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Build help and troubleshooting => MU524 Vari-Mu Tube Compressor => Topic started by: soundguy937 on August 18, 2025, 02:42:02 AM

Title: Stuck on step 7
Post by: soundguy937 on August 18, 2025, 02:42:02 AM
Got to step 7 with my latest kit and the voltage between tp1 & tp2 keeps jumping up and down by hundredths of vdc. I did let the tube warm for at least 15 mins before testing. What would cause the voltage to flutter? Is it possible it is the tube?

Thanks

P.S. I got the U6 part for the stereo right kit today, Thanks a million
Title: Re: Stuck on step 7
Post by: soundguy937 on August 18, 2025, 03:39:35 AM
UPDATE: my DMM keeps fluttering between 1-4vdc in small to large increments between tp1 & tp2. All the other prior tests seems to be working correctly. 
Title: Re: Stuck on step 7
Post by: JPK on August 20, 2025, 03:40:35 PM
Try without any input signal.
Was the DMM stable when you measured the V+ voltage?
Remove U4 and measure the voltage on TP1 and TP2 against 0V.
Is the voltage accross TP1 and TP2 more stable?
Title: Re: Stuck on step 7
Post by: soundguy937 on August 23, 2025, 01:14:14 AM
looks like I am stable at the v+ although I am now noticing its reading negative instead of positive 15.6vdc, V- reads 15.5vdc positive vs. the setup saying it should be negative.

V+= -15.6vdc
V-= 25.5vdc

with U4 removed
TP1 to 0V= 32.3vdc
TP2 to 0V= 28.4vdc
TP1 to TP2= maybe slightly more stable, but still jumps around from a few hundredths to a couple tenth of a percent of voltage +-
Title: Re: Stuck on step 7
Post by: JPK on August 23, 2025, 04:39:11 PM
V+= -15.6vdc
V-= 25.5vdc
Do you put the black probe on 0V?
Check your DMM batteries and check your lunchbox voltages.
Voltage variations between TP1 and TP2 are normal. They should stabilize once thermal equilibrium is reached.
Title: Re: Stuck on step 7
Post by: soundguy937 on August 23, 2025, 06:39:50 PM
okay now with black probe on 0V I'm reading :

V1= 15.68vdc
V2= -15.65vdc

all my tests between TP1 & TP2 have been after at least 15-25min of warming up.

TP1 & TP2 after 30min and U4 still removed varies between +-1 to 2 hundredths of a VDC. 

With U4 replaced shortly after shut down of last test and waiting another 30min I am seeing my DMM with fresh batteries jump around again not stabilizing between 0.00 to +-0.05 VDC. Unfortunately it takes twice as long to mostly stabilize than what I seen in my first kit and the setup recommendation waiting time. Is this to be expected?
Title: Re: Stuck on step 7
Post by: JPK on August 24, 2025, 09:18:48 AM
Can you change the tube with the one from the other unit and check if it stabilizes better?
Title: Re: Stuck on step 7
Post by: soundguy937 on August 25, 2025, 12:23:33 AM
Unfortunately it does not appear a tube swap made a difference in stabilizing the voltage.
Title: Re: Stuck on step 7
Post by: JPK on August 25, 2025, 03:32:52 PM
Check that the input transformer is in the correct orientation.
Check that TR1 and TR2 have the correct value.
Check that R3 to R10 have the correct value.
Check that the voltage on B+ is stable.
Check the input transformer coils resistance (in circuit, input pot on max): 

Check the input resistance (between XLR pins 2&3, input pot on max): 4.6k

Title: Re: Stuck on step 7
Post by: soundguy937 on August 26, 2025, 04:19:53 AM
Input transformer is in correct orientation.

TR1=202 2k
TR2=102 1k

B+ stable at 45.5VDC

R3= 3k3
R4= 3k3  
R5= 10k 
R6= 10k  
R7= 10k  
R8= 470R 
R9= 10k  
R10= 10k  

input transformer values (input on max)

2-4= 37.2 ohms
3-5= 37 ohms
7-9=100.1 ohms
8-10= 99.1 ohms

input resistance pins 2&3= 4.61 k ohms

everything mostly checks out excepts input transformer 8-10 being 99.1ohms. 
Title: Re: Stuck on step 7
Post by: JPK on August 26, 2025, 03:22:56 PM
All OK.
Check again the tube heater (step 4 of the setup guide).
I am sorry, I am short of ideas. Do you want to sent it to us for a check at the lab?
Title: Re: Stuck on step 7
Post by: soundguy937 on August 27, 2025, 03:45:16 AM
Pretty much spot on. Goes to about 2.VDC in about 5 seconds & within 10 seconds goes to 6.43VDC and is stable.

At this point I think I am ready to send it to your lab :(, just let me know the best way to do so. I'm bummed but this is obviously beyond me. Either a faulty part or a faulty installation of one by me somewhere.
Title: Re: Stuck on step 7
Post by: JPK on August 27, 2025, 09:52:54 AM
Contact me here: https://www.soundskulptor.com/en/content/6-contact
Title: Re: Stuck on step 7
Post by: soundguy937 on September 18, 2025, 04:39:22 PM
My unit should have arrived late last week. Do you have an eta when you or your staff will be able to take a look at it?
Title: Re: Stuck on step 7
Post by: JPK on September 18, 2025, 05:48:33 PM
Hello, your module is repaired.
There were two problems.
The first was the broken 2n7000 (be careful of static electricity, this component is very sensitive) and the second was the defective solder on the tube's PCB. I think the solder you're using isn't the best quality for proper soldering. I noticed some residue due to the solder. The connection to the tube wasn't perfect (there was noise in the tube and the tube's balance voltage was impossible to achieve).

The module has been calibrated and reassembled. It's ready to use!
Title: Re: Stuck on step 7
Post by: soundguy937 on September 18, 2025, 06:48:38 PM
Thanks a million, definitely taking notes on my mistakes.