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LA502T Tube Optical Compressor / Re: test procedure - having trouble

« Last post by doktar on Today at 02:51:30 AM »

If I'm understanding correctly, for the first set of measurements I'm adjusting the gain reduction then adjusting the output gain to the voltage you specified (1K tone).

15V no GR - 0VDC
10.6V -3db (one LED) .03VDC
1.5V -20db (all LEDs) .39VDC

U4 and U3 voltages are almost exactly as you specified.
U4 0V to .08V
U3 .76V to .19V

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LA502T Tube Optical Compressor / Re: test procedure - having trouble

« Last post by JPK on October 12, 2026, 05:09:12 PM »

What DC voltage do you get on TP8 (against0V) 
- for Vout = 15VAC (no gain reduction),
- for Vout = 10.6V (-3dB),
- for Vout = 1.5V (-20dB).

What DC voltage do you get on U4 pin13 (against0V) when you turn Trimmer TR1 from 0 to the max. Should go from 0.08V to 0V approx.

What DC voltage do you get on U3 pin2 (against0V) when you turn Trimmer TR2 from 0 to the max. Should go from 0.75V to 0.2V approx.

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LA502T Tube Optical Compressor / Re: Release time, and a couple other oddities

« Last post by Perihelion on October 09, 2026, 08:50:36 PM »

1. The decay is set by the cell itself, it is not reflected in the LEDs which are driven by the side chain. It is not very fast, about 200-300ms.

Ok!

2. Not normal. It should only change the gain by 0.5dB.
Check the solder joints and components value of R44, R46, C21, C22.

R44 metered as 4.67kohm and R46 metered right at 100kohm. C21 is a Wima 0.1 and C22 is a Wima 0.22. The solder joints look pretty good all around. 

3. Normal. There is a minimal attenuation caused by the input transformer and the attenuation network.

Ok!

4. Not normal. Can you identify the source of the noise by pressing a pen or something against the components, most probably one coil. 

Ah yes now that you mention it it's obviously inductor whine. It sounds exactly like my GPU, just not as obnoxious.

Check if any component is getting abnormally hot.

Nothing is abnormally hot. The ICs and U9 have a little bit of warmth to them, but nothing is anywhere close to the heat of the tube.

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LA502T Tube Optical Compressor / Re: test procedure - having trouble

« Last post by doktar on October 08, 2026, 07:46:56 PM »

After repeating, Im now getting about 23V max in step 5. So good there

Step 6 - with peak at 0 - there is 0v AC/DC on TP5.
  With peak adjusted to 10.6V - i get only -3 LED illuminated, but can adjust because all trimmers are down.
  With peak at 1.5, All LEDS are illuminated.  Still no adjustment.
   I'm closer now, but seems like slightly higher GR meter readings.  The -20 illuminates around 2.2 V

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LA502T Tube Optical Compressor / Re: test procedure - having trouble

« Last post by JPK on October 08, 2026, 05:43:25 PM »

Step 5- I only get about 15-16V max when it should be 20V
How many AC Volts max (against 0V) do you get on TP4 and TP5? (should be 11-12VAC)

Then in step 6- I get 3 gain reduction LEDS on with all 3 trimmers all the way down (CCW) and cannot adjust to only 1 LED (-3)
With PEAK at 0 What DC voltage do you get on TP8? (should be 0V).

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LA502T Tube Optical Compressor / test procedure - having trouble

« Last post by doktar on October 07, 2026, 10:23:14 PM »

After completing my build, I tested voltages, they are all within spec.
 
When I try to calibrate,  Step 5- I only get about 15-16V max when it should be 20V

Then in step 6- I get 3 gain reduction LEDS on with all 3 trimmers all the way down (CCW) and cannot adjust to only 1 LED (-3)

Gone through the proceedure twice and same results.

Unit sounds as it should. 

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LA502T Tube Optical Compressor / Re: Release time, and a couple other oddities

« Last post by JPK on October 06, 2026, 03:39:50 PM »

1. The decay is set by the cell itself, it is not reflected in the LEDs which are driven by the side chain. It is not very fast, about 200-300ms.
2. Not normal. It should only change the gain by 0.5dB.
Check the solder joints and components value of R44, R46, C21, C22.
3. Normal. There is a minimal attenuation caused by the input transformer and the attenuation network.
4. Not normal. Can you identify the source of the noise by pressing a pen or something against the components, most probably one coil. 


Check if any component is getting abnormally hot.

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LA502T Tube Optical Compressor / Re: Release time, and a couple other oddities

« Last post by Perihelion on October 01, 2026, 05:07:51 AM »

Here is the 502 on a snare channel: 

https://youtube.com/shorts/raAIkmTgPdc?is=uK2wjD3vC5Dufp2W

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LA502T Tube Optical Compressor / Release time, and a couple other oddities

« Last post by Perihelion on September 30, 2026, 10:34:40 PM »

Heya,

Just assembled and calibrated my 502T and have a couple questions about things that may or may not be an issue. All test points checked out good, and there were no meter calibration issues. I did the D1-D4 shorting mod and the r16 cap mod.



Quirky thing 1:

The release sounds fast, really fast, like all-buttons-in 1176 fast. The GR meter also shows this. It doesn't exhibit a logarithmic release like you'd think an opto comp would. I can link a video in a follow-up post.



Quirky thing 2:

Switching to 5ms attack from 10ms or 20ms adds about 3-4 dB of gain reduction on a constant 1kHz sine wave input. I understand a faster attack would do more GR on a music signal, but I've never seen that effect on a sine wave.



Quirky thing 3:

A 1.000v 1kHz sine wave at the input comes out at 0.924v on the output, compressor engaged with no GR, no output gain. Bypassing the compressor restores the output to 1.000v.



Quirky thing 4:

There is an audible 14kHz whine coming from the unit itself when powered on. With the cover off I can hear it about a half meter away. I didn't want to get my ear too close but it sounds like it's coming from the center of the board around the tube or opamps.



The unit otherwise operates like it should. All of the pots and switches do what they're supposed to do.



All of this kinda adds up to something shorted in the side chain or maybe a bad U7 or U8, but if you have any advice as to where to look first I'd definitely appreciate it. Thanks!

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LA502T Tube Optical Compressor / Re: Some wrong parts in the kit.

« Last post by UrbanOsterman on September 24, 2026, 11:08:06 AM »

Thanks
Great Product! Works perfect now.