Hi, I'm having problems to get this board to work. The board is a rsag7.820.2194, schematic attached.
After replacing this blown components: V810 (20N60) - R820 R821 (0.22/3W) - C915 (470uF/16V) and finally the fuse F801 (6.3A/250VAC), I've plugged it to my isolation transformer and I'm getting 5VS out of pin 11 from connector XP805, so far so good, but when I bridge pin11 (5VS) with pin 12 (STB), to make the psu to start up, nothing happens. At the cathode of diode VD903 (FR104) I'm getting 18V instead of 20V.
I've checked the optocouplers, the V831 V832 (10N60) and the output rectifiers and they're all fine.
After the mains bridge rectifier, I'm getting 330V instead of 300V, as marked in the schematic.

Any ideas of what could be happening??

Thank you very much.

 

Does VCC go to about 18 volts when you link 5 volts to STB ? Are you getting around 300 to 400 volts at PFC ? (This powers the main inverter.)

Les.

 

Hi Les, before linking 5V to STB, the VCC pin 5 of N901 measures 18V, after linking 5V to STB, it goes down to 15.3V and the VCC of ICs N811 (NCP33262) and N831 (NCP1396) goes to 13.7 volts, instead of the 20 volts listed on the schematic.
PFC measures 326 volts.

Thanks for your time.

 

The VCC I was looking at was the one on the emitter of V903 (I don'y know if this is the same at pin 5 of N901.) I was expecting it to go from near zero volts before 5 volts was connected to STB to about 17.4 volts when it was connected. I am assuming it is this VCC that causes the main part of the power supply to work. I think 326 volts on PFC is about right. (I have not worked on any power supplies with power factor correction.)

Les.

 

Les; the VCC on the emitter of V903 is the same at pin 5 of N901

 

My simple understanding of how VCC was controlled by the standby signal must be wrong. I can't work out how the main power supply part is controlled.

Les.

 

How is your smoothing caps?

 

How is your smoothing caps?

+1
And the rest of caps.

 

Hi,

Is this a brand new board or an old board you are trying to fix?

 

Hey guys, it is working now!!
The pin 5 of IC N831 (NCP1396) it's getting 0.86V from the voltage divider, but as the VCC is a bit lower than it should, I had to do something I do not think right.
This is what pin 5 does

The Brown--Out circuitry (BO) offers a way to protect the resonant converter from low DC input voltages. Below a given level, the controller blocks the output pulses, above it, it authorizes them. The internal circuitry, depicted by Figure 44, offers a way to observe the high--voltage (HV) rail. A resistive divider made of Rupper and Rlower, brings a portion of the HV rail on pin 5. Below the turn--on level, the 26.5 mA current source IBO is off. Therefore, the turn--on level solely depends on the division ratio brought by the resistive divider.

I've replaced the 8k2 resistor from the voltage divider whit a 10k so now the output of the voltage divider is 1V instead of 0.86, and now the circuit is running, but I'm still concerned for the low VCC (14V).
The output voltages are stable and working just fine. The TV is back to life, I do not know for how long

Thank you very much for your time guys

 

Hello again,

I asked if this was a repair because if it was it could be a bad cap or two. The electrolytic caps in power supplies are known to fail and that affects the voltage getting to the rest of the circuits. If you have one or more bad electrolytic caps that could be what caused the problem in the first place. Replacing them would be the best solution if that is the case, but happy to hear you got it working anyway.

One of my TV's had bad caps so it would not turn on most of the time. After replacing the caps, the TV worked fine ever since then (maybe a year or two ago now).
Also one of my computer power supplies would shut off by itself for no apparent reason. After changing the caps it worked fine again.

In both cases the caps were checked and it was found that the ESR went way up and the capacitance went down considerably in the main electrolytic caps. After changing that solves both of those issues so it works again.

LATER:
I just checked my notes and found that the TV had been done about 3 years ago and the computer power supply almost 8 years ago. The TV was in constant use since then but not the power supply, but when the power supply was used the computer did not shut off anymore for no reason.

 

Hi MrAl, I've replaced the caps too, but there were no changes on VCC.
Anyway thanks for your help.

 

Hi MrAl, I've replaced the caps too, but there were no changes on VCC.
Anyway thanks for your help.

Hello again,

Oh ok, too bad, i guess this one will remain a mystery then unless you care to go digging a little more.