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CP4500 Stereo Bus Compressor / Make up gain no longer working

« Last post by MRMNW on Today at 01:29:32 PM »

Hi,
The unit has been working fine up until recently, now the makeup gain does nothing. 

I'm getting a reading of -1.42v at TP12 as per step 1.2 on the voltage checks and it doesn't change when moving the makeup gain on step 1.3.

Any help appreciated

Thanks

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

« Last post by doktar on October 23, 2026, 06:34:07 PM »

I believe I've blown out the LED's, when testing on the IC's I accidentally shorted pins with the meter probe,  I've ordered some replacements and will report back.

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

« Last post by doktar on October 16, 2026, 12:14:01 AM »

Here are the correct voltages on TP8

- for Vout = 15VAC (no gain reduction),= 0 VDC
- for Vout = 10.6V (-3dB), .050 VDC 
- for Vout = 1.5V (-20dB).  .592 VDC


the -3 LED illumanites at 12.8 V, and both trimmers are all the way down, so can't adjust to 10.6V.


However, now I only can get 2 LEDs to illuminate, no matter where the trimmers are at.

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

« Last post by Perihelion on October 15, 2026, 05:10:01 PM »

Ok I had the time to do some more probing and here's what I have found: 

Firstly, I re-calibrated the meter with the attack switch on 10ms.

Now with a 1kHz 1.000v input signal and 0.711v on the output, the -3dB LED on the meter is just lit (perfect!)

With this configuration and switching the attack to 5ms, the output drops a little bit to 0.675v, which is about a 0.5dB drop, much less than I initially thought. However on the meter the -6dB LED is now firmly lit.

(This is true on the entire meter range. For example if I'm doing 9dB of gain reduction in 10ms mode, switching to 5ms will show 12dB of reduction, even though the actual output change is about -9.5dB).

The other thing I have noticed is that in 5ms mode the GR LEDs are slightly dimmer than in 10 or 20 mode.

I've already re-flowed everything in the side chain circuit, so I'll take a closer look at the meter PCB and the 10-pin header.

If the meter is not accurate at higher speeds, that's perfectly fine! Like I said all of the switches seem to do what they're supposed to.


In regards to the coil whine, I might take you up on a replacement sktps2. The whine stops if the board is removed.

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

« Last post by Perihelion on October 15, 2026, 02:34:39 PM »

https://imgur.com/a/dLkYro1
Here's an album with a few pictures of the build, if it helps.

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

« Last post by JPK on October 14, 2026, 04:06:35 PM »

The first set of measurements is like step 6 in the setup Guide: Start with a 15VAC output voltage then adjust PEAK to get the 10.6V and 1.5V output and measure the DC Volts on TP8.
If this is what you did, then your values are good.

Please describe again the issue, I am lost!

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

« Last post by JPK on October 13, 2026, 06:02:33 PM »

2. Do you measure the gain reduction from the LEDs or the actual gain reduction on an external meter?
It is true that the LED meter is not accurate when the decay time is increased. This is because it is driven by the side chain, not by the signal itself.

4.  We haven't encountered this problem yet. Is the inductor whine annoying? If it is the case, we can exchange the module.

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

« Last post by doktar on October 13, 2026, 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 .08V max - 0V min
U3 .719V min .198V max

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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.