Sharp TV 1984 model 20J81C - fuse blown.

[derek]

The main power supply fuse has blown and I'd like to fix this myself
if I can find the components at fault. It had a burning smell at the
time it blew, however apart from the fuse itself I can find no signs
of burned caps, diodes or resistors etc. I have tried to find a
schematic in the library RCA guides but can only get one for model
20D74C, however this power supply schematic does not seem to be quite
the same as my set. Does anyone know which guide number would contain
my model(20J81C)?
I have some electronics tech experience but am not a TV technician and
lack any equipment such as a scope or variac supply; I do however have
an isolation transformer - only 250VA, would this suffice? So far I
have used the series light bulb technique; at first the lamp is bright
but after a minute or so it dims and then starts on and off, about
once a second with a clicking sound coming from either the main filter
cap C706 (470micF) or the neighbouring HOT (T602).
At this time the voltage across C706 rises from 40V to 80V - with the
lamp still in series.
Can anyone suggest likely components to test, and how exactly with
only a VOM?
In the meantime I'm off to buy a new set but still want to crack this!
Reply only to the NG please.
Thanks, Derek

 

[Art]

SAMS Photofact should have a cross reference for your 20 year old Sharp TV.
The burning smell you refer to may be from the LOPT, Yoke, or other device
within the set. Suspect possible problems within the deflection and power
supply circuits. Main situation you may encounter is the availability of
components for a set this old, especially from Sharp. IMHO.

 

[RonKZ650]

The Sharps of this vintage had a big problem with high voltage transformers
going bad. My guess is the transformer is bad, shorting the horiz output
transistor then blowing the fuse.
Rom

 

[b]

Derek,

Good on you for wanting to have a go at saving this set, but I have
been there, done that in the past, and I have to say that any old set
like this, having a power supply or line output stage problem (i.e. a
serious fault) is simply not worth investing time and money in. You
could end up spending hours repairing the power supply only to find
that some other critical problem , say, the yoke or Horizontal output
section immediately causes it to go up in smoke again.
Save your time and money on this one. Ditch it. After all, you've had
your money's worth out of the old girl ;-)
Ben

 

[derek]

Thanks guys; regretfully I think you are right about the probable
fault(s) and the impracticality of repair. It relates to that other
post about the future of TV repair doesn't it - very sad. I hate this
throw away society!
Oh well - I'm not going to waste money on any rapidly obsolescing
high-end product when I rarely watch TV. Thinking of a cheap
Sanyo,Sylvania or something called Prima (Chinese I guess?)
Derek

 

[derek]

Thanks guys; regretfully I think you are right about the probable
fault(s) and the impracticality of repair. It relates to that other
post about the future of TV repair doesn't it - very sad. I hate this
throw away society!
Oh well - I'm not going to waste money on any rapidly obsolescing
high-end product when I rarely watch TV. Thinking of a cheap
Sanyo,Sylvania or something called Prima (Chinese I guess?)
Derek

 

[derek]

Thanks guys; regretfully I think you are right about the probable
fault(s) and the impracticality of repair. It relates to that other
post about the future of TV repair doesn't it - very sad. I hate this
throw away society!
Oh well - I'm not going to waste money on any rapidly obsolescing
high-end product when I rarely watch TV. Thinking of a cheap
Sanyo,Sylvania or something called Prima (Chinese I guess?)
Derek

 

[b]

Derek,

If you don't want to spend a lot of money on a Tv, I would advise
against going for the supermarket/el-cheapo new sets- they will almost
certainly develop some problem, usually immediately after the warranty
expies :-(. If I were you ,I would look at investing the same amount
of cash (or less!) in a good name brand second hand one which is not
too old (ie. less than 10 yrs!) (from a reliable retail source if
poss.).

For a set such as a Sony, JVC, Philips etc, there will be far more
technical support and its common faults are already known to the
repair trade. That way, if anything does go wrong, you're not left in
the lurch and you'll likely see more life out of your purchase. Try
posting here for recommedations for reliable used models (don't forget
to say what part of the world you're in!) Here in Europe, Philips
ITT/Nokia and Grunding (RIP!) are amongst the best used buys and easy
to get spares for (IMHO)

Good luck
Ben

 

[Chaos Master]

derek said:

Thanks guys; regretfully I think you are right about the probable
fault(s) and the impracticality of repair. It relates to that other
post about the future of TV repair doesn't it - very sad. I hate this
throw away society!
Oh well - I'm not going to waste money on any rapidly obsolescing
high-end product when I rarely watch TV. Thinking of a cheap
Sanyo,Sylvania or something called Prima (Chinese I guess?)
Derek

Get a TV from a known brand like Sony, Philips...
You will get more TV for what you spend.

[]s