I suspect this voltage regulator is bad, (came out of a Zenith TV that
is dead, no attempt to power up), since I have B+ voltage on the
primary of the switching transformer, and nothing on the secondary. I
have the datasheet for the IC, but looking at it can't figure out how I
can do simple DMM check on it, rather than repalce and hope. Anyone out
here have a sugestion on how to test to determine if this part is
actually bad? Thanks all.
P.S. Just a beginner here.

On 17 Oct 2005 07:05:56 -0700, put finger to keyboard
and composed:

I suspect this voltage regulator is bad, (came out of a Zenith TV that
is dead, no attempt to power up), since I have B+ voltage on the
primary of the switching transformer, and nothing on the secondary. I
have the datasheet for the IC, but looking at it can't figure out how I
can do simple DMM check on it, rather than repalce and hope. Anyone out
here have a sugestion on how to test to determine if this part is
actually bad? Thanks all.
P.S. Just a beginner here.

IME the most common failures in these types of regulators are CE
shorts in the output transistors.

Test for a SC between pins 2&4 and 3&4. You should measure two diode
drops in one direction, and OC in the other. If you have a zener
tester, then do a zener test between pins 5 and 1.

-- Franc Zabkar

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

wrote:
I suspect this voltage regulator is bad, (came out of a Zenith TV that
is dead, no attempt to power up), since I have B+ voltage on the
primary of the switching transformer, and nothing on the secondary. I
have the datasheet for the IC, but looking at it can't figure out how I
can do simple DMM check on it, rather than repalce and hope. Anyone out
here have a sugestion on how to test to determine if this part is
actually bad? Thanks all.
P.S. Just a beginner here.

You need to start by posting the complete model number and year of
manufacture. In Zenith sets depending on the year and picture tube in
use, the picture tube will short and cause catastrophic failure of the
power supply components, in other years there are simply two high esr
capacitors which will cause a dead set.

Odds are since you did not specify any burnt resistors, blown fuse or
transistors, that it is the one with the bad capacitors.

As to testing it, you need to verify that it has the main b+, start up
voltage, starts to oscillate, and continues to oscillate and regulate
in circuit. If it shorts the fuse will be blown.

David

I'm not at home, so I don't have the model # with me, but the main fuse
was found blown. I replaced it, but nothing happened at power up. I'll
check again, but I'm pretty sure I found B+ on the regulator, and
primary of the switching transformer as said. I'm assuming it isn't
oscillating since there's nothing on the secondary. Am I thinking about
this logically? This may be a no-brainer question for most of you, but
I have a new Fluke meter with frequency measurement that I haven't used
yet, just got it. Is this how I would check to see if the regulator is
oscillating? Thanks to all that have replied. Great info.

On 18 Oct 2005 03:13:58 -0700, put finger to keyboard
and composed:

I'm not at home, so I don't have the model # with me, but the main fuse
was found blown. I replaced it, but nothing happened at power up. I'll
check again, but I'm pretty sure I found B+ on the regulator, and
primary of the switching transformer as said. I'm assuming it isn't
oscillating since there's nothing on the secondary. Am I thinking about
this logically? This may be a no-brainer question for most of you, but
I have a new Fluke meter with frequency measurement that I haven't used
yet, just got it. Is this how I would check to see if the regulator is
oscillating? Thanks to all that have replied. Great info.

Check whether the startup resistor, Rs, (page 3 of the datasheet) is
OC:

http://www.cpcares.com/pdf/STR53041.PDF

The datasheet also shows that a winding of the flyback transformer is
used to synchronise the PSU's oscillator to the horizontal frequency.
Is the FBT OK? Have you checked the HOT?

-- Franc Zabkar

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

I checked page 3 of data sheet, but not sure what the startup resistor,
R's is. Page 3 was a test circuit. I have checked pin 1 and 5 and have
resistance. I also checked between pins 2/4, 3/4 as you suggested. Pin
2/4 seems to be OK, Pin 3/4 is infinite resistance both directions.
Since pin 3 is the input, I am again assuming this IC is bad. I checked
the HOT when I first opened the set and it's good. I haven't checked
the flyback yet. The model # of the set is C25A24T. Would you agree
with my findings? Thanks alot.

"Golf" wrote in message
ps.com...
I checked page 3 of data sheet, but not sure what the startup resistor,
R's is. Page 3 was a test circuit. I have checked pin 1 and 5 and have
resistance. I also checked between pins 2/4, 3/4 as you suggested. Pin
2/4 seems to be OK, Pin 3/4 is infinite resistance both directions.
Since pin 3 is the input, I am again assuming this IC is bad. I checked
the HOT when I first opened the set and it's good. I haven't checked
the flyback yet. The model # of the set is C25A24T. Would you agree
with my findings? Thanks alot.


Checking the resistance of pins on the IC won't do you a whole lot of good,
check for voltage at them and compare it to what should be there.

On 18 Oct 2005 15:20:36 -0700, "Golf" put finger to
keyboard and composed:

I checked page 3 of data sheet, but not sure what the startup resistor,
R's is.

One end is connected to the + terminal of the main filter capacitor
(200V or 400V), the other end connects to pin 2 of the regulator. This
resistor kickstarts the oscillator. Take care to discharge the main
filter cap when probing in this area.

Page 3 was a test circuit. I have checked pin 1 and 5 and have
resistance. I also checked between pins 2/4, 3/4 as you suggested. Pin
2/4 seems to be OK, Pin 3/4 is infinite resistance both directions.

I'm a little confused by the equivalent circuit on page 1. It looks
like the emitter of the PNP transistor should be connected to the base
of the NPN transistor, but the drawing is unclear. My initial post
assumed there was no connection. If the two *are* connected, as is
suggested by the application circuit, then pins 2,3,4 would the BCE of
an NPN transistor, and you would then test it in the normal way.

Since pin 3 is the input, I am again assuming this IC is bad. I checked
the HOT when I first opened the set and it's good. I haven't checked
the flyback yet. The model # of the set is C25A24T. Would you agree
with my findings? Thanks alot.

These sorts of faults are usually diagnosed by disconnecting the line
stage and substituting a dummy load. I use a 240V 100W lamp.
Unfortunately your test circuit gets its feedback from the FBT, so I
don't know how it will run without it. The supply may just squeal, or
it may not oscillate. Hopefully nothing will break.

-- Franc Zabkar

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