Magnavox 25TR12 C102 Chassis 25P606-00AA wont power off

[GoodFeeling]

I am working on a Magnavox 25TR12 C102 (Chassis 25P606-00AA). Originally had
a problem where the set wont power on, had low standby voltage. Replaced
C432 (known problem) and fixed cold solder on 2 res next to the cap. Now,
the set automatically powers on when plugged in. It didnt do it right after
the replacement, so they may not be related. Nevertheless, I cant get it to
power off. Tried checked low voltage points but seems to be fine when its
on, guess the voltage must be backfeeding. Anyone have this issue before?
Thanks in advance.

Tim

 

[kip]

Check the relay ...!!!!

kip

 

[GoodFeeling]

Relay? How do I get there?

 

[kip]

Ok Take it to a repair shop..
kip

 

[GoodFeeling]

I thought you were referring to the news relay, not the electronic
component. I didnt imagine someone actually recommending such a fix. As far
as the problem, the relay is working fine, I have had a low voltage problem
and would like to know if there is a way to force the TV off so I can check
my voltage points.

 

[sofie]

GoodFeeling:
AGAIN..... check the relay (electronic component in television).
Make certain that it is de-energizing and that the contacts actually open
up.

 

[RonKZ650]

Well, if the set uses the 100uf 200v cap for C432, it doesn't use a
relay. The STR30130 regulator IC is switched on/off electrically. You
should always have about 150vdc on pin 3 and the output on pin 4 sits
about 15vdc when set is off and 130vdc when on. It's possible this IC
is bad, and also possible if C432 was open to have burnt resistors near
the IC.

 

[kip]

Well for your info Relays do stay stuck on..Very common.
Next time let us know what you have done in the way of diagnosing
so we wont reply with a stupid suggestion.

kip

 

[Chuck]

I am working on a Magnavox 25TR12 C102 (Chassis 25P606-00AA). Originally had
a problem where the set wont power on, had low standby voltage. Replaced
C432 (known problem) and fixed cold solder on 2 res next to the cap. Now,
the set automatically powers on when plugged in. It didnt do it right after
the replacement, so they may not be related. Nevertheless, I cant get it to
power off. Tried checked low voltage points but seems to be fine when its
on, guess the voltage must be backfeeding. Anyone have this issue before?
Thanks in advance.

Tim

If this is the chassis that has Q410 next to the regulator ic, check
to see if it is shorted. Chuck

 

[GoodFeeling]

RonKZ650,

Thanks for the response, here is what I have come up with. I checked the
voltage at the regulator IC, and on pin 4, I have a little over 130vdc, when
the set is off (the set doesnt turn off, the display just goes a little dark
and the sound goes off), then the voltage turns down to around 120vdc on
pin 4. Does this sound like a bad reg IC or could it be another component
around it. I really need to understand what is actuallly dropping the
voltage, is it the Reg IC or should the input drop? Thanks for your help.

 

[GoodFeeling]

Chuck,

There is a Q410, but not right next to the reg IC, I will check if it is
shorted though, thanks!

 

[GoodFeeling]

Checked Q410, it tests fine.

 

[GoodFeeling]

Update: Found that R432, a 1000 ohm resistor was open, so I replaced it.
Turn on the set and it smoked almost instantly. This res goes right to pin 2
of the IC. Does I am guessing that means the IC is bad. Does that sound
correct?

 

[nvic]

I agree, the contacts could be stuck closed (welded, open coil, etc.)

 

[RonKZ650]

Q400 is the one that sits right next to the IC, make sure it's not bad,
which is possible. If it's good then I think your regulator STR30130 is
bad. A burnt 1k resistor to pin 2 should turn off the IC, so if it's
still putting voltage out on pin 4 it's bad it looks like from my
perspective. The way it works is the input voltage (pin 3) stays a
constant aprox 150-160vdc regardless of whether the set is on or off.
Pin 2 is your on/off control, it sits around 15v when set is off along
with the output on pin 4. When pin 2 is turned high when the set is
turned on, pin 4 follows and outputs your 130v supply.

 

[Bill S]

Check turn-off circuit. Usually just burns up R432 1K 1/2W,
but occasionally propagates further into D421, Z405, Q402.

 

[GoodFeeling]

Hi Bill,

In the schematic, it says I should have 134vdc on the base of Q420, I have
160vdc. Is that to much? or is that normal? Thanks.

 

[GoodFeeling]

Replaced STR30130, still toasts R431...

 

[GoodFeeling]

New information: If I replace R432 and turn on the TV, it wont smoke... as
soon as I try to turn it off, the screen dims and R432 starts to get hot and
will smoke.

 

[masterteks]

Check/change the optocoupler ic346, check r432 may be open,c431,q400