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MU524 Vari-Mu Tube Compressor / Re: Different output levels

« Last post by Elektrovolt on August 20, 2024, 02:28:42 PM »

Thanks JP,

I was already suspecting the input potentiometer tolerance, they measure a bit different from each other. The transformers are mounted correctly and all audio path resistors are correct also.
The pots measure:
Full values are 9,7 kohms and 10,8kohms
Wiper set exactly in the middle: 0,95 kohms and 1,08kohms.
So there is a difference between both.

I do have some good quality log pots 25k from a Sound Devices mixer. Would that work too, (with an 18k resistor between pins 1 and 3) or should they really be 10k?

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MU524 Vari-Mu Tube Compressor / Re: Different output levels

« Last post by JPK on August 19, 2024, 06:52:45 PM »

The input pot difference could be caused by the tolerance on the pot resistance value (20%). You can check the resistance value of the pots between the 2 side pins, pot at max.

Regarding the 20VAC output it might be OK. Simply check that the input transformer is mounted in the correct orientation, label facing towards the back of the module.

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MU524 Vari-Mu Tube Compressor / Different output levels

« Last post by Elektrovolt on August 18, 2024, 06:31:52 PM »

Hi all!

I've finished a pair of MU524s last week and did the calibration routine four times on each of them.
the difference in output level and gain reduction is quite a lot, I need to set the input pot on of of them 1 dot to the left to get the same levels on both.

In step 6 of the setup manual (audio path test) the 1v signal gives 3,177 VAC on the left one and 3,450 VAC on the right unit.
GR is different too and output levels differ around 1,2 dB.

I do not mind that two units have little differences in settings, but around one dot seems a bit much.
All voltages are within specs, except for the output level when the out pot is CW which gives over 20VAC on both units which seems a lot.

What could cause the difference in signal levels and the high output?

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MU524 Vari-Mu Tube Compressor / Re: MU524- output potentiometer voltage, meter adjustment, and ringing in audio

« Last post by JPK on August 18, 2024, 06:10:35 PM »

Actually this is fine. The 12VAC value mentionned in a previous post was for one single phase of the output. When measuring accross the two phases, you can reach 20V. Sorry for the confusion.

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MU524 Vari-Mu Tube Compressor / Re: MU524- output potentiometer voltage, meter adjustment, and ringing in audio

« Last post by Elektrovolt on August 16, 2024, 10:52:48 PM »

The same thing happens with both MU524's I just finished. The signal level measures about 20,3 volts when measured as described in the last part of setup guide step 6. 
I am using a Fluke 179 which is true RMS, if that makes a difference here.
The voltages are within specs, no ringing or meter issues here.

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MP573 Microphone preamplifier / Re: Blown capacitor

« Last post by JPK on August 14, 2024, 06:21:37 PM »

Yes it happens. Electrolytic capacitors are the most fragile components, especially if their temperature is high.

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MP573 Microphone preamplifier / Re: Blown capacitor

« Last post by fabi on August 14, 2024, 04:17:15 PM »

Yes, they look pretty good now:

TP2: -9.1
a: 2.6
b: 4.0
c: 22.5
d: 13.5
e: 2.0
f: 12.9
g: 0.9
h: 4.0
i: 0.4
j: 3.4
k: (cannot measure, it's plugged)
l: 2.9


Mmm, could it be just bad luck with that blown cap? Or do you think there's another issue? Just FYI I have another MP573 working just fine in the same lunchbox, so not sure why it happened

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MP573 Microphone preamplifier / Re: Blown capacitor

« Last post by JPK on August 12, 2024, 11:27:40 AM »

These voltages must be measured against TP2.
So they don't look so bad.
Can you redo these measurements relatively to TP2 from A to L ?

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MP573 Microphone preamplifier / Re: Blown capacitor

« Last post by fabi on August 11, 2024, 11:51:10 PM »

I changed the cap, now the measurements:

TP2: -9.1
a: -6.4
b: -5.1
c: 13.4
d: 4.4
e: -7.1v


I unplugged the unit at this point, since the measurements seemed bad. Now TP2 seems fine, but not the others. If you need some other measurement please let me know, the card is over the table now.

Thanks!

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MP573 Microphone preamplifier / Re: Blown capacitor

« Last post by JPK on August 10, 2024, 06:00:30 PM »

Could be caused by a bad input?
No.

I would start by just replacing the cap which is the most likely to fail.
The other parts replacements are only if this doesn't work.