TV with short circuit - how do I find the short?

[news.verizon.net]

A friend of mine gave me an old 27 inch TV that no longer works. Upon
inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
soon as the TV was plugged in again. I am assuming that there is a short
circuit on the board somewhere. How would I go about finding the short? Is
it possible the power cable or its connection to the circuit board is
related to this short?

Thanks in advance.

 

[Jon Slaughter]

A friend of mine gave me an old 27 inch TV that no longer works. Upon
inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
soon as the TV was plugged in again. I am assuming that there is a short
circuit on the board somewhere. How would I go about finding the short?
Is it possible the power cable or its connection to the circuit board is
related to this short?

Thanks in advance.

Simple way? Put in a much larger time delay fuse then look for the smoke.

 

[Arfa Daily]

A friend of mine gave me an old 27 inch TV that no longer works. Upon
inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
soon as the TV was plugged in again. I am assuming that there is a short
circuit on the board somewhere. How would I go about finding the short?
Is it possible the power cable or its connection to the circuit board is
related to this short?

Thanks in advance.

Does the set have a 'conventional' mechanical on / off switch as well as a
standby switch ? If so, does the fuse blow when this switch is set to off ?
If yes, you might have a short circuit noise filtering cap across the
switch. If the fuse only blows when the main power switch is on, then it
gets a bit more complicated. It could still be a noise filtering cap on the
'back' side of the switch. It could also be a faulty posistor, which was
quite common on a lot of sets for violent fuse blowing. However, if it's
none of those things, it gets a lot more complicated as, if the set is
anything up to say 10 years or so old, it will employ a switch mode power
supply. These are dangerous to inexperienced hands, and notoriously fickle
with their faults. Violent 'instant' fuse blowing is usually down to a short
circuit chopper transistor or hybrid, or sometimes the bridge rectifier, but
that is often not the end of the story. There may be assorted damaged
resistors, diodes, control IC if it uses one, feedback opto, and the whole
lot may have been caused in the first place by a faulty electrolytic in the
primary side.

If the fuse does not blow instantly (ie set needs to be taken out of
standby) or violently, then the chances are that the fault is downstream of
the power supply, most likely in the HOP stage somewhere. However, when this
is the case, most well designed switchers will detect the excess load, and
shut down to a 'safe' mode where they will squeal, tick or chuff, rather
than blow fuses. A bit more info is needed really, before any 'reasoned'
diagnoses can be made.

Arfa

 

[Samuel M. Goldwasser]

A friend of mine gave me an old 27 inch TV that no longer works. Upon
inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
soon as the TV was plugged in again. I am assuming that there is a short
circuit on the board somewhere. How would I go about finding the short? Is
it possible the power cable or its connection to the circuit board is
related to this short?

If you have electronics experience, especially around high voltage and
potentially lethal equipment, then go to the TV Repair Guide at the
Web sites below, especially for the SAFETY information. A blown fuse
can have many causes.

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[[email protected]]

Does the set have a 'conventional' mechanical on / off switch as well as a
standby switch ? If so, does the fuse blow when this switch is set to off?
If yes, you might have a short circuit noise filtering cap across the
switch. If the fuse only blows when the main power switch is on, then it
gets a bit more complicated. It could still be a noise filtering cap on the
'back' side of the switch. It could also be a faulty posistor, which was
quite common on a lot of sets for violent fuse blowing. However, if it's
none of those things, it gets a lot more complicated as, if the set is
anything up to say 10 years or so old, it will employ a switch mode power
supply. These are dangerous to inexperienced hands, and notoriously fickle
with their faults. Violent 'instant' fuse blowing is usually down to a short
circuit chopper transistor or hybrid, or sometimes the bridge rectifier, but
that is often not the end of the story. There may be assorted damaged
resistors, diodes, control IC if it uses one, feedback opto, and the whole
lot may have been caused in the first place by a faulty electrolytic in the
primary side.

If the fuse does not blow instantly (ie set needs to be taken out of
standby) or violently, then the chances are that the fault is downstream of
the power supply, most likely in the HOP stage somewhere. However, when this
is the case, most well designed switchers will detect the excess load, and
shut down to a 'safe' mode where they will squeal, tick or chuff, rather
than blow fuses. A bit more info is needed really, before any 'reasoned'
diagnoses can be made.

Arfa

If you're not familiar with TV circuits I'm afraid that this is going
to be a difficult undertaking for you. Whatever you do though, don't
try the larger fuse trick. That advice was just plain stupid, and so
is the person for giving it. Lenny. .

 

[Ancient_Hacker]

Yep, it's likely to be a short, but not a simple thing like a paper
clip that fell across the power wires.

Most likely a capacitor or transistor shorted, one of the bigger ones,
like the power supply switching-mode transistor or its drivers.
When that shorts it can blow the horizontal output transistor, or vice-
versa. Plus maybe weakening or blowing out nearby diodes and
capacitors.

Worse yet, the real cause of the first transistor blowing is often yet
some OTHER part being a bit below par, something
not easily diagnosed, as it can be something subtle like a capacitor
having a somewhat high ESR. So even if you
replace the bad transistors ($$$) the underlying cause is likely to
blow them again in a millisecond or maybe a week.

And you can't just replace one bad component at a time, as the other
bad ones are likely to blow the new component toute-suite.

To do this right you need a source of gentle line voltage, like a
Variac, lots of spare parts, lots of patience.

I'd recommend you make the best of a bad situation and throw the TV
off a very tall building. Taking all appropriate safety precautions
of course.

 

[PanHandler]

Simple way? Put in a much larger time delay fuse then look for the smoke.

Electronics run on smoke. When the smoke gets out they stop running.

 

[Clint Sharp]

Hardly. There's probably a rectifier built out of diodes somewhere near
where the 120V comes in. Probably one shorted. BE CAREFUL, YOU CAN EASILY
KILL YOURSELF by touching the anode on the tube.

You are more likely to kill yourself messing around with the rectifier
diodes, especially if the reservoir capacitor is still charged.

 

[ian field]

A friend of mine gave me an old 27 inch TV that no longer works. Upon
inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
soon as the TV was plugged in again. I am assuming that there is a short
circuit on the board somewhere. How would I go about finding the short?
Is it possible the power cable or its connection to the circuit board is
related to this short?

Thanks in advance.

This can be as simple as a short circuit diode in the bridge rectifier, or
as complicated as any one or more of a number of parts in the SMPSU. In CRT
TVs the degauss posistor can also cause fuse blowing.

Note that the 'ground reference' rail in a SMPSU is actually a couple of
hundred volts negative and can easily supply lethal current!

One trick you can try instead of blowing more fuses and possibly making the
damage worse, is wire a 60W mains voltage bulb across the fuse holder - this
will limit the current to a value that avoids further damage and provides an
opportunity to measure voltages (or at least see where there aren't any that
should be). Note that once you have the bridge rectifier working it might be
possible to store a hefty charge on the big electrolytic capacitor - that
can give a very nasty bite!

In theory you should use an isolating transformer to power the set while
working on it, but the important thing is to *AVOID* touching anything
earthed while inadvertently touching any live part!

 

[Steve Ashman]

Does the set have a 'conventional' mechanical on / off switch as well as a
standby switch ? If so, does the fuse blow when this switch is set to off
? If yes, you might have a short circuit noise filtering cap across the
switch. If the fuse only blows when the main power switch is on, then it
gets a bit more complicated. It could still be a noise filtering cap on
the 'back' side of the switch. It could also be a faulty posistor, which
was quite common on a lot of sets for violent fuse blowing. However, if
it's none of those things, it gets a lot more complicated as, if the set
is anything up to say 10 years or so old, it will employ a switch mode
power supply. These are dangerous to inexperienced hands, and notoriously
fickle with their faults. Violent 'instant' fuse blowing is usually down
to a short circuit chopper transistor or hybrid, or sometimes the bridge
rectifier, but that is often not the end of the story. There may be
assorted damaged resistors, diodes, control IC if it uses one, feedback
opto, and the whole lot may have been caused in the first place by a
faulty electrolytic in the primary side.

If the fuse does not blow instantly (ie set needs to be taken out of
standby) or violently, then the chances are that the fault is downstream
of the power supply, most likely in the HOP stage somewhere. However, when
this is the case, most well designed switchers will detect the excess
load, and shut down to a 'safe' mode where they will squeal, tick or
chuff, rather than blow fuses. A bit more info is needed really, before
any 'reasoned' diagnoses can be made.

Arfa

Thanks for the advice, but I think I have made an informed decision to cease
work on this TV as too many people have told me on the group and in real
life that I am more than likely to end up touching something that could kill
me. The set does not have a mechanical on off switch, but the fuse blows as
soon as the TV is plugged in, before the standby switch is pressed. As I
said, I am most likely discontinuing work on this TV. I found a similar one
in working condition for 25 dollars on craigslist. Thanks again.

 

[Arfa Daily]

Thanks for the advice, but I think I have made an informed decision to
cease work on this TV as too many people have told me on the group and in
real life that I am more than likely to end up touching something that
could kill me. The set does not have a mechanical on off switch, but the
fuse blows as soon as the TV is plugged in, before the standby switch is
pressed. As I said, I am most likely discontinuing work on this TV. I
found a similar one in working condition for 25 dollars on craigslist.
Thanks again.

This is probably a wise decision. No one on here would seek to put you off
for the sake of it, but everyone is telling the truth and looking out for
your safety, when they warn about the dangers of TV sets in inexperienced
hands. Much modern equipment makes use of switchmode power supplies, and
these are truly potentially *very* dangerous, and require specialist
workshop equipment such as an isolation transformer and variac, as well as a
detailed knowledge of the workings of them, to even begin to be safe to work
with. They can also suffer from problems and cascade failures, which can
make the 'strongest' and most experienced of us weep ...

If you are new to experimental electronics, don't let this put you off.
There is much hobbyist fun to be safely had with low voltage and battery
operated equipment.

Arfa

 

[ian field]

This is probably a wise decision. No one on here would seek to put you off
for the sake of it, but everyone is telling the truth and looking out for
your safety, when they warn about the dangers of TV sets in inexperienced
hands. Much modern equipment makes use of switchmode power supplies, and
these are truly potentially *very* dangerous, and require specialist
workshop equipment such as an isolation transformer and variac, as well as
a detailed knowledge of the workings of them, to even begin to be safe to
work with. They can also suffer from problems and cascade failures, which
can make the 'strongest' and most experienced of us weep ...

If you are new to experimental electronics, don't let this put you off.
There is much hobbyist fun to be safely had with low voltage and battery
operated equipment.

Arfa

..............and never throw out wall wart PSUs when the kit they power gets
replaced, they're great for home brew projects.

 

[Dave]

Simple way? Put in a much larger time delay fuse then look for the smoke.

Or just use foil. He he he.

 

[ian field]

In all the responses to this posting, the one thing I believe no one has
mentioned is the need to obtain a schematic and servicing information for
the TV. Without the schematic, you are flying blind. Some of us could
make a start at trouble shooting a shorted power supply by a visual check
of the components on the various circuit boards and making resistance
checks on the obvious power rails. Sometimes wisdom comes from experience
and the wisdom helps to identify dangers. But there is usually no
substitute for a schematic if the damage is not visible.

Good luck and stay safe.

Ken Fowler, KO6NO

These days the turnover of new designs is so rapid for TVs & monitors etc
that each new model is only in production for a few months before the next
one comes along, by the time it wears out the manufacturer might as well
have forgotten it ever existed! Very few manufacturers even bother to reply
to requests for service information, the few that do will refer you to
distributors - most of which only deal with trade.

The OP said he found something similar in GWO for $25 - unless you know
where to look a schematic can easily cost that much and may even turn out to
be a very fuzzy scan of a creased and stained original.

 

[Arfa Daily]

In all the responses to this posting, the one thing I believe no one has
mentioned is the need to obtain a schematic and servicing information for
the TV. Without the schematic, you are flying blind. Some of us could
make a start at trouble shooting a shorted power supply by a visual check
of the components on the various circuit boards and making resistance
checks on the obvious power rails. Sometimes wisdom comes from experience
and the wisdom helps to identify dangers. But there is usually no
substitute for a schematic if the damage is not visible.

Good luck and stay safe.

Ken Fowler, KO6NO

Although this is fundamentally true for a lot of equipment, unless you have
a detailed understanding of switchmode power supplies, the schematics will
be of little use to you. There are many techniques used nowadays, for power
saving and power factor correction, which really muddy the waters around the
front end of a switcher. Even the way that the primary side high voltage
rail is derived, may be at best not obvious, and often just downright
confusing, if a PFC IC is used, and the supply is a 'universal' type with
automatic input voltage sensing. Locating a suitable 'ground' point on the
primary side can be a very dangerous undertaking, and often, the schematics
can be misleading in this respect.

Given this particular poster's declared skill level, a schematic might have
helped to identify if there was a noise filtering cap at any appropriate
point just inboard of the power lead, but beyond that, I think that he would
probably have been on a loser with this particular repair, so posession of
service info would have been irrelevant.

But yes, in general, I would agree that if you have the skills to be working
on a particular piece of equipment, schematics are certainly desirable.

Arfa