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

« Last post by guenter1910 on Today at 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....

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by JPK on Today at 10:10:37 AM »

- Remove all the socketed IC's and and check it it is still the same.
- Check the value of R39 with the ohmmeter (1 ohm). It should almost look like a short.
- Did you need to remove a component during the build?

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by JPK on Today at 10:05:55 AM »

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

« Last post by JPK on Today at 10:04:29 AM »

Try tapping on components and connectors with a pen to try to locate the defective connection.
Resolder all solder joints that look suspicious.
Monitor a constant sine signal with your AC voltmeter at points TP1, TP2, TP3, TP4, TP5, TP6 to try locate the stage where the problem occurs.

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

« Last post by JPK on Today at 09:57:31 AM »

Check you didn't swap Q1 & Q2.
Check the voltage on R46 top pin (7.4V).
Check the solder joints on R45, R46, R47.
Check the direction of C23.

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LA502 Optical Compressor / V- Rail returns low

« Last post by Namy on Today at 02:04:05 AM »

Hi!

So, relatively new DIYer. I have just built my first LA502! Yay!

In testing, the v+ rail looks great (+15.7v)

Second test of the v- rail returns… .787 mv.

I have checked all polarized pieces (caps, diodes, etc), and they’re fine. I’ve looked for bad solder joints across the board, and particularly between pins 13, 14, 5, —> OV / V-, but I don’t see anything of concern. I also double checked connectivity and resistance on those joints, and everything checks out.

I’m sure it’s something silly, but I’m still learning how to fully understand the schematics and I’m not sure where to turn my attention. Any help would be appreciated!


(PS: I am using an XT500 with an API Lunchbox 6B. The XT500 seems to fully check out, though there might be something I missed)

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

« Last post by guenter1910 on August 20, 2026, 07:06:17 PM »

Intermittendly the signal gets distorted and very quiet. Sometimes it disappears and comes back after a while and sounds ok. This happens on and on. It seems to be contact problem (?) - I was thinking of the tube circuit and soldered the connections of the tube pcb again but this does not help. It happens on both inputs line and mic.
Any hints?
Thank you in advance!
Guenter

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

« Last post by hanssongt on August 20, 2026, 10:10:41 AM »

Thank you for getting back so quickly.

Q2 is in the correct position.

R51:
15.77V near C24
15.92V near connector

R48:
131mV near R41
1V near C23

So something seems off when looking at the measurements at R48?

Thanks,
Tobias

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by noeqplease on August 20, 2026, 06:30:57 AM »

UPDATE:

I spent some time away from testing this compressor. Glad I did.
I saw some new test equipment, so I ordered a few new items to replace my aging oscillator and old as hell multimeter.

So, in the end it turns out that it was my test equipment that was not happy with this compressor.

Today I tested the LA 502 compressor I thought was bad, and it wasn't. At all.

Lesson: Always test your testing equipment, and always test your wires, leads, alligator clips, etc.

Thank you for your help and patience. Time to order a few mic preamps now

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

« Last post by JPK on August 18, 2026, 11:04:36 AM »

Check that Q2 (2SK170) is in the correct position A.
Can you measure the DC voltage (against 0V) on both sides of R51: near connector (16V) and near C24 (15V) ?
Can you measure the DC voltage (against 0V) on both sides of R48: near C23 (8V) and near R41 (1V) ?