Hi!
I have a problem with a faulty PSU in the TV set (as in title) and honestly I'm stuck on it and could use a bit of advice if you don't mind…
The PSU's schematics is attached (psu1.png, psu2.png):
It produces +5VSB (standby), but doesn't produce any other voltage (12V, 24V, backlight). To test it, I connected +5VSB to S/B input (and to ENA input to disable the backlight).
After a bit of measuring I came to these conclusions:
1. +5VSB is produced and stabilized correctly (4.68V with S/B unconnected, 5.19V with S/B connected to +5VSB, is stable under load)
2. T904 (flyback transformer which produces +5VSB) has an additional winding to power the PFC, 12+24V converter and CCFL inverter (Vdd-S)
3. Vdd-S is only 4.78V, which is way too low (should be around 14-15V)
4. Disconnecting it (at Q907's emitter) and providing 14V externally causes the PSU to start and work properly
During my testing, I disconnected everything (except from C946) from D916's cathode, leaving only T904, R959, D916 and C946 in this fragment, and caught some oscillograms:
Cathode of D916 (voltage on C946):
- the voltage correctly goes up to around 15V
Anode of D916 (input of D916):
- we can see 15V pulses (OK)
Pin 4 of T904 (output of the transformer):
- we can see 15V pulses and it doesn't differ from the anode of D916 (OK)
…but it got more interesting when I loaded C946 with a 1kΩ resistor (which should draw around 15mA):
Cathode of D916:
- we can see that the voltage goes up (I think it's due to the less cycles being skipped by TNY277 because of empty C950 – if I understand the mechanism of regulation in TNY277 correctly), but quickly falls down to 2.5V
Anode of D916:
- positive pulses are strongly clamped
Pin 4 of T904:
- we can see proper positive pulses around 15V
R959 seems to be dropping the voltage because the duty cycle of these positive pulses is too low (too many cycles are skipped by TNY277, so positive pulses occur too rarely), but how can it be too low if +5VSB is regulated properly?
After loading the +5VSB output to skip less pulses, the voltage on C946 went up – loading +5VSB with 1A makes it go up to around 10V, which is still too low and I doubt this PSU needs over 1A to be drawn from +5VSB to work…
And now I'm stuck. I checked (and even blindly replaced) D916, R959, C946, but they seem fine. The duty cycle cannot be higher, because then +5VSB wouldn't be regulated.
What am I missing here? What's the trick?
I have a problem with a faulty PSU in the TV set (as in title) and honestly I'm stuck on it and could use a bit of advice if you don't mind…
The PSU's schematics is attached (psu1.png, psu2.png):
It produces +5VSB (standby), but doesn't produce any other voltage (12V, 24V, backlight). To test it, I connected +5VSB to S/B input (and to ENA input to disable the backlight).
After a bit of measuring I came to these conclusions:
1. +5VSB is produced and stabilized correctly (4.68V with S/B unconnected, 5.19V with S/B connected to +5VSB, is stable under load)
2. T904 (flyback transformer which produces +5VSB) has an additional winding to power the PFC, 12+24V converter and CCFL inverter (Vdd-S)
3. Vdd-S is only 4.78V, which is way too low (should be around 14-15V)
4. Disconnecting it (at Q907's emitter) and providing 14V externally causes the PSU to start and work properly
During my testing, I disconnected everything (except from C946) from D916's cathode, leaving only T904, R959, D916 and C946 in this fragment, and caught some oscillograms:
Cathode of D916 (voltage on C946):
- the voltage correctly goes up to around 15V
Anode of D916 (input of D916):
- we can see 15V pulses (OK)
Pin 4 of T904 (output of the transformer):
- we can see 15V pulses and it doesn't differ from the anode of D916 (OK)
…but it got more interesting when I loaded C946 with a 1kΩ resistor (which should draw around 15mA):
Cathode of D916:
- we can see that the voltage goes up (I think it's due to the less cycles being skipped by TNY277 because of empty C950 – if I understand the mechanism of regulation in TNY277 correctly), but quickly falls down to 2.5V
Anode of D916:
- positive pulses are strongly clamped
Pin 4 of T904:
- we can see proper positive pulses around 15V
R959 seems to be dropping the voltage because the duty cycle of these positive pulses is too low (too many cycles are skipped by TNY277, so positive pulses occur too rarely), but how can it be too low if +5VSB is regulated properly?
After loading the +5VSB output to skip less pulses, the voltage on C946 went up – loading +5VSB with 1A makes it go up to around 10V, which is still too low and I doubt this PSU needs over 1A to be drawn from +5VSB to work…
And now I'm stuck. I checked (and even blindly replaced) D916, R959, C946, but they seem fine. The duty cycle cannot be higher, because then +5VSB wouldn't be regulated.
What am I missing here? What's the trick?
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