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MP573 Microphone preamplifier / Second hand MP573 with -6dB level drop

« Last post by sdiazmun on Today at 08:07:07 PM »

Hi!!

Im just received a second hand MP573 (I have other three), and after compare them with "Smaart", the last one has a -6dB drop across the entire frequency spectrum, and that difference remains even when modifying the gain or the output potentiometer.

Where should I start looking for the point of failure?

Thanks in advance
Regards 
Sergio

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on Today at 11:19:16 AM »

Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4
You can debug without the front panel PCB connected. This will help to narrow down the search area.

Just to confirm
Normal. 2 pins are connected together. 10kohms max between the 2 other pins.

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CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by JPK on Today at 11:09:08 AM »

Contact me here for a repair.

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 18, 2026, 11:01:38 PM »

I'm stumped, Now I lost ac voltage at TP1 and I still get 1.64vac at TP3 and 0.055vac at TP4
I still have 30vdc at TP1 and 2 as well.
Just to confirm, what value should I be getting on P1? the impedance stays at 0 when turning the pot on the right 2 pins when looking at the pot from the rear, It varies between 0 and 10k on the left 2, I get 10k on th outer 2 pins

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CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by vwyf11 on April 18, 2026, 04:57:30 AM »

Well I think if there’s nothing left to check, I give up.  I’ve replaced Q7 three times and still am coming up with the same results.  I’ve looked over the schematic and retraced everywhere for continuity.  Every part I’ve pulled out has tested fine.  All of the Q7 transistors have tested identical out of circuit.  I haven’t had trouble like this with other diy kits so I’m feeling a little defeated.  There must be something small I’m overlooking somewhere.

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 17, 2026, 09:21:04 PM »

I get no signal at TP4 with the front pcb removed, power on. With and without U2. Solder joints and component values seem to be good.

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 17, 2026, 09:43:04 AM »

So the issue is in the final gain stage around U1.2.
Check the value, direction and solder joints of U1, R14 to R16, R18, C9, C10, C11.
Do you get a signal on TP4 when the front PCB is completely removed, with power on?
Also try with U2 removed.
All voltages should be measured against 0V.

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CP554 Diode bridge compressor / Re: Cant get 3v at step 5

« Last post by vwyf11 on April 16, 2026, 04:52:21 PM »

Ok thanks for clearing all that up!  Definitely gives me a better understanding

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by Ben on April 16, 2026, 04:19:05 PM »

I have 1.22vac between TP1 and TP4 and 0.058vac between TP0v and TP4 with U4 removed

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MU524 Vari-Mu Tube Compressor / Re: Low output voltage step 6

« Last post by JPK on April 16, 2026, 10:59:14 AM »

TP4 = .054vac
TP1 to TP3 all good except TP4?

Do you say that TP4 is back to 1.22VAC when U4 is removed?