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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.

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

« Last post by JPK on September 16, 2026, 05:20:51 PM »

I am sorry for the mistake (still a new product). We have sent the correct switches.

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

« Last post by UrbanOsterman on September 16, 2026, 04:49:52 PM »

I just received a LA502T kit and I started to build it:
But there were some wrong parts included in the kit.
The 4 switches were of the wrong type.
If you can send me the proper ones, I can send you
money for them.
I can keep the wrong types so I dont have to send them back.
Regards
Urban Osterman

 
 

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CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« Last post by steveholm on September 13, 2026, 10:35:03 PM »

LED comes on fine. 

I went ahead and ordered a new SK25 and I'll reach back out if I have more troubles after that one is built. Thank you!

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CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« Last post by JPK on September 12, 2026, 09:34:46 AM »

Does the LED lights up?
Check Q7 and Q9 with the DMM in diode mode. It is possible to test Q7 while in circuit and test half of Q9, the BC junction.
Test transistors
Generally DOA's are very difficult to troubleshoot because of the density and all the feedback paths. I would recommend to build a new one.

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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by JPK on September 12, 2026, 09:11:34 AM »

Actually, this is the normal frequency response of the original 1073. About -1dB at 20kHz.
I guess that the modern reissues are modified.
You can make the frequency response flat up to 20kHz by changing the value of 2 caps:
C47 at 100pF (instead of 470pF),
C60 at 47pF (instead of 150pF).

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CP5176 FET compressor / Re: SK25 output collapses and develops large negative DC offset at higher input

« Last post by steveholm on September 11, 2026, 07:28:37 PM »

I have also tested all the resistors, and they all match the spec. The diodes are working in the correct directions as well.

Steve