[smas22]

Hi! i just completed my 502 and everything seems to be working(powers on, passes signal, compresses etc). When i went to set up the unit, i could not read 26V gain at 100% gain and could only muster 12.8 V between pins 2 and 3. I tested the points listed here in the original forum post and all the readings were correct. My suspicion is there is a faulty chip or something is up with my dmm or 500 rack ( Cranborne 500r8 ). What do you think the issue is? Thanks!

[JPK]

First you must make sure you have a real 1VAC on your input. It must be checked with the compressor connected.
The total gain is defined by R6-R7. Check you have the correct resistors here.

[smas22]

Hi! I measured 1VAC on the output while testing. R6 and 7 are both the correct value.

[JPK]

Do you get the 10V+ on TP1 in the last point of 3. Try to follow the signal
https://www.soundskulptor.com/forum/index.php?topic=19.msg21#msg21

[smas22]

Yes i get 10V+

[JPK]

The signal is good on the output TX primary.
Can you measure it on the secondary, across R17 ?

[smas22]

I may have discovered an issue. I wasn't getting any voltage across r17 and i checked the resistor and it is supposed to be the 1k .5W but i am only measuring a 22 ohm resistance across R17. The resistor is labelled as a 1k though. is it faulty?

[JPK]

The resistor is in parallel with the secondary of the transformer. So you are actually measuring the transformer.
Check the transformer solder joints.
Measure the transformer primary resistance : ohm-meter between TP1 and 0V : 5.5 ohms

[smas22]

i reflowed the solder joints on the output transformer, i read a resistance of 4.6 to 5 ohms 

[JPK]

Redo the setup guide step #5 but do the voltage measurement across R17 instead of the output.
Also you can try to use a lower than 1kHz frequency, like 100Hz, because some voltmeters cannot go very high in frequency.

[Florian]

Hi,

I just finished my build of the 502 and I have exacty the same issues until here. Exact same readings as smas22. The output won't go over 12.7V max.

I measured the voltage over R17 and it is 26.8V when gain is at 100. But against 0V it ist 12.3V before and 12.6V after R17. Also, R17 gets hot very quickly at full gain and there is a hearable ringin tone coming from the transformator.

Dumb question, now that I can't measure the resistance of R17: is it 1k (bn bk bk bn bn) or 110ohm (bn bn bk bk bn)?

[JPK]

The output measurement must be done between the hot and cold outputs (XLR pin2 & 3), not against 0V because it is floating.
If you get the 27V across R17 then you are OK. You can do the setup by measuring 15VAC here. It is the same as the output.
It is normal that the resistor gets hot when sending 27V because it makes 0.73W dissipation.

[Florian]

Oh bummer, I measured between tip and shaft of the output jack, instead of tip and ring. Thanks für the heads up, Jean-Pierre.

I've now successfully calibrated the unit and it sounds awesome! I just noticed quite a bit of noise on the release phase on percussive sounds. Is that to be expected on opto compressors (it's my first one)?

[JPK]

This compressor has noise but it can be minimized by working at higher levels.
If you can, set your AD/DA convertors to work at higher level that the basic +4dBu (we set ours to work at +24dBu).