41

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« Last post by egelmett on June 24, 2025, 01:58:54 PM »

No. Regardless of the devices placement, either next to the other MU524, or by itself in a separate chassis, the noise begins after the device is warmed up for a while.

Again, I swapped the DC/DC converters...and many other components.

What do you charge for troubleshooting the MU524?

Thanks!

Ed

42

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« Last post by JPK on June 24, 2025, 09:04:56 AM »

Great news !
For the meter you can increase its sensitivity by decreasing the resistor R34. Try 3k3 or 2k2.

43

MP573 Microphone preamplifier / Re: C34 Film Capacitor Placement

« Last post by musicarr on June 23, 2025, 11:50:32 PM »

Perfect. Thanks for the info and confirmation.

44

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« Last post by greyskull on June 23, 2025, 09:12:06 PM »

Salut JP!

In a last ditch effort, after some digging through the forum, I tried cleaning the valve pins and socket and it all now works and I was able to calibrate the unit within spec, other than the GR meter which I maxed the trimmer out on just under 6 db!

I'm a million miles ahead of where I was, but if you have any suggestions as to how to fix that last bit, I'll be forever grateful!

Merci!

Guy

45

MP573 Microphone preamplifier / Re: C34 Film Capacitor Placement

« Last post by JPK on June 23, 2025, 05:05:45 PM »

It fits into the 2 holes on the left that best match the cap size. The PCB can accept a larger capacitor.

WIMA 0,47 100 is the right cap.

46

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« Last post by JPK on June 23, 2025, 05:01:56 PM »

Does the noise in the left unit depend on the presence of the right unit next to it?

47

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« Last post by JPK on June 23, 2025, 04:57:21 PM »

Reversed input transformer ? The label must face towards the back.
What voltages do you get in the Voltage checks here.

48

MP573 Microphone preamplifier / C34 Film Capacitor Placement

« Last post by musicarr on June 22, 2025, 11:53:15 PM »

Hi,

The C34 Film Capacitor has two pins, but the PCB has three holes. Which holes should the capacitor go in?

Also, just want to confirm that the "WIMA 0,47 100-" is indeed the C34 capacitor. Is that correct?

Thanks.

49

MU524 Vari-Mu Tube Compressor / Re: MU 524 Stuck on Step 7

« Last post by greyskull on June 22, 2025, 07:26:20 PM »

Hi JP. I've had a little more time to devote to this and am starting going through the setup again, before sending off to you.

It has now become clear that an issue must exist elsewhere, as when I reach step 6 > 6 (Check the output voltage has risen to 3-4 V AC) the output does NOT rise and remains at 1.1 V. If I turn the output pot all the way up, it reaches 13.44 V AC.

I have carefully been through the board to check all component values and orientations are correct. Is there an area you can suggest I should look?

Many thanks again.

50

MU524 Vari-Mu Tube Compressor / Re: Light Hiss/Noise

« Last post by egelmett on June 18, 2025, 02:42:17 PM »

FYI - Still having issues.

I have two MU524. The "Left" one is the one with intermittent snaps and pops and noise - it comes and goes. It happens whether in "IN" or "Linked" modes.

The "Right" unit is my control source. It's silent...no noise. It did have a problem with the P1 pot going bad - replaced.

What I did on both was to tape the entire underside (toward the components) of the cover plate leaving only the screw holes and their conductive pads exposed. That worked for the "Right" unit.

But, for the "Left", it's been a long-running process of swapping components between the two devices to try to find the component responsible for the intermittent noise.

Along the process, I did swap the voltage converters, but was not the cause.

Other attempts:

Lightly sanded the board contacts (to the 500 box), lightly added solder to the header pins connecting to the front plate (thinking poor connectivity), tried three different 500 chassis, tried moving positions in Midas L6 (and removing other devices around it), Swap tubes, swap C4 with C7, replace all electrolytic capacitors, replace C17, remove and resolder T1 & T2, swapped U3 & U1 between units, swap tube mount and socket between units, replace C10, C11 & C12, reflow ALL solder joints.

I will still swap U2 & U4, one at a time, to continue the process.

I'm probably forgetting other maneuvers as it's been a year-long process...could it be a bad trace on the PCB? That's where my head is going. It only starts when the device is well warmed...but a reflow of ALL solder hasn't helped...neither has a very tight mount of the support posts to the chassis (thinking ground issues).

I'm about ready to hit JP up for his repair service...I'm so damn frustrated at this point!