[seanlew98]

Hi all, I've a problem with my MU524, the voltages didn't seem quite right upon first step of setup but I went through with the calibration anyways just in case. The compressor is working and might I say sounds pretty damn good, but I feel something is still not right given that I couldn't get the setup parameters right and it's also a lil low on the output.

Lemme run you through the details.

SETUP

Outer pins of tube PCB would only go up to 2.486 VDC MAX, even after half an hour.

Tube doesn't glow at all, same tube glows in my orange micro terror amp. Tube gets barely warm after a long time being powered on, it's basically as warm as the output transformer and the Q4 U U6 area.
Swapped 12AX7 tube from amp into MU524, voltage got up to around 4.45VDC but nowhere near 6v even after long. Swapped back to stock tube for rest of test.

V+ 15.72 VDC

V- -15.69 VDC

B+ 45.28 VDC

1.0 VAC Sine tone bypass, when compressor is in it drops to 0.523 VAC (!!!)

output pot maxed out reads only 5.927 VAC

rest of the setup was okay except for step 10 TR5 Couldn't hit 3.0 VAC, was about 2.43 VAC

Also scrubbed through some other similar posts and measured:
- 6.19 VDC between V- test pin and the right side pin of the tube holder PCB (marked H).
- Q4 is warm
- C32, C33 not reversed

Troubleshooting:

TPI
43.57 VDC
0.232VAC

TP2
43.97 VDC
0.124 VAC

TP3 = TP4
0.244 VAC

output 0.489 VAC

changed conditions

TP3
0.09 VAC

TP4
1.058 VAC

TP5
1.037 VAC

TP6
-0.773 VDC

Output 2.1 VAC

GR Meter 1.5dB

Here are some pics for reference!
And yes, followed the steps in detail for both calibration and troubleshooting, including knobs & switches positions & removing and attaching of JMP1.

Also, I had to desolder the two headers on the meter PCB (J1B & J2B) cause I soldered em on the wrong side (yikes). They're resoldered on fine with some pins coming out but reposition levelled with the rest, and the pots & switches were fully working in the final assembly so I don't think that would be the issue. Also since the power side (?) isn't on this PCB.

Also, bit of an embarrassing thing, when I got to the relay step on the build guide, I thought IC's was similar having soldered IC's in previous builds so I just soldered them in and only say the IC sockets later on. Could that be the problem? Seemed unlikely to me since I have fully functioning and great sounding compression.

Please help

[seanlew98]

*update*

Reflowed practically everything one leg at a time on IC and transistors to not overheat, and exactly the same, 2.54 VDC on the two outer legs of tube PCB, no glow.

[JPK]

It looks like the soft start of the tube heater always remains on "soft".
Try shorting R39 (18R) with a little piece of wire and check if the tube heater voltage (step 4) looks better.

[seanlew98]

shorting as in connect both leads of of R39? Will try in a bit!

[seanlew98]

OMG Tube's glowing for the first time ever!

Step 4 Voltage is @ 6.19 VDC.

Do I leave the wire in? What's next? Didn't leave it powered on for long just in case.

EDIT:

I calibrated it and each step was right in the Trip pot range with no difficulties.

One thing tho, there seems to be a hiss coming from the output knob, I've seen it reported before as well. Any remedy for that?

[JPK]

It would be preferable to repair the soft start to extend the tube life.
I am not 100% sure but from your picture, I suspect Q4 being reversed. The writings on the transistor but be facing towards R38. Please check.

[seanlew98]

It is indeed reversed. Lemme fix that real quick!

Flipped Q4 back and removed the bridge wire on R39 and it's working! You're a wizard man!!

[seanlew98]

Tho one thing I was wondering about before, I think I might have a microphonic tube.

Did some testing with my 12ax7 tube in for comparison. I did however noticed after the fact that the 12ax7 isn't glowing while the ECC189 was. The levels on the 12AX7 are more akin to the "soft" start mode before I bridged the resistor too.

What do you think? Is it still just noise from my rack since it's so "microphonic"?

https://drive.google.com/drive/folders/1EvaXznCp-66F9AlMgMM0UjU_vV91IWNP?usp=drive_link

EDIT: PS* Also from some searching I've understood that a "microphonic tube" isn't really what it is, the tube seems okay but is it normal for it to pick up so much noise around it? Just wanting to see if it's the source of the hiss.

Just measured the noise level with output knob all the way up and it's around 0.014VAC. I am however using a Midas L-10 tho, getting the link audio X-10 mod for it with the high end external PSU. Would report back on whether noise levels improve.

Thanks!

[JPK]

2mV of noise at the output is normal with IN and OUT pots at max. 14mV seems a lot.
All tubes are more or less microphonic, this also is normal. Taping on the tube can create an output voltage of 50-60 mV.

[seanlew98]

yeapp, shall report back in a couple of days once I've built my new rack PSU. At least then we'd know if it's the Midas

[seanlew98]

Built my X-10 rack and tested, still the same noise, maybe down by .5dB at most so effectively no change. Also tested this with a RND chasis, same issue noise still very prominent. Is it possible it's the tube? Or any suspect within the other parts that may have caused this?

Thanks!

[JPK]

Can you measure the output noise voltage with the pots at max and the jumper removed?
This to make sure the noise doesn't come from the side chain.

[seanlew98]

Took it out of the rack with the jig to measure today, noticed that now when the cover plate was off the noise is quite low (at least compared to before) , at around 0.010 VAC, identical with both jumper on/off. I then remounted the side plate and basically discovered the same issue user ckalinec & egelmett had. I'm quite sure with the old power supply noise level was equal sideplate on or off, maybe I missed it. But basically what happened was sideplate being over the unit has marginally lower noise, but when fastened with screws tight the noise level goes up a couple of times.

I tried tapping the parts around the voltage regulator and it does indeed make more noise, especially when I rest a finger on the PCB edge portion of the regulator. Now I just have a couple o-rings resting between the screws & side panel but it seems to stick out into the next slot, won't be a problem for the short run. I did however have my suspicions and being an impatient guy ordered both a replacement tube & the voltage regulator.

Will see if that improves anything when they arrive!

Oh forgot to mention but basically with the screws on the least noisy way the noise dropped to about 0.002VAC on the output, much more usable now. Noise levels on my DAW loopback (nominal with no unit in between) is about -48dB at the highest, and -70dB output pot minimum position, which is where I typically use my compress mode at.

[JPK]

You could have a noisy regulator. It could be intersting to replace it.
The noise that can be measured with or without the cover plate is very high frequency induced and shouldn't affect the audio.

[seanlew98]

Replaced the regulator today with the one you linked in the other post. Noise dropped by at least 25dB, now at nearly -90dB and even with output maxed out it's barely audible. Super stoked.

Anyone else having noise issues should look into replacing the voltage regulator.

Cheers!

[JPK]