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Panasonic TV - TC-25V30R - fuse blows repeatedly

W

W.i.k.i.M.a.n

Jan 1, 1970
0
Hi,

I had my tv repaired some time ago. All the functioning
was fine but the technician had replaced the main fuse with a
thick wire and I started using the tv without knowing about that.

Recently, it went bad again. I decided to check it myself. I opened
the casing and instantly spotted a broken/open resistor 3.3 ohm, 15W.
I replaced it and powered the set. It started but after about 5/6
seconds went down with explosion and lightning. The same resistor
broke open again. At this time I spotted the thick wire at the
main fuse holder.

I bought the tv's circuit diagram, read SER FAQs, and went on to
check for any bad power transistor in the supply section (since power
supply input read 50 ohms indicating that bridge or posistor is not
the problem IMO). HOT was good but another one, C4706, high voltage
switching transistor in the regulator section of SMPS, was bad, both
B-E and C-E were shorted.

I replaced the resistor, C4706 transistor, and fuse (T4.0A), and
powered the set. It ran for about 7/8 seconds, screen was brightened
white with a black horizontal line in the middle. And then it turned
off again with explosion and lightning. I rechecked, this time only
the fuse was blown.

Being an electrical engineer, I'm not considering an option to get it
repaired by someone else. For this, I need the following information:

- Is this start-up scenario typical with some common cause and
remedy?
- I think of first confirming if the SMPS is good, and for this I
should disconnect all other circuit (all load of SMPS) and check if
fuse still blows, and if it doesn't, are the output voltages correct
(according to the diagrams, SMPS should give 140v, 12v, and 5v DCs)?
- Any other ideas of how to proceed?

Thanks in advance,
W.i.k.i.m.a.n
 
A

Art

Jan 1, 1970
0
Suggest getting the appropriate service manual from Panasonic and doing some
rather delicate diagnosis rather than the hammer and ice technique you now
employ. Being an
"Electrical Engineer" should have precluded the approach you are currently
using!! Otherwise you may want to actually employ the services of a
'Professional Electronic Technician', with television experience, to repair
the set properly: IMHO
 
W

W.i.k.i.M.a.n

Jan 1, 1970
0
Art said:
Suggest getting the appropriate service manual from Panasonic and doing some
rather delicate diagnosis rather than the hammer and ice technique you now
employ.

Thanks for the advice. But the circuit diagram I have is complete with
part numbers and all. A service manual doesn't have much more info
than this, AFAIK.
Being an
"Electrical Engineer" should have precluded the approach you are currently
using!!

As I already mentioned, "hammer and ice technique" is not what I want.
That's why I have posted here. I don't think a step-by-step approach
with the help of sci.electronics.repair experts is "hammer and ice".
Otherwise you may want to actually employ the services of a
'Professional Electronic Technician', with television experience, to repair
the set properly: IMHO

W.i.k.i.M.a.n said:
Hi,

I had my tv repaired some time ago. All the functioning
[Snipped]
Thanks in advance,
W.i.k.i.m.a.n

I only want to do it myself because I think I can do it. And my being
a EE is going to help me a lot. I understand that I'll have to do a
lot of labor, reverse engineering, understanding the circuits, and
other grunt stuff like that, but I'm here to do it. The only thing I
lack is practical experience. If that were there, I wouldn't have
posted here. AFA professional technician is concerned, I live at a
place where it is hard to find one, (Yes! not even Panasonic repair
shops are professional enough at my location), so its much more
preferrable for me to cash my EE skills.

W.i.k.i.m.a.n
 
S

Sofie

Jan 1, 1970
0
W.i.k.i.M.a.n:
Since you are an EE I am certain that you are obviously aware that most SMPS
designs can NOT be tested or even safely operated without loads
approximating the actual circuits they power.
After you get ALL the faulty parts replaced in the SMPS you need to focus
your attention on the Horizontal Deflection circuitry including the HOT,
shaping caps, flyback derived B+ sources, etc, etc...... if you have a coil
ringer you need to test the flyback too. You will also need to test the
electrolytics in the SMPS and other areas of the television circuitry with
and ESR meter...... nothing else will conclusively test electrolytics for
high esr..... even though the electrolytics may test for the proper
capacitance they can still have high esr and not perform properly in many
circuits.
 
A

Asimov

Jan 1, 1970
0
"Art" bravely wrote to "All" (02 Jun 04 06:07:35)
--- on the heady topic of "Re: Panasonic TV - TC-25V30R - fuse blows repeatedly"

Ar> From: "Art" <[email protected]>

Ar> Suggest getting the appropriate service manual from Panasonic and
Ar> doing some rather delicate diagnosis rather than the hammer and ice
Ar> technique you now employ. Being an
Ar> "Electrical Engineer" should have precluded the approach you are
Ar> currently using!! Otherwise you may want to actually employ the
Ar> services of a 'Professional Electronic Technician', with television
Ar> experience, to repair the set properly: IMHO

Oh, come on! Give him a break already. Didn't you read the part about
him having a PET repair it previously and to now find he had replaced
the mains fuse with a piece of nomex? What kind of a professional is
it that pulls a stunt like that?!!! The op is right not to trust his
area PET with even wiring a lamp base. It is you who seems out of it.

A*s*i*m*o*v

.... :<) What does that wire do? =8<Q (oh!)
 
J

Jerry G.

Jan 1, 1970
0
All the advice you have been getting from the others is very true. The power
supply would have to be first serviced, and then the horizontal output
stage. There are other components that are off value, and must be replaced.
There is also a chance that the flyback transformer has been damaged,
especially that the HOT was shorted. If there is even one shorted turn in
the flyback, it can cause your description. But, you can be sure that there
is a lot more damage than just the flyback.

The people who service these sets professionally, go through the proper
training and theory of how these sets work. The factory service techs are
extensively trained on the particular models that they will be servicing.
These sets are fairly complex, and can sometimes be a challenge for even the
most experienced techs.

In many cases it is not cost effective to service the set yourself, unless
you are properly trained, and have the necessary test gear to work on the
set. Infact, many of the smaller shops will jobber out the more complex
problems to the factory service people.

I would think that after changing the various blown components a number of
times and also considering the purchase of the service manual, you are
probably approaching the cost of having an authorized service tech service
the set for you. This is not even accounting for your time.

In the many years I have been involved in the service business, I found it
very rare for the non experienced to be able to repair any of the complex
faults in these sets. I have only seen them succeed with the more simple
faults, such as changing a switch or control, or a simple part that they
were told to change that is a standard type of problem.

--

Jerry G.
=====


Hi,

I had my tv repaired some time ago. All the functioning
was fine but the technician had replaced the main fuse with a
thick wire and I started using the tv without knowing about that.

Recently, it went bad again. I decided to check it myself. I opened
the casing and instantly spotted a broken/open resistor 3.3 ohm, 15W.
I replaced it and powered the set. It started but after about 5/6
seconds went down with explosion and lightning. The same resistor
broke open again. At this time I spotted the thick wire at the
main fuse holder.

I bought the tv's circuit diagram, read SER FAQs, and went on to
check for any bad power transistor in the supply section (since power
supply input read 50 ohms indicating that bridge or posistor is not
the problem IMO). HOT was good but another one, C4706, high voltage
switching transistor in the regulator section of SMPS, was bad, both
B-E and C-E were shorted.

I replaced the resistor, C4706 transistor, and fuse (T4.0A), and
powered the set. It ran for about 7/8 seconds, screen was brightened
white with a black horizontal line in the middle. And then it turned
off again with explosion and lightning. I rechecked, this time only
the fuse was blown.

Being an electrical engineer, I'm not considering an option to get it
repaired by someone else. For this, I need the following information:

- Is this start-up scenario typical with some common cause and
remedy?
- I think of first confirming if the SMPS is good, and for this I
should disconnect all other circuit (all load of SMPS) and check if
fuse still blows, and if it doesn't, are the output voltages correct
(according to the diagrams, SMPS should give 140v, 12v, and 5v DCs)?
- Any other ideas of how to proceed?

Thanks in advance,
W.i.k.i.m.a.n
 
D

David

Jan 1, 1970
0
For troubleshooting, you need to find out if the deflection is pulling
too much current, or a defect in the SMPS.

Before going any further: STOP if you do not have an isolation
transformer to plug the tv into during troubleshooting. Do not
continue without an isolation transformer, and preferably a variac to
vary the ac line input.

Look at the schematic and determine what the feedback path is for the
SMPS. i.e. Which voltage is monitored for regulation. Make sure
there is a load on that voltage. The 5v will have to be connected as
it runs the system control. The probably regulation is done with the
140v. Unsolder the HOT. Solder in 40 watt 130V (not 120V) light bulb
across the 140v line for a known resistive load. This will allow you
to find out if the SMPS has a problem or the deflection stage. You
can then determine if the SMPS is regulating properly or the problem
is in the deflection/Hv stage.

Beyond that, you really need someone with experience there in person
to explain what is going on and how to troubleshoot that type of
failure if the SMPS is in fact running correctly under the test load.

David


Art said:
Suggest getting the appropriate service manual from Panasonic and doing some
rather delicate diagnosis rather than the hammer and ice technique you now
employ.

Thanks for the advice. But the circuit diagram I have is complete with
part numbers and all. A service manual doesn't have much more info
than this, AFAIK.
Being an
"Electrical Engineer" should have precluded the approach you are currently
using!!

As I already mentioned, "hammer and ice technique" is not what I want.
That's why I have posted here. I don't think a step-by-step approach
with the help of sci.electronics.repair experts is "hammer and ice".
Otherwise you may want to actually employ the services of a
'Professional Electronic Technician', with television experience, to repair
the set properly: IMHO

W.i.k.i.M.a.n said:
Hi,

I had my tv repaired some time ago. All the functioning
[Snipped]
Thanks in advance,
W.i.k.i.m.a.n

I only want to do it myself because I think I can do it. And my being
a EE is going to help me a lot. I understand that I'll have to do a
lot of labor, reverse engineering, understanding the circuits, and
other grunt stuff like that, but I'm here to do it. The only thing I
lack is practical experience. If that were there, I wouldn't have
posted here. AFA professional technician is concerned, I live at a
place where it is hard to find one, (Yes! not even Panasonic repair
shops are professional enough at my location), so its much more
preferrable for me to cash my EE skills.

W.i.k.i.m.a.n
 
A

Asimov

Jan 1, 1970
0
Hi,

Use a 100 watt lamp in series with the powerline next time to limit
the current in case of a problem. In my opinion if the screen filled
up white this means you have both deflection and HV. The black bar is
still an unknown at this point.

Now the question may be what about the audio section, the video, etc?
Is the regulated supply too high? For example 150 volts on the 130
volts line. This could make the HV go way high and then some xray
safety feature is causing the mains fuse to blow. Try disabling the
HOT and see if some other circuit is drawimg too much power. Work at
diagnosing the problem 1 step at a time. Start by checking the
electros around the horizontal driver and HOT.


W.i.k.i.M.a.n said:
Hi,

I had my tv repaired some time ago. All the functioning
was fine but the technician had replaced the main fuse with a
thick wire and I started using the tv without knowing about that.

Recently, it went bad again. I decided to check it myself. I opened
the casing and instantly spotted a broken/open resistor 3.3 ohm, 15W.
I replaced it and powered the set. It started but after about 5/6
seconds went down with explosion and lightning. The same resistor
broke open again. At this time I spotted the thick wire at the
main fuse holder.

I bought the tv's circuit diagram, read SER FAQs, and went on to
check for any bad power transistor in the supply section (since power
supply input read 50 ohms indicating that bridge or posistor is not
the problem IMO). HOT was good but another one, C4706, high voltage
switching transistor in the regulator section of SMPS, was bad, both
B-E and C-E were shorted.

I replaced the resistor, C4706 transistor, and fuse (T4.0A), and
powered the set. It ran for about 7/8 seconds, screen was brightened
white with a black horizontal line in the middle. And then it turned
off again with explosion and lightning. I rechecked, this time only
the fuse was blown.

Being an electrical engineer, I'm not considering an option to get it
repaired by someone else. For this, I need the following information:

- Is this start-up scenario typical with some common cause and
remedy?
- I think of first confirming if the SMPS is good, and for this I
should disconnect all other circuit (all load of SMPS) and check if
fuse still blows, and if it doesn't, are the output voltages correct
(according to the diagrams, SMPS should give 140v, 12v, and 5v DCs)?
- Any other ideas of how to proceed?

Thanks in advance,
W.i.k.i.m.a.n

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?
 
A

Art

Jan 1, 1970
0
Jerry: Very Well Put!! thanks /aj
Jerry G. said:
All the advice you have been getting from the others is very true. The
power
supply would have to be first serviced, and then the horizontal output
stage. There are other components that are off value, and must be
replaced.
There is also a chance that the flyback transformer has been damaged,
especially that the HOT was shorted. If there is even one shorted turn in
the flyback, it can cause your description. But, you can be sure that
there
is a lot more damage than just the flyback.

The people who service these sets professionally, go through the proper
training and theory of how these sets work. The factory service techs are
extensively trained on the particular models that they will be servicing.
These sets are fairly complex, and can sometimes be a challenge for even
the
most experienced techs.

In many cases it is not cost effective to service the set yourself, unless
you are properly trained, and have the necessary test gear to work on the
set. Infact, many of the smaller shops will jobber out the more complex
problems to the factory service people.

I would think that after changing the various blown components a number of
times and also considering the purchase of the service manual, you are
probably approaching the cost of having an authorized service tech service
the set for you. This is not even accounting for your time.

In the many years I have been involved in the service business, I found it
very rare for the non experienced to be able to repair any of the complex
faults in these sets. I have only seen them succeed with the more simple
faults, such as changing a switch or control, or a simple part that they
were told to change that is a standard type of problem.

--

Jerry G.
=====


Hi,

I had my tv repaired some time ago. All the functioning
was fine but the technician had replaced the main fuse with a
thick wire and I started using the tv without knowing about that.

Recently, it went bad again. I decided to check it myself. I opened
the casing and instantly spotted a broken/open resistor 3.3 ohm, 15W.
I replaced it and powered the set. It started but after about 5/6
seconds went down with explosion and lightning. The same resistor
broke open again. At this time I spotted the thick wire at the
main fuse holder.

I bought the tv's circuit diagram, read SER FAQs, and went on to
check for any bad power transistor in the supply section (since power
supply input read 50 ohms indicating that bridge or posistor is not
the problem IMO). HOT was good but another one, C4706, high voltage
switching transistor in the regulator section of SMPS, was bad, both
B-E and C-E were shorted.

I replaced the resistor, C4706 transistor, and fuse (T4.0A), and
powered the set. It ran for about 7/8 seconds, screen was brightened
white with a black horizontal line in the middle. And then it turned
off again with explosion and lightning. I rechecked, this time only
the fuse was blown.

Being an electrical engineer, I'm not considering an option to get it
repaired by someone else. For this, I need the following information:

- Is this start-up scenario typical with some common cause and
remedy?
- I think of first confirming if the SMPS is good, and for this I
should disconnect all other circuit (all load of SMPS) and check if
fuse still blows, and if it doesn't, are the output voltages correct
(according to the diagrams, SMPS should give 140v, 12v, and 5v DCs)?
- Any other ideas of how to proceed?

Thanks in advance,
W.i.k.i.m.a.n
 
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