Electronics

Newbie · Guest

I'm hoping somebody can help me with this TV set. I am new to
electronics repair and have read quite a bit on the net but the
recommendations are fairly generic.

This set blows the fuse instantly upon applying power. I have checked
the board for hairline cracks with no avail (at least to my eyes). I
believe I have verified everything up to the HOT (Q4401), but anything
is possible for a newbie. The HOT is high on C-E in both directions,
but on B-E it reads 0.4 ohms in both directions (on 200 ohm DMM scale).
Does this mean the HOT is bad? If so, what are the next steps (I do
not want to simply replace the HOT and see if it happens again, which
I've been told previously).

Thanks in advance for any help!

Guest

Obviously you did not verify the power supply which is where you should
have started when diagnosing a blown fuse. It looks like from you
information that you have tried several fuses. Time to call on someone
who knows what they are doing and has a basic understanding of proper
troubleshooting technique to have a go at the set, in person.

You may need to have someone who has more experience working in your
shop give you a hand with the repair and help to bring you up to speed
on troubleshooting and repair.

Continuously replacing a blown fuse is not proper troubleshooting.

Newbie wrote:

Newbie · Guest

I apologize for not being more clear in my original post. I believe I
have verified everything up to the HOT (Q4401) beginning with the main
power supply.

Because the set went out while the TV was running, I figured it was
worth a shot to replace the fuse once and apply power. Because the
fuse blew immediately, I started by looking at the components beginning
with the PS. Everything appear normal until the HOT...

Guest

The fuse is only there to protect against hard shorts on the primary
side of the power supply (on sets like this using a true SMPS). You
missed something there, try again.

Proper technique is to replace the fuse, bring the unit up slowly on a
variac while monitoring current. If the total current the tv would
normally draw is exceeded before you reach 120V ac, then stop and do
not cause additional damage.

Alan Harriman · Guest

On 6 Feb 2005 13:13:45 -0800, "Newbie" <jagtree@yahoo.com> wrote:

Newbie · Guest

Thanks for the replies guys. I know that the bridge rectifiers are in
good shape (with no power on the board, CR1,2,3,4 all show around 130k
ohm resistance).

What's the best way to proceed in diagnosing the chopper output and any
other damaged components withhout introducing any further damage?

Guest

#1 do a google groups search for ctc169 in sci.electronics.repair for
lots of information.

The diodes could very well be breaking down with full voltage applied,
so the ohm meter reading may not tell the whole story.

Capacitors are common failures in the SMPS and cause multiple
additional parts failures. C4108 is a critical value so use an OEM
part only.

Best way is to assume a part is bad if you cannot accurately and with
100% confidence be assured it tests good, replace it. All the small
parts cost very little in that set.

Even with the replace it to be sure the total parts count should still
be under 12 after finding the parts that are known bad.

C · Guest

Newbie wrote:

Arnold stewart · Guest

Remove the HOT. If the B-E still mesures .4 ohms in both directions,
it's shorted. Normally reads between 15 - 30 ohms or more.

Newbie wrote:

Arnold stewart · Guest

Arnold stewart wrote:

	Electronics Forum Index -> Repair	All times are GMT
Page 1 of 1