Sound Skulptor forums
Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: CGMix on October 18, 2023, 02:40:19 PM
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Ok I'll try to keep it short, but I do want to include some other findings. I went through the troubleshooting guide before I posted just so some of the questions don't get redundant. But like the title says, my problems begin at step six. When I turn the unit on, I don't get the jump to 3-4v. It stays at 1v. I did go ahead and move forward, just to see what other numbers I got. I was able to get most of the voltages throughout the setup guide. If I remember correctly (it got late and my memory is foggy) I got to step 10 and was unable to get to the 3.6v. I ran out of turns on the TR4. I think one more voltage was off but I can't remember at the moment. Anyway, I got all the way to the end, and the input and output sections work just fine. I get gain up and down from both knobs, just no compression when turning up the input section, only gain. Where should I begin to look?
Edit:
Here are my test numbers
TP1 = TP2 41 VDC
TP1 = TP2 UNSTABLE
TP3 = TP4 0.51 VAC
OUTPUT = 1.032
TP3 0.23 VAC
TP4 2.86 VAC
TP5 2.61 VAC
TP6 -3.1VDC
METER -6db
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TP1 = TP2 UNSTABLE
This is for VAC, right?
Remove U4 and check again step 6. Does it change?
Remove the front panel and measure the AC voltage on TP3 and TP4. It should be 1.8VAC.
Is the input transformer T1 in the correct direction, label facing to the back?
Did you check this : Make sure that capacitor C12 do not touch the transformer daughterboard pins. It is very close and if the pins can pierce the insulation the circuit will oscillate.
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Ok, I ran the tests you mentioned, and also double checked the transformer direction. The transformer was good, and I did not get any change in voltage after removing U4, and did not show any voltage across T3 and T4. That was with the front panel removed. I went ahead and ordered another C12 because I do have a pin that is very close to the capacitor. I believe the pin touches the capacitor so there's a good chance it could be damaged.
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After removing C12 and replacing it, I still get the same results. I did want to mention though, that once I dial in my 1k tone at 1.0VAC, and I switch the compressor on, it does increase the voltage slightly. Instead of a 3-4v increase, I get a .003-.004v increase. Since it's off by 1000, do I have a component value off somewhere?
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Check the value of these components, by colour code or marking:
Pot P1 = 10kB
Pot P2 = 2k2C
trimmer TR1 = 2k
trimmer TR2 = 1k
R5 = 10k
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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
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Seems like maybe I got something reversed or mixed up.
Check that you didn't swap the potentiometers.
Also for TR1 the BOM calls for a 2k trim pot but says the marking should say 203
202 is the correct marking. The current partslist (Last modified : 15/02/2023) is correct.
I stopped at TR2 because the lowest value I could get was about 8V
This is not compatible with your previous TP1 = TP2 41 VDC. Which one is right?
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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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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?
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.
Here I can only adjust tr2 (bal1) to a minimum of 8.9 & a max of 25.72.
Are these VDC? If true, this is a huge imbalance. Check the components value and solder joints of TR1, TR2, R3 to R10, Q1, Q2 and tube.
What are the DC voltages on TP1 and TP2 against 0V?
Check again after removing U4.
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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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but I noticed the light for the VU meter doesn't light up with U4 removed
This is not normal. Check that V+ comes to pin8 of U4 and pin2 (down-left) of J3a.
Between 0V and TP1 I get 36.75 VDC, between 0V and TP2 I get 45.6 VDC.
One half of the tube is not conducting.
Check the tube connections.
Check the input transformer (in circuit):
- Resistance between pin7 and pin10: 186 ohms (sorry, not 286),
- Resistance between pin2 and pin4: 36 ohms,
- Resistance between pin3 and pin5: 36 ohms,
Try removing Q2 (not Q5) and check again.
If it is the same then you may have a defective tube. Contact us.
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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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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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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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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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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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Sorry typos, the resistance between pin7 and pin10 of the input transformer should be around 186 ohms.
And the transistor to remove was Q2, not Q5.
The TL431A is easy to find and cheap.
You can continue testing with U4 removed. This disconnects the side chain from the audio path and your issue is in the audio path. You need to be able to balance the DC volages on TP1 and TP2 (get the same DC value by acting on TR2) .
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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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The transformer looks good.
Are the DC voltages on TP1 and TP2 (against 0V) still very different from each other with Q2 removed?
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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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Too many issues, maybe connected.
Let's try with the meter light issue:
What are the DC voltages (against 0V) on each side of R12 with the compressor IN?
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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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Check if V+ comes to J3a pin2
Top left pin (square pad) is pin1, bottom left is pin2.
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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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Now, can you pass step 6 of the setup guide?
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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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Now you should test with Q2 in place.
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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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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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