CRT TV repair, power supply defect.

[Vernon Paul]

I have a phillips 29 inch tv(model PT4423) which lost power suddenly.

After replacing a shorted chopper transistor in the switching
circuit and two open resistors the TV came back to live with another
problem.

When ever a picture is pumped into the TV's AV sockets the picture
shrinks vertically before shutting down. Further attempts to on the tv
after that is not possible, unless the power is recycled. I've replace
the main filter capacitor, main bridge rectifier, output regulator,
PWM ic and a few zener diodes but the problem is still there. Next I
plan to replace the opto-coupler.

What do you think is the problem? Could there be a problem with the
deflection circuit?

Thanks-a-million.

[Jamie]

You say chopper transistor? Is this a Thyristor, Mosfet or a bipolar
transistor?

In any case, it almost sounds like you have insufficient power being
generated.

Did you use exact replacement parts or SUBS like NTE ?

Also, is the circuit board brown from over heat in that area?

Jamie

[Vernon Paul]

On Sep 15, 9:21 am, Jamie

Jamie

the chopper transistor is a MOSFET, The exact parts were used in the
replacement.

> In any case, it almost sounds like you have insufficient power being
> generated.

but almost everything in the power supply circuit has been replaced.
I suspect the mosfet has taken out one or two more components in the
switching circuit that I'm missing or could there is a problem with
the defection circuit?

[Phil Allison]

"Vernon Paul"

>
> When ever a picture is pumped into the TV's AV sockets the picture
> shrinks vertically before shutting down.

** And using the sets own tuner - what happens ??



.... Phil

[Vernon Paul]

the set shuts down as well

[Phil Allison]

"Vernon Paul"
"Phil Allison" :

> "Vernon Paul"
>
>
>
> > When ever a picture is pumped into the TV's AV sockets the picture
> > shrinks vertically before shutting down.
>
> ** And using the sets own tuner - what happens ??
>

the set shuts down as well

** Suspect a problem associated with the deflection yoke.

Maybe a shorted cap, VDR or similar.

Or the dreaded yellow glue .......



.... Phil

[Vernon Paul]

Hmm...since there is picture for a few seconds before it shuts down I
don't think its the yoke.

Sorry but what is VDR?

[Phil Allison]

"Vernon Paul"
"Phil Allison"
"Vernon Paul"
> "Phil Allison" :
>
> > "Vernon Paul"
>
> > > When ever a picture is pumped into the TV's AV sockets the picture
> > > shrinks vertically before shutting down.
>
> > ** And using the sets own tuner - what happens ??
>
> the set shuts down as well
>
> ** Suspect a problem associated with the deflection yoke.
>
> Maybe a shorted cap, VDR or similar.
>
> Or the dreaded yellow glue .......
>

Hmm...since there is picture for a few seconds before it shuts down I
don't think its the yoke.

** You need to lose attitudes like that if your ever expect to fix
anything.

Sorry but what is VDR?

** Voltage Dependant Resistor.



... Phil

[Franc Zabkar]

On Wed, 14 Sep 2011 17:56:26 -0700 (PDT), Vernon Paul
<vernon...@gmail.com> put finger to keyboard and composed:

>I have a phillips 29 inch tv(model PT4423) which lost power suddenly.
>
> After replacing a shorted chopper transistor in the switching
>circuit and two open resistors the TV came back to live with another
>problem.
>
> When ever a picture is pumped into the TV's AV sockets the picture
>shrinks vertically before shutting down. Further attempts to on the tv
>after that is not possible, unless the power is recycled.

I'm not familiar with that chassis, but you didn't say whether the B+
was OK.

Another thing to check would be the components in the line stage, eg
the tuning capacitor.

I'd also verify the secondary loads on the FBT, perhaps by monitoring
the current in the fusible resistors.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.

[Vernon Paul]

On Sep 15, 12:07 pm, Franc Zabkar <fzab...@iinternode.on.net> wrote:
> On Wed, 14 Sep 2011 17:56:26 -0700 (PDT), Vernon Paul

> <vernondop...@gmail.com> put finger to keyboard and composed:

Thanks Franc,

will check the B+

I think its the L7.3 A chasis since the service manual downloaded from
the net seem to match, but there is no marking of the chasis number on
the PCB.

[Jamie]

It's possible.

On the flyback you most likely have a B+ or some may call it a C+
since this source is derived from a tap on the fly back.

It's common to monitor one of these sources for a quick shut down,
because it covers the flyback area problem and supply going to it. How
ever, caps do go bad here and the ripple being seen could be just enough
to shut it down when the circuit gets loaded.

This is a good time to use your scope, if you have one. First you
test the gate signal to the MosFet you replaced. My guess is it should
be a PWM type signal and should vary when load varies. You want to see
what the duty cycle is. what I have done in the past to test for this is
to put a low load R on the output of the supply while I am watching
the scope to see if the duty system increases to maintain output. Of
course, you could also just simply watch the output with a DMM as you
alternate your external load R.

While you're probing around with the scope, see how clean the signal
is at the switching supply's output after rectification and filtering.

I've seen bad UF switching type diodes/rectifiers go bad, they test ok
with a meter but have slow recovery issues under operation. Make sure
you replace these types with equally fast units. A good indication of
this is one operating hot.. This is a good time for a hand held IR meter. :)

Next, check the Secondary supplies coming from the flyback for any
ripple around 15khz.. You shouldn't see much at all. if you do, it could
be bad caps or slow recovering diodes.


P.S.
since the original MosFet was shorted, have you checked to make
sure the switching transformer leads are well soldered in? This was a
common problem back then and depending where the lead was that had a
cracked joint, it could cause intermitting back EMF on the MosFet and
short it.

Also, you may want to test for the Drain voltage and signal via the
scope to make sure you are not getting any over voltages that look like
spikes.

That should give you enough to work on.

Jamie

[Vernon Paul]

On Sep 15, 11:13 pm, Jamie

Wow Thanks Jamie,

very informative post. Will check the things you mentioned and let
you know the outcome.

Vernon

[Arfa Daily]

"Jamie" <jamie_ka1lpa_not_v...@charter.net> wrote in message
news:xrocq.23161$oz6....@newsfe11.iad...

The only thing that I would add is to be very VERY careful when you start
waving a 'scope around the primary side circuitry of a switcher. Make very
sure that you know where your ground reference is, as it is very likely that
the ground lead on your probe will be common with the 'scope's chassis,
which will in all likelihood, be bonded to the earth of the line power
feeding it. The bang that this can produce, and the trail of catastrophe
that ensues, can be spectacular ...

Arfa

[Phil Allison]

"Arfa Daily"

>
>
> The only thing that I would add is to be very VERY careful when you start
> waving a 'scope around the primary side circuitry of a switcher. Make very
> sure that you know where your ground reference is, as it is very likely
> that the ground lead on your probe will be common with the 'scope's
> chassis, which will in all likelihood, be bonded to the earth of the line
> power feeding it. The bang that this can produce, and the trail of
> catastrophe that ensues, can be spectacular ...

** It is ESSENTIAL to use an isolation transformer in the AC supply for
such work.

Then you can ground the negative side of the main filter caps and work as
normal.



.... Phil

[jango2]

On Sep 18, 11:57 am, jango2 <crow_slap...@yahoo.co.uk> wrote:
> What happens if you use a video source that's portable, runs of
> batteries, and isn't connected to mains or anything else (such as a
> portable dvd player)?

off*

[Vernon Paul]

An update on my repair situation. I decided to follow all the
instruction gathered here and some from Sam, beginning with the
easiest first. This is what I got. By reducing the G2 and brightness
all the way down the TV works fine, no shutdown. Even with a decent
brightness and G2 cranked up and decent picture available, it does not
shutdown. However when there is some bright scene there is signs of
tendency to shutdown. When the the brightness and G2 is cranked up too
much it shutdowns. The voltages from the power supply seems to be
stable during the shutdown, not sure about the currents though.

Is the problem more likely in the power supply or deflection side?

[Franc Zabkar]

On Wed, 21 Sep 2011 21:00:20 -0700 (PDT), Vernon Paul
<vernon...@gmail.com> put finger to keyboard and composed:

Is there an ABL (automatic beam limiter) circuit? I have seen ABL
failures that have caused shutdown.

[jango2]

On Sep 22, 12:36 pm, Franc Zabkar <fzab...@iinternode.on.net> wrote:
> On Wed, 21 Sep 2011 21:00:20 -0700 (PDT), Vernon Paul

> <vernondop...@gmail.com> put finger to keyboard and composed:

Sometimes called x ray protect.