DIYbanter - View Single Post - Samsung TXB-1930 TV not starting reiliably

lsmartino

ha escrito:

This is a cheap 19" with a 1994 date code. It has recently become
unreliable when turning it on. Sometimes, it comes on just fine.
Sometimes, there is no picture, no sound, no HV crackling, and no
power-on thump (from the flyback?), but there is a relay clicking
repetitively at about once per second. It produces the same symptoms
with both the remote control and the power switch. Once it starts
clicking, it can take many attempts to stop it with either the remote
control or the power switch. I would have guessed it was a problem in
part of the power supply. Does anyone have the schematic for this TV,
particularly the power supply and the remote control? Samsung says
they no longer have it available. Is this the right area to be
looking? Are there any other likely common problem areas to check?

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!