[tvservice] Re: FW: Re: New Post--Fuse keeps blowing; Dead TV

  • From: "graytv" <graytv@xxxxxxx>
  • To: <tvservice@xxxxxxxxxxxxx>
  • Date: Tue, 27 Apr 2004 09:15:25 -0400

Shorted collector to ground is wrong.

Horizontal output transistors in circuit should normally read, with black
(-) to ground:
Emitter to ground short- possibly a coil or very low ohm resistor between
emitter and ground, but generally emitter is ground.
Base to ground short - reading through horizontal driver transformer
windings to ground.
Collector to ground no short - should be very high ohms to infinity,
depending on design, however the inintial reading may be skewed by
capacitors charging up, hold the probes in place for 10 seconds.

Out of circuit the only low ohm reading should be from base to emitter.
Collector to base and collector to emitter should be high ohms.

If after you removed the transistor, there was still a reading from the
collector pad to ground, then there is a short somewhere along the B+ line.
Colud be the flyback, timing caps, filtering caps, rectifiers, or in some
cases (Panasonics especially), there could be a zener diode to ground off
the B+

You mentioned ohming the transistor. Don't you have a diode checker setting
on your multimeter to properly check a transistor junction?. Also, a flyback
(or any coil for that matter), cannot be checked by simply ohming the
winding. You need to have an inductor tester to ring the flyback. Even then
this is not an infallible test for flybacks, as they can fail in ways that
cannot be tested without specialized, expensive equipment.

When you disabled the B+, did you check to see if the voltages were there?
This set, along with most other newer sets, run in an always on standby
state for many of the power supply voltages, waiting for the microprocessor
to put the set into the run mode. At the very least you shoud have a standby
5v, and with this set you should also have B+.

hth, Russ Gray, A+, Pres.
Gray's Electronics Inc.
201-C Commonwealth Blvd.
Port Orange FL USA 32127
386-322-0904  voice
386-322-3809  fax
graytv@xxxxxxx  email

----- Original Message ----- 
From: "Mike Selnick" <selnick@xxxxxxxxxxx>
To: <tvservice@xxxxxxxxxxxxx>
Sent: Monday, April 26, 2004 8:21 PM
Subject: [tvservice] FW: Re: New Post--Fuse keeps blowing; Dead TV


>
> Further to my message below, I checked all the pin-to-pin readings on the
> flyback xformer and one checked out ok.  Bad horiz output tranistor?  See
> readings below.
>
> ----Original Message Follows----
> From: "Mike Selnick" <selnick@xxxxxxxxxxx>
> Reply-To: tvservice@xxxxxxxxxxxxx
> To: tvservice@xxxxxxxxxxxxx
> Subject: [tvservice] Re: New Post--Fuse keeps blowing; Dead TV
> Date: Mon, 26 Apr 2004 19:56:35 -0400
>
> Looked like a nice call at first.  I disabled B+, gave it a new fuse,
> plugged into 120vac, and the fuse did not blow.  The horizontal output
> transistor emitter and collector outer leads both seemed to be shorted to
> ground on the ohmmeter.  After I removed the transistor, I did some
ohmmeter
> checks on it:  Emitter to collector reads 7k ohms, emitter to base reads
5k
> ohms, and base to collector reads infinity.  (Reverse ohmmeter polarity
> gives infinity for the same three readings.)
>
> Do you still think it's a bad transistor or that other faults are
grounding
> the emitter and collector?
>
> ----Original Message Follows----
> From: "graytv" <graytv@xxxxxxx>
> Reply-To: tvservice@xxxxxxxxxxxxx
> To: <tvservice@xxxxxxxxxxxxx>
> Subject: [tvservice] Re: New Post--Fuse keeps blowing; Dead TV
> Date: Mon, 26 Apr 2004 10:06:25 -0400
>
> First thing I would do is disable the B+ to the flyback and sacrifice
> another fuse.
>
> If it doesn't blow then check the horizontal output transistor for a
short.
> If it's ok then you might have a bac flyback.
>
> If the fuse still blows, then check the secondaries for shorts.
>
> BTW, a good check to see if the primary side is ok, after disconnecting
the
> B+, is to check pin 1 of U4101 for -40v to hot ground. If that is good
then
> the regulator circuit is almost certainly (never say never), working
> properly.
>
> hth, Russ Gray, A+, Pres.
> Gray's Electronics Inc.
> 201-C Commonwealth Blvd.
> Port Orange FL USA 32127
> 386-322-0904  voice
> 386-322-3809  fax
> graytv@xxxxxxx  email
>
>
> ----- Original Message -----
> From: "Mike Selnick" <selnick@xxxxxxxxxxx>
> To: <tvservice@xxxxxxxxxxxxx>
> Sent: Saturday, April 24, 2004 5:25 PM
> Subject: [tvservice] Re: New Post--Fuse keeps blowing; Dead TV
>
>
>   > Thanks, Russ.  Hello List members.  My first time aboard.  It's been
> decades since I did TV repair work.
>   >
>   > I'm working on a 31-inch RCA GE 31GT659, Code 45104, Chassis CTC 186/7
>   > bought from Sears in Feb. 1996.  Set is completely dead.  Fuse f4001
is
>   > open.
>   >
>   > I replaced the 5A. fuse and 2.3-ohm 15-watt power resister r4001 and
> plugged the power cord into 120VAC power.  Even *before* I had a chance to
> turn
> the set ON, the Fuse immediately opened and f4001 resister was very hot to
> the touch.  A good on-line troubleshooter recommended (for similar model
RCA
> GE set) that ic U4101 (STK730-010) is probably shorted and should be
> replaced.
>    > I did quick ohmmeter checks of all the diodes and resistors in the
> power
> supply and they look ok.
>   >
>   > Question: What jumps out as the most likely faulty component to
replace
> and how would you test it beforehand?   ( B+ filter?  Degauss relay k4201
or
> thermistor rt4201, Switch Mode xformer T4101, Hybrid Switch Mode Reg U4101
> ic, Closed caption filterQ3302?)
>   >
>   >
>   > ----Original Message Follows----
>   > From: "Hoyt's TV" <hoytstv@xxxxxxxxxxx>
>   > Reply-To: tvservice@xxxxxxxxxxxxx
>   > To: <tvservice@xxxxxxxxxxxxx>
>   > Subject: [tvservice] Re: New Post
>   > Date: Sat, 24 Apr 2004 09:36:29 -0400
>   >
>   > I think you have the honor of making the first post, Mike.
>   > I signed up for the list on March 21st and yours is the first I've
seen
> from
>   > it.
>   >
>   > Russ Hoyt
>   > Hoyt's TV
>   > Exeter, NH
>
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