RevennaFox
- Sep 6, 2020
- 1
- Joined
- Sep 6, 2020
- Messages
- 1
I'm working on an old 19" Samsung TV from 1992, after cleaning and replacing the electrolytic capacitors I'm still having trouble getting it to power up. Like most tube TVs it has a fail safe circuit to shut down the set to prevent excess X-ray emissions and damage to the flyback transformer. Hitting the power button, the TV turns on and I get a raster on screen but after about 2-3 seconds the fail safe circuit triggers and shuts down the set.
So I start by checking voltages, on turning the set on the anode voltage raises to about 40kV(!!!) before the fail safe circuit trips. The anode voltage is supposed to be 27kV. Checking farther back in the chain I look at the input to the voltage regulator Q801 that supplies 125V to the flyback transformer. Input pin is supposed to be 154VDC, I'm reading 385VDC. Something is really messed up here.
Now I start checking the value of the resistors in the power supply. Since the low voltage stuff (everything from D901 down) seems to be OK I'm ignoring it. I checked all of the power resistors around the voltage regulator; R802, R804, R805, R806, R807, all within the specified 5%. Diode D802 checks out as well.
At this point I'm starting to wonder if it's possible that high voltage is leaking back into the power supply from somewhere else, like in the deflection circuitry. I've been staring at the schematic for a couple of hours but nothing really jumps out. I have a couple of questions here. Is this possible? Is it possible that the flyback transformer is shorted? If I replace the flyback, will the replacement be damaged if the problem is elsewhere?
Forgive my ignorance on this subject, I've only done a few TVs, and I usually have the opposite problem. Schematic/service manual is attached, I also have the factory service manual from Samsung and I'll upload if asked, bu the Sams' version is a little easier to read I think.
So I start by checking voltages, on turning the set on the anode voltage raises to about 40kV(!!!) before the fail safe circuit trips. The anode voltage is supposed to be 27kV. Checking farther back in the chain I look at the input to the voltage regulator Q801 that supplies 125V to the flyback transformer. Input pin is supposed to be 154VDC, I'm reading 385VDC. Something is really messed up here.
Now I start checking the value of the resistors in the power supply. Since the low voltage stuff (everything from D901 down) seems to be OK I'm ignoring it. I checked all of the power resistors around the voltage regulator; R802, R804, R805, R806, R807, all within the specified 5%. Diode D802 checks out as well.
At this point I'm starting to wonder if it's possible that high voltage is leaking back into the power supply from somewhere else, like in the deflection circuitry. I've been staring at the schematic for a couple of hours but nothing really jumps out. I have a couple of questions here. Is this possible? Is it possible that the flyback transformer is shorted? If I replace the flyback, will the replacement be damaged if the problem is elsewhere?
Forgive my ignorance on this subject, I've only done a few TVs, and I usually have the opposite problem. Schematic/service manual is attached, I also have the factory service manual from Samsung and I'll upload if asked, bu the Sams' version is a little easier to read I think.
