lsmartino wrote:
I had the same problem with a TXB-1940 Samsung TV which uses the same
chassis as yours. It was caused by Q902 (KSC1507 or NTE198), C974
(electrolytic 10uF x 25V) and C970 (electrolytic 100uF x 25V). Q902 is
part of the standby power supply. It takes 151V from the main filter
cap of the PSU, and reduces it to 12.5 V. Samsung designers in their
infinite wisdom decided to leave that transistor without a heatsink,
which makes the transistor to work very hot and to overheat each time
the TV is off *and* plugged to the AC. Over time, the electrical
characteristics of the transistor changes and it can´t deliver the
amount of power needed to sustain the micro while making the relay to
turn on as soon the user press the Power On button. This causes the
realy to cycle as you noted. Once the TV manages to turn on, the
standby power supply is overriden and the TV starts to operate ok.

It would also be a good idea to check and replace R902 (5.6 K), R905
(2.2 K), R901(5.6 K) and R903 (68K). These resistors work very hot
while the TV is at standby and they probably have shifted in value too.
The schematic calls for 2 Watt resistors, but I replaced them by 5 Watt
ones. It was a bit tricky to mount them, but they produce a lot less
heat than the factory originals, and that helps to preserve C970 too.

Overall, that TV seems to be produced to self destruct after several
years of use. Critical transistors aren´t properly heathsinked, and
because of that they become unrelieable. Check also Q603 (KSC2073) and
Q602 (2SA940) for bad solder joints. They are the output pair of the
sound amplifier, are not heatsinked, and they work *hot*. You will see
the PCB darkened around them and the solder joints gray dull. Resolder
them if needed.

KSC1507 is not a critical component, so you can use an equivalent part.
I strongly suggest you to heatsink it. I did that with my TV and I
haven´t had a single problem since then.

Good luck!

I replaced Q902 with an MJE2361 on a heat sink (it was the only NPN HV
power transistor my local electronics store had in stock) and also C970
and C974, as you recommended. That seems to have fixed it! Thank you.

This TV does not see much use, so there was a lot of blackening under
R901/R902/R903/R905 and none under Q602/Q603 (consistent with your
explanation). All of these resistors are still in spec, so I did not
replace them (even though I had purchased 5W replaceements). On my
chassis, R905 is 1.0K by measurement and by reading the color code.

The quality of your reply gave me the confidence to purchase the parts
before even opening the TV. Your analysis showed that you clearly
understood the circuit and the failure mechanism. From your
description, I could easily guess at a probable circuit diagram. Even
if I had had a circuit diagram, it would have taken serious
troubleshooting for me to isolate the problem. This is the first time
that I remember fixing a failure due to a partially degraded
semiconductor component. My most common failures are due to bad
electrolytics or completely zapped semis.

Again, thanks.