21

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?

22

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).

23

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.

24

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.

25

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....

26

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?

27

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.

29

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

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

30

LA502 Optical Compressor / V- Rail returns low

« Last post by Namy on August 21, 2026, 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)