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Tip for Toshiba TV verical repair

G

Gene Gardner

Jan 1, 1970
0
I inherited a Toshiba model CF2750K, chassis# TAC 9011, with the notorious vertical
problem...compressed to center and upper half...bottom half dark. After reading some
of the posted hints, I arbitrarily replaced the 1uf-50v capcitor in the verical section.
It seemed to be the only 1uf in the vertical section, but it was C-305 rather than
the C-301 or C-342 mentioned by others. One mentioned that the quality of the capacitor
was important, so I replaced it with a paper-tubular (mylar?) 1uf-200v. But apparently,
the set may have had two defects, because when I turned it on, it now had no vertical
deflection at all (bright horizontal white line in middle). I believe the handling of
the PC-board had caused a lingering problem to appear. I arbitarily re-soldered the
seven in-line pins on IC-301 which I believe is probably the Vertical power IC. It
had a large heat-sink.
The set now worked about normally, except that the vertical linearity was a little
disappointing...a slight widening of spacing progressing from bottom toward top, with
and exaggerated widening on the top inch of the screen. I could only find one pot named
vertical height, but could find nothing for vertical linearity. There were other
un-named pots in another section but did not to turn them, and they were sealed. They
did not have the expected 300 series number.
I did shunt an extra 660uf across an existing 2200uf (300 series) and shunted an
extra 25uf across an existing 10uf (300 series)...both in the vertical section.
Adding these made no significant improvement. Incidentally, capacitors of about 3/8"
diameter can be tack-soldered on the bottom side (just enough space) without removing
the original (electrolytics almost never short).
Does anyone have any hints as to improving the vertical linearity on this model?Is it
possible that the high quality of my 1uf capacitor was TOO high? (one poster suggested
that the circuit was very critical to the quality of the 1uf capacitor)
 
J

Jfet

Jan 1, 1970
0
Sorry, you mentioned that electrolytics almost never short?

That wouldn't be correct. They certainly do fail, usually in the shorted condition.

You may have other caps that are leaking, try using a hair dryer and heat near the other
caps and see if any of the picture has altered. If it gets better, you know you have a
bad cap. If you freeze it (using cold spray), you may find the faulty cap short out
completely and may blow the vertical drive IC.

Typically, the vertical drive IC generates a lot of heat, most of the caps around it fail
because of the heat.

If you removed a 1uF electrolytic, most likely it was polarized. Did you purchase a mylar
cap that was polarized?? A tantalum cap would be of the best quality, and values
typically don't exceed 1uF. Tantalums are typically polarized.

I would also change the vert drive IC, I don't know if you mentioned you did (couldn't
tell from your post). It is possible this is the route cause of the trouble, causing
other components to fail in a cascading effect.


Gene Gardner said:
I inherited a Toshiba model CF2750K, chassis# TAC 9011, with the notorious vertical
problem...compressed to center and upper half...bottom half dark. After reading some
of the posted hints, I arbitrarily replaced the 1uf-50v capcitor in the verical section.
It seemed to be the only 1uf in the vertical section, but it was C-305 rather than
the C-301 or C-342 mentioned by others. One mentioned that the quality of the capacitor
was important, so I replaced it with a paper-tubular (mylar?) 1uf-200v. But apparently,
the set may have had two defects, because when I turned it on, it now had no vertical
deflection at all (bright horizontal white line in middle). I believe the handling of
the PC-board had caused a lingering problem to appear. I arbitarily re-soldered the
seven in-line pins on IC-301 which I believe is probably the Vertical power IC. It
had a large heat-sink.
The set now worked about normally, except that the vertical linearity was a little
disappointing...a slight widening of spacing progressing from bottom toward top, with
and exaggerated widening on the top inch of the screen. I could only find one pot named
vertical height, but could find nothing for vertical linearity. There were other
un-named pots in another section but did not to turn them, and they were sealed. They
did not have the expected 300 series number.
I did shunt an extra 660uf across an existing 2200uf (300 series) and shunted an
extra 25uf across an existing 10uf (300 series)...both in the vertical section.
Adding these made no significant improvement. Incidentally, capacitors of about 3/8"
diameter can be tack-soldered on the bottom side (just enough space) without removing
the original (electrolytics almost never short).
Does anyone have any hints as to improving the vertical linearity on this model?Is it
possible that the high quality of my 1uf capacitor was TOO high? (one poster suggested
that the circuit was very critical to the quality of the 1uf capacitor)


Sorry, you mentioned that electrolytics almost never short?

That wouldn't be correct. They certainly do fail, usually in the shorted condition.

You may have other caps that are leaking, try using a hair dryer and heat near the other
caps and see if any of the picture has altered. If it gets better, you know you have a
bad cap. If you freeze it (using cold spray), you may find the faulty cap short out
completely and may blow the vertical drive IC.

Typically, the vertical drive IC generates a lot of heat, most of the caps around it fail
because of the heat.

If you removed a 1uF electrolytic, most likely it was polarized. Did you purchase a mylar
cap that was polarized?? A tantalum cap would be of the best quality, and values
typically don't exceed 1uF. Tantalums are typically polarized.

I would also change the vert drive IC, I don't know if you mentioned you did (couldn't
tell from your post). It is possible this is the route cause of the trouble, causing
other components to fail in a cascading effect.
 
S

Sofie

Jan 1, 1970
0
Gene:
Replace (remove the old caps, don't shunt them) the remainder of the C3xx
caps in and around the vertical output chip and look especially for C301, it
is hiding about 2 inches away from the rest of the C3xx caps and is toward
the center of the board near a large chip (jungle IC)..... and definitely
replace it.... usually 1uf @ 50V .....
Most of the electrolytic caps, with age and heat will suffer from high ESR,
degraded capacity, leak corrosive electrolyte and many times will become
electrically leaky or even short...... because of this you need to actually
remove the caps, don't shunt them. Unless you have an ESR meter to
properly test them, just replace them all, there are only about 6 to 8 units
so it is fairly easy and cheap.
When replacing the electrolytics be certain to observe polarity.
 
L

Leonard G. Caillouet

Jan 1, 1970
0
Ditto Sofie's comments. Also, clean any electrolyte from the board and
resolder all connections in the circuit. The 200v cap may have a high ESR
and may also cause problems. Use the original value electrolytics.

Leonard
 
R

RonKZ650

Jan 1, 1970
0
Like mentioned earlier, look for another 1uf. There are 2 identical 1uf or
2.2uf caps used (depending on model), one is near the vertical output IC, the
other is close to the larger osc/video jungle IC. Look at the color and
size/shape of the original and compare with other C300 series in the set until
you find it's matching cap.
Funny thing is these caps normally check OK on both an ESR meter as well as a
cap checker, but cause linierity problems. You need to replace them both.
Ron
 
S

Shoreline Electronics

Jan 1, 1970
0
These are the 3 MUST change caps for that model.

C308 220uf 50v, C305 1 Uf 50v, C301 1 Uf 50v


As others mentioned , one of them is usually in the center of the board next
to the large signal processing IC.



--
==========================
Jeff Stielau
Shoreline Electronics Repair
344 East Main Street
Clinton,CT 06413
860-399-1861
860-664-3535 (fax)
[email protected]
========================
"If you push something hard enough it will fall over."
Fudd's First Law of Opposition - Sir Sidney Fudd
 
G

Gene Gardner

Jan 1, 1970
0
[email protected] (Gene Gardner) writes:


Thanks everyone...my third problem is fixed! Your instructions were absolutely right.
The last vertical linearity problem was cured by "finally" locating C-301. And yes,
both C-305 and C-301 were the 1uf-50v which were "brownish-maroon" in color. This
79 year old brain is a slow learner: I wasted a lot of time replacing several other
electrolytics before the last poster (Shoreline Electonics) reinforced the other posts
and persuaded me that there was a C-301 far away from the vertical section in a sectioned
off part near a very large IC about the size of a microprocessor. This was so different
from my experience many years ago. I would be curious as to the theory of how such a
beautifully linear vertical sweep can be generated, and with no vertical linearity pot
or vertical centering pot required. And after all this time (mfd Nov 1991) the picture
display is near perfect. I'm also amazed that the (probably 16 year old Toshiba, which
had only 48 channels) that I am now replacing, still has a good picture...I could almost
say very good....and has never had a component replaced. The only repair was a solder
joint on the PC board where thermal-fatigue at a 2-watt power resistor connection became
intermittent and tapping or flexing made it easy to find (similar to the problem I mentioned
in my original post concerning the Vert Out power module).
 
J

Jason D.

Jan 1, 1970
0
[email protected] (Gene Gardner) writes:


Thanks everyone...my third problem is fixed! Your instructions were absolutely right.
The last vertical linearity problem was cured by "finally" locating C-301. And yes,
both C-305 and C-301 were the 1uf-50v which were "brownish-maroon" in color. This
79 year old brain is a slow learner: I wasted a lot of time replacing several other
electrolytics before the last poster (Shoreline Electonics) reinforced the other posts
and persuaded me that there was a C-301 far away from the vertical section in a sectioned
off part near a very large IC about the size of a microprocessor. This was so different
from my experience many years ago. I would be curious as to the theory of how such a
beautifully linear vertical sweep can be generated, and with no vertical linearity pot
or vertical centering pot required. And after all this time (mfd Nov 1991) the picture
display is near perfect. I'm also amazed that the (probably 16 year old Toshiba, which
had only 48 channels) that I am now replacing, still has a good picture...I could almost
say very good....and has never had a component replaced. The only repair was a solder
joint on the PC board where thermal-fatigue at a 2-watt power resistor connection became
intermittent and tapping or flexing made it easy to find (similar to the problem I mentioned
in my original post concerning the Vert Out power module).

Bravo good going!

That color is right on accounting for color fading. It is usually
funny red (almost lipstick red) but it can be that dirty red. Very
stinky caps indeed when soldered. Phew.

C301 is the ramp generator circuit using large IC's own fixed contant
current source to make a linear ramp, centering pot still used in that
chassis. Then comes out as ramp signal to the vertical IC that does
own few magics along with own retrace circuit built in.

Resolder that vertical output IC also if you have already done and
change 100uF 35V cap that has + side pointing to it by a diode that is
supplied by 26V source. That's retrace cap, vertical retrace kick is
boosted to appox 50V using that charged cap as temporary battery in
series with it's own 26V supply.

Cheers,

Wizard
 
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