Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

[angelos]

Hi Guys,
This one is a real head scratcher. My +5V STBY is fluctuating. The PFC circuit does not operate but is also pulsing. I dont get +400V on my mains smoothing cap but rather 330V DC which is correct since our incoming supply is 220V AC.
I have narrowed it down to a PFC circuit problem but dont have the spares to repair it without having to wait months for my spares to arrive. I suspect that the OB2262 chip which operates the PFC circuit might be to blame but i dont measure any funny resistance measurements on that 6 pin chip with regard to ground. I have scoped my output from the OB2262 and what i see is a series of pulses followed by nothing and then it starts all over again. I even measured on the collector of the power mosfet Q6 that drives the PFC coil and there is a large pulse (x100 probe used). What i noticed is that the Vcc supply to OB2262 is fluctuating from 22V down to about 12V. That is pin 5. I have added the circuit diagram for the PFC for you to check as well as top and bottom images of the power supply. The power supply is a IPS150.

Would you also suspect the OB2262 chip? Or is there aother test i can do to prove it?

[jetadm123]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

I suggest you first try replacing the startup cap on pin 5. It's probably something small like 22-47uf. From your schematic, it appears to be the cap to the left of zener diode Z4.

[angelos]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

Yes, i am not in front of the tv right now but if i did not replace it then i firstly measured the capacitance and it was spot on and then i measured its ESR and it was 1.2 ohm which is also spot on.
Just of note, i forced the power supply on (5V to STB pin) and then my mains smoothing cap voltage went to 420V DC but was fluctuating slightly as well as the other voltages. Does this mean that the PFC circuit only activates when Standby-on is activated?
Where should i look for fault???

[angelos]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

I did a search on this forum on the IPOS 150 power supply and found a lot of information especially about a round diode on a heatsink that was shorted causing the PSU to trip. I never saw this diode so i cant wait for tomorrow so that i can be in front of the set and sift thru the info.. will post a update.

[yohnsee]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

I suggest you first try replacing the startup cap on pin 5. It's probably something small like 22-47uf. From your schematic, it appears to be the cap to the left of zener diode Z4.

Hello,
it may sound old, but I would change all the output caps without hesitation, under that weird looking heatsink-cage. I once repaired a Schaub-Lorenz TV, had the same odd placement of caps.
What were the engineers thinking? "Yo, let's put the caps where they get dried out the fastest. Maybe under this hot aluminium? - yeah, that's a good idea" -clap, clap.

Edit: there can be many reasons, why a PFC doesn't start. Of course, it is often caused by component failure on the hot side, but in some cases a short on the cold side can cause a shutdown, due to overload. A quick measurement from cold side supply lines to GND can answer this question.

[R_J]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

A lot of power supplies don't operate the pfc in standby mode, only when the power on command is applied does the pfc operate
The OB2262 does operate in burst mode in standby which would explain your scope reading
The cap I would change would be EC5 47µf/50v
Q5 supplies VCC to the PFC circuit to turn it on, If the 18 volts on EC5 is fluctuating that could cause the pfc circuit to start up once and a while.

[angelos]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

Hi Guys,
It was that round diode on the secondary, a HER304 it was short circuited. Replaced it and power supply started up. Yes, i will replace the caps that were not previously changed to avoid further issues. Regards....

[nomoresonys]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

Nice fix.

[yohnsee]

Re: Sansui 37" LCD TV Model SV-3715TLCD Standby voltage tripping

Good job. Like to hear such easy fix once in a while.