Hi JPK, hi folks,
I got my cp5176 few months ago in used condition. An old model (pre 2017). I realized a lot of irradiation-like hiss (lots of harmonics of 50Hz and some strange modulated HF content). With input=10, output=10, no signal i get 500mV noise. In the forum I read that 2-3mV is normal with this setup. So i tried to track it down. No real success. Then I also modded the compressor following the newer schematic (with pnp transistor as 2nd amplifier stage) and new components to get rid of V+/V- noise (I got the Fredenstein Bento 6S) as well as possible bad solder joints or so. Same thing, 500mV, so this was not the problem. See: i did some tests to give you an idea. (Using newer schematic terminology)
1. bypassing R6 to get 0V to Q2 base -> silence (~2mV noise)
2. connect 0V to Q1 drain -> silence (~3mV noise)
3. connect 0V to TP1 -> 40mV noise
4. connect 0V to TP1 and remove Q1 -> 70mV noise
5. connect input transformer pins 5 and 8 -> 40mV noise
6. connect input transformer pins 1 and 4 to 0V -> 500mV noise
7. no mod -> 500mV noise
So it seems to come in at the input amp. I also checked the feedback line (via R9, R14). Signals go through the compressor as expected. Its just the noise. Do you have an idea what the problem might be, is this a known issue or shall i do some more tests?
Thanks so much for your help.
Generally I'm not against a new try with a new kit, I would like to make sure that I dont run into same problems again.
Finix