Re: Panasonic TV - TC-25V30R - fuse blows repeatedly

From: Sofie (sofie_at_olypen.com)
Date: 06/02/04 

Date: Wed, 2 Jun 2004 11:20:46 -0700

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. 

-- 
Best Regards,
Daniel Sofie
Electronics Supply & Repair
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"W.i.k.i.M.a.n" <wikipediholic@yahoo.com> wrote in message
news:e8ca9c8.0406011905.26d1a866@posting.google.com...
> 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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