[finix]

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

[JPK]

Do you get the correct DC voltages (A to G) on the preamp?

[finix]

Yes I just rechecked and all DC are OK. Power: -16V, 16V, 5.5V. See the DC voltages below.

[JPK]

Check the potentiometer P1: center pin to each side pin going from 0 to 10k.
Check the DC resistance of primary and secondary of the input TX: 2k2 : 2k2
Check the connection between TX case and 0V
Check the solder joints of the transformer.

[finix]

1. P1: center to side go both from 0.1 ohms to 10.3 kOhms in log shape
2. coil A: 2.084 kOhms, coil B: 2.06 kOhms
3. input transformer case to 0V is 0.0 Ohms
4. solder joints are all OK

[JPK]

The noise could be caused by a HF oscillation. Check that C9 and C16 have good solder joints.
An oscilloscope could be useful here.
It could also come from the input transformer. It is a 1:1 ratio so the compressor could work without it. You can try to remove it and send an unbalanced signal to pin 8.

[finix]

So i found out what the problem is: It comes from switching PSU of the bento 6s. THe electrolytic capacitors are too slow to damp these ~MHz oscillations. I put fast foil capacitors extra between V-, gnd and V+ to get rid of the oscillations. Now it works with maximum 10mV noise. Thank you for your help.