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MP573 Microphone preamplifier / MP573 weird behavior (no volume after 20)

« Last post by camarada78 on August 21, 2025, 10:04:16 PM »

I have a MP573 on my bench that used to work normally.

It then got no sound at all. It was the transformer that had an open tap. I got a new one and that was fixed. 

But now i have a weird problem:

It works normally and with beautiful sound with main knob at 10 or 20 volume, after that it has no sound. From 30 and above it has no sound. if i return to 20, it works normally.

Innitially i thought it was some open resistors, but they appear to be fine. 

Does someone had this issue before?

Thanks,

Have a good day.

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CP4500 Stereo Bus Compressor / Re: CP4500 Cannot level match and incorrect voltages in step 1.6

« Last post by Gian on August 21, 2025, 12:42:19 PM »

Great, thanks. I'll get back here when I have a replacement for the 1646.

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EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by Pierro on August 21, 2025, 10:54:44 AM »

Meme si la LED s'allume avec le DMM?
J'ai déja remplacé 2 fois Q1 avec précautions

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EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by JPK on August 21, 2025, 09:50:49 AM »

J'en deduis que :
- La LED est HS, en court-circuit,
- Q1 est HS, coupé.
Il faut changer les 2.

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EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by Pierro on August 20, 2025, 10:24:50 PM »

C'est surement car j'avais fait les tests avec la LED cout-circuitée...

J'ai refait les tests et j'obtiens:

avec signal entrant 1000HZ 2VAC:
TP1: 2,35 VAC. 0 VDC
TP2: 2,4 VAC. 0,4 mV DC
TP3: 0,24 VAC. 0 VDC

Et de nouveau lorsque je court-circuite Q1 l'EQ s'active mais pas la LED verte.
E R21 est bien connectée a 0V quand SW3 est on, sans court-circuiter la LED.

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EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by JPK on August 20, 2025, 04:05:09 PM »

TP3 -15 V DC
Ceci n'est pas du tout normal.
Cette tension devrait se retrouver aussi sur TP1, peut-être en fonction de la position de P5.
Il y a un court circuit quelque part entre le signal et le 15V. Cherchez les ponts de soudure.

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CP4500 Stereo Bus Compressor / Re: CP4500 Cannot level match and incorrect voltages in step 1.6

« Last post by JPK on August 20, 2025, 03:52:08 PM »

I would say you only have one bad 1646. All the other IC's seem to be good.
Re-socket the IC's and and make a fresh total voltage check (with the front PCB in) of channel 1, step1.1 to 1.7.

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MU524 Vari-Mu Tube Compressor / Re: Stuck on step 7

« Last 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?

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EQP501 Passive Equalizer / Re: Son faible et pas d'EQ

« Last post by Pierro on August 18, 2025, 02:33:23 PM »

Avec la LED court-circuitée et SW3 on, R21 est bien connectée au 0V.

TP1: 96 mV DC
TP2: 0,4 mV DC
TP3 -15 V DC

Le DOA rien a changé, il a l'air correct

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CP4500 Stereo Bus Compressor / Re: CP4500 Cannot level match and incorrect voltages in step 1.6

« Last post by Gian on August 18, 2025, 11:30:54 AM »

Very much appreciate your help JPK.

Both Bridge diodes were in the correct orientation.
Removed all chips.

Added only one chip on the board, the THAT1246 (U1) inserted:
TP1=0VDC, TP2=0VDC, but TP2=0.9VAC

Then left THAT1246 in U1, and added THAT1646 in U7 (cool one):
TP1=0VDC, TP2=-0.232VDC (TP2=0VAC)

Inserted second THAT1646 and this one got super hot, but I pulled it after a few seconds (have put it aside).

Put good THAT1646 in U7, left THAT1246 in U1,
U7, Pin 1=0VDC, Pin 8=0VDC

Then, added each OPA2134 one at a time in U6. Same values for each OPA2134
TP1=0VDC, TP2=0VDC, Pin 1=0VDC, Pin 7=0VDC

What's the next steps?