Hej,
Przeklejam swojego posta z grupy dyskusyjnej transceivera QDX. Może wy coś podpowiecie...?
Schemat:
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.
- 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?