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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 22, 2026, 01:50:33 AM »

Wait, if r39 in circuit should be 1ohm, 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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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by Namy on August 22, 2026, 01:48:12 AM »

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

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by Namy on August 22, 2026, 01:27:56 AM »

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?

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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.

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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....

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LA502 Optical Compressor / Re: V- Rail returns low

« Last post by JPK on August 21, 2026, 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 August 21, 2026, 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 August 21, 2026, 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.