Sound Skulptor forums
Build help and troubleshooting => LA502 Optical Compressor => Topic started by: superhessu on December 24, 2023, 04:33:28 PM
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Hi all,
Got my 502 unit built a couple months ago and all's been working well, but a week ago I noticed that my recording levels were way off. After checking cables and other basic stuff I came to the conclusion that it's the 502 that's not behaving correctly. Problem seems to be that the gain reduction simply stops reducing gain from time to time, and as a quick fix to that I've tapped the unit which seemed to fix the issue. It's now gotten worse though, and there's no gain reduction happening at all anymore, the LED's don't light up no matter what the settings are.
I did get some life to the LED's by simply turning the PEAK knob rapidly from side to side, leading me to suspect that maybe the potentiometer is busted? I took the unit apart and tried measuring the resistance of the PEAK pot from the backside of the PCB, from the three legs of the pot. I'm getting very erratic readings from it and finding it difficult to measure, seeing all kinds of numbers with my cheap multimeter from 0 to 10K.
From the GAIN potentiometer I'm measuring about 4.7K and 0 (depending of the pot setting), without difficulties.
Am I looking at a busted PEAK potentiometer, or should I look for something else instead? I'm going to try and re-solder some of the joints next to see if that'll do anything.
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Am I looking at a busted PEAK potentiometer, or should I look for something else instead?
The resistance between the 2 side pins should be stable around 16k
The resistance between the center pin and the lower pin should go from 16k to 32k then decrease rapidly to 0 when rotating clockwise.
The issue could also come from the solder joints on the pot.
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Hi,
I re-soldered the PEAK potentiometer connections but the problem persists.
I will look for the resistance values soon and see if I'll get anything similar like you suggested.
Meanwhile, I was able to investigate the issue further by using a 1 kHz signal generator. I recorded the signal while at the same time turning the PEAK pot from 0 to max, the resulting record should be attached here as a picture;
(https://1drv.ms/i/s!AiNnowB4pdeDhuROKTXYN_xia6Stdw?e=BglOyk)
EDIT: The image doesn't seem to load, here's the direct link: https://1drv.ms/i/s!AiNnowB4pdeDhuROKTXYN_xia6Stdw?e=BglOyk
There's some gain reduction starting to happen at around 5 second mark (the PEAK knob itself pointing at about 30), but then the reduction suddenly dies away for a while, until at around 15 second mark (knob pointing about 60) it kicks back in and continues to function correctly to the end.
I haven't found the other settings (ratio, attack etc) to make any difference to the issue, they seem to work as intended.
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Am I looking at a busted PEAK potentiometer, or should I look for something else instead?
The resistance between the 2 side pins should be stable around 16k
The resistance between the center pin and the lower pin should go from 16k to 32k then decrease rapidly to 0 when rotating clockwise.
The issue could also come from the solder joints on the pot.
Hi,
I disassembled the unit and measured the pot from the PCB solder side, see picture: pins.jpg (https://1drv.ms/i/s!AiNnowB4pdeDhuRPHy3bT5cngkZPUQ?e=8jcgen)
Going by the pin numbers I put to the picture, I'm getting these readings:
Pot at 0 / turned all the way CCW
1-2: 16,3 Kohm
2-3: 0,8 ohm
1-3: 16,3 Kohm
Pot at max / turned all the way CW
1-2: 93 Kohm
2-3: 83 Kohm
1-3: 16,3 Kohm
So it seems my readings are way off from where they should be? Any suggestions moving forward?
Meanwhile, I'll try and see if I spot something suspicious on the PCB.
EDIT: Overall pictures from the front and back: Front (https://1drv.ms/i/s!AiNnowB4pdeDhuRQGItiXcvZCMIPbQ?e=Lxb9qm), Back (https://1drv.ms/i/s!AiNnowB4pdeDhuRRioL8PBC0QXuxNw?e=9TdTPh)
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It looks like a defective potentiometer.
Contact me here (https://www.soundskulptor.com/en/content/6-contact) for a replacement.
when you remove the potentiometer, do not try to save it, you risk damaging the pcb. Simply cut the pins and the bracket.
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Hi,
Received & installed the new potentiometer and the unit seems to be working normally now, thanks Jean-Pierre!