to 6VDC?

Yes it slowly rises from about 1.6 DC to 6VDC


Once I take the compressor out of bypass & engage it I measure 1.258 VAC
Check that you didn't reverse the input transformer: label facing to the rear.

Yes the label is facing the rear of the module with the torx screws facing up.



Here I can only adjust tr2 (bal1) to a minimum of 8.9 & a max of 25.72.
Are these VDC?

Yes it's goes from 8.87minimum to 25.72maximum VDC when I turn TR2


Check the components value and solder joints of TR1, TR2, R3 to R10, Q1, Q2 and tube.

I double checked the components, TR1 which is marked 202 but only goes to 1.3K max TR2 is marked 102 and measures 1.0K max
I checked every resistor before assembling anything for their actual value and they all measured within spec of what they were marked. I checked them in the circuit & their markings are what they are supposed to be from R3 to R10. When I checked their value with my DMM in the circuit they all measured what they are supposed to be except for R3 to R5. R3 & R4 are both 1.999K instead of 3.3K. R5 measured 1.41K instead of 10K. I checked the markings on Q1 & Q2 and both are MPS A29. I don't know how to check the value of Q1 & Q2 with my DMM in circuit. I also don't have a tube tester but the tube is marked as ECC 189 as specified. I checked all solder joints and for good measure resoldered them on these components. I got the same results as before.


What are the DC voltages on TP1 and TP2 against 0V?

Between 0V and TP1 I get 36.75 VDC, between 0V and TP2 I get 45.6 VDC. 


Check again after removing U4.

After removing U4 I get the same results for 0V between TP1 & TP2, but I noticed the light for the VU meter doesn't light up with U4 removed

Hopefully this info sheds some light on the problem.
Thanks for your time.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 19, 2023, 08:29:07 AM »

The pots were the first things I checked & looks to be correct with 10k in P1, 2K2 in P2 & 2MB in P3. Maybe I haven’t described my measurements well so I’ve redone them. 

Step 4: with the everything set as described in step 3 I measure 6.15 DCV between the side pins of tube holder. 

Step 5: with the black probe on 0V & red probe on +V I measure 15.40V. 
With the red probe on V- I measure -15.31V. 
With the red probe on B+ I measure 46.08V. 

Step 6: in this step I am connecting my probes between pins 2 & 3 of the output of my DAW & measure a 1K tone at .998VAC. I plug that into the input of my api rack & measure pins 2 & 3 on the output xlr as .998 while in bypass. 
Once I take the compressor out of bypass & engage it I measure 1.258 VAC when the out pot is at 0. I measure 14.66VAC when the out pot is at its 10. 

Step 7: in this step I connect the black probe to tp1 & the red probe to tp2 with my dmm set to dc. Here I can only adjust tr2 (bal1) to a minimum of 8.9 & a max of 25.72. I stop doing measurements here. I hope this makes more sense now.

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MU524 Vari-Mu Tube Compressor / Re: My Problems begin at step 6 of setup

« on: December 17, 2023, 02:49:02 PM »

I’m also having a problem at step 6. I’m in the process of going through the components to check for errors but thought maybe I could ask here to narrow it down. I get 1.7779V when I take the unit out of bypass & the max of my output pot reads 19V. I noticed that 1.7 is half of what it should be & 19 is about double what it should be. Seems like maybe I got something reversed or mixed up.
Also for TR1 the BOM calls for a 2k trim pot but says the marking should say 203. My trim pot marking is 202. I had assumed 203 was a typo because 202 is the correct marking for a 2k trim pot.
I tried to go on through the calibration but I stopped at TR2 because the lowest value I could get was about 8V before turning the pot stopped changing the value. Your help is much appreciated.

Thanks