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MP573 Microphone preamplifier / MP573 Build - No Output with XLR Connector

« Last post by morast on August 31, 2026, 01:35:06 PM »

Hi,

i finished my 2nd MP573 build and I'm having troubles getting it to work & would need some advice / guidance.


When the DI board is inserted, JMP3 removed, everything works fine. However, with no DI board, JMP3 on + Mic/Signal via XLR, there is no output.

Tests / measurements I did so far:

DC Values A-K all have the correct voltage.

AC Values with multimeter on 440hz Sine wave via XLR Input:
* In+ to In- : 0,6V
* C2 to C3: 0,6V
* TP2 to T1 Pin 7: Here things get whacky, AC Voltage on multimeter is jumping from 6V to 13V, also Led starts to light up, as soon as i have one(!) multimeter probe connector on T1 Pin7, Led jumps between green and red, corresponding to the 6-13V range, around 2x per second.

Hints:
* The kit was missing C3, which i then substitutet with a ceramic cap "n47" 100V. (see attached pic).


* RLY1 edge got burnt (see attached pic2). I don't remember if it was from soldering iron (unlikely) or due to faulty connection...




Thanks for you support & cheers from Austria

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by JPK on August 25, 2026, 06:53:28 PM »

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MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« Last post by JPK on August 25, 2026, 06:52:18 PM »

Your Q2 may be damaged, try replacing it.

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by Namy on August 25, 2026, 12:47:05 AM »

SOLVED:

Nevermind! I took a pause from examining the LA 502, and instead took a closer look at the XT 500. I noticed a bunch of starved connectors and reflowed most of the solder, and lo and behold the LA502 now returns with +- 16v on the respective rails!

sometimes the error isn’t the item being tested, but your tools, folks!

Hopefully that’s the only issue I run into, but I’ll start a new thread if I encounter problems during the rest of setup. 

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MP512 Microphone preamplifier / Re: Clipping/distorting noise on High Z input

« Last post by hanssongt on August 24, 2026, 10:15:24 AM »

Looks like the components are inte right place and soldered correctly. Voltage on R46 top pin measured to 7.7V.

What do you think about the measurements on R48? That's whats has been off so far.

Thanks,
Tobias 

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by Namy on August 23, 2026, 07:25:55 PM »

Ah! Makes much more sense. Was scratching my head trying to figure out what I wasn’t understanding about the schematic!

So, I tested the v- test pin to pin 14 (-16v) on the XT500, as well as the 502’s card slot. Both registered 1 ohm. (Well, the xt500 pin has an issue I need to fix, but tested from another point on it).

If I’m reading the schematic right, these are the two points on either side of r35, so should tell me if anything is funky with that resistor or its connectivity. 

What should I check next?

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by JPK on August 22, 2026, 04:52:17 PM »

Sorry !!! I mean't R35, not R39.
Double check R35. 
Check you didn't break the via which connect the bottom and top pads together. In particular on the bottom pin of R35 (near R17).

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MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« Last post by JPK on August 22, 2026, 04:44:04 PM »

I can measure an AC Voltage on all TP's except TP2 and 3.
This is because there is a large DC offset which fools your meter. It is normal.

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by Namy on August 21, 2026, 08:57:03 PM »

Awesome, thanks! I’ll give this a go when I’m next able to work on it.

There was one component that got removed: I accidentally put R33 in the spot for R35, but I eventually clipped it out and put a replacement for R33 into the correct spot. I’ve extensively tested R35, but not found anything to indicate it’s a damaged slot - I can trace power through it in both directions to other components, with proper resistance. I’ll double check (and maybe reflow) it, though.

(for better ease of reading, I've compiled my follow ups into one post)

EDIT: I just checked R39 in-circuit. It returns 130 ohms, but that's also what the parts list says it should read. Am I not checking the resistor itself, but its connection to one of the pins?

EDIT: With the ICs out, V+ registers at 16.42v and V- at .646v

EDIT: Wait, if R39 in circuit should be 1 ohm, but it's registering as way more in circuit, that means there is supposed to be a path of lesser resistance that it's not finding... hrm. I'll study the schematic.

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MP566-II Tube microphone preamplifier with HPF / Re: Signal gets heavy distorted and very quiet.

« Last post by guenter1910 on August 21, 2026, 04:13:11 PM »

I can measure an AC Voltage on all TP's except TP2 and 3. I applied a 1 kHz sin wave and I can hear it ok. I can hear the changes in gain and distortion and I see it on the frequency analyser. But there is no measurable AC voltage on TP 2 and 3. Although I can see it on the oscilloscope feature of the multi meter. The signal is there but quite low compared to TP 4 for example, but it seems to have some kind of a DC offset - it is not on the 0 line of the oscilloscope. All other signals on the other TP's are at the 0 line. I don't know if this is normal, as my testing right now with mic, line and HiZ inputs was all ok....