Hej,

Przeklejam swojego posta z grupy dyskusyjnej transceivera QDX. Może wy coś
podpowiecie...?

Schemat: https://www.qrp-labs.com/images/qdx/schem3.png

Subject: [QRPLabs] My QDX went almost deaf (something's wrong with op-amp bias?)

Hi,

Like in subject -- one day my QDX worked, the other day it went almost
deaf. I can receive only strongest signals, and the signal level is very
low even with high gain settings.

Audio filter sweep (done with the dummy load) shows USB at -46 dB.
Previously (when it worked) it was -14 dB.

https://ibb.co/qNLGrsB

I measured DC voltages on IC7 and IC9 amplifiers. All inputs are around
0.895 V and all outputs are around 0.815 V. Precisely:

- IC7A in+ (pin 3): 0.895 V
- IC7A in- (pin 2): 0.897 V
- IC7A out (pin 1): 0.815 V

- IC7B in+ (pin 5): 0.894 V
- IC7B in- (pin 6): 0.896 V
- IC7B out (pin 7): 0.815 V

- IC9A in+ (pin 3): 0.893 V
- IC9A in- (pin 2): 0.896 V
- IC9A out (pin 1): 0.815 V

- IC9B in+ (pin 5): 0.892 V
- IC9B in- (pin 6): 0.896 V
- IC9B out (pin 7): 0.815 V

I guess they should be more around 2.5 V...

Voltage supply is correct:

- IC7 vcc (pin 8): 4.994 V
- IC7 gnd (pin 4): 0 V

- IC9 vcc (pin 8): 4.994 V
- IC9 gnd (pin 4): 0 V

IC10 (PCM1804) gives 2.509 V on both VCOML and VCOMR outputs. This voltage
drops on R39 (and R36) -- after R39 it's 2.009 V.

Capacitors C71 to C74 are not shorted. Their inputs (from IC4 side):

- C71: 0.353 V
- C72: 0.356 V
- C73: 0.355 V
- C74: 0.352 V

I suspected that C71...C74 might be shorted only partially, but they don't
show anything on the meter (it has 60 Mohm range).

I also suspected that op amp inputs might be partially shorted to ground,
but there's around 5.8 Mohm between their inputs and ground.

Does anyone have any other tips on what could have failed?

--
- Czy córka przedstawiła wam już swego przyszłego męża ?
- Ona jest bardzo nieśmiała. Na razie przyprowadza go do domu, tylko
wtedy gdy nas nie ma.