Toshiba CRT A80ERF031X13 Shorted, appreciate advice

[[email protected]]

This set came in with bright red screen and retrace lines. The CRT has
a red Cathode to filament short. I disconnected both sides of the
filament circuit from the flyback and ran the tube filament from a DC
supply at 6.0V, and although the picture was watchable it still had
some red smeariness through it. I've advised the customer that we can
try to remove the short but it may open the filament. But the set is
worthless either way. I don't have a rejuvenator but I would like to
give this a shot before pronouncing it DOA. Has anyone had any luck
doing this? Would I have a better chance with say a CR70? Or can I
duplicate what the CR70 would do in this instance? I would hate to
destroy the tube by using a procedure possibly not proper for this
particular problem. With a low ohms meter I can possibly isolate which
side of the filament the short is on if that would help. Thanks for
any advice. Lenny.

 

[hr(bob) [email protected]]

This set came in with bright red screen and retrace lines. The CRT has
a red Cathode to filament short. I disconnected both sides of the
filament circuit from the flyback and ran the tube filament from a DC
supply at 6.0V, and although the picture was watchable it still had
some red smeariness through it. I've advised the customer that we can
try to remove the short but it may open the filament. But the set is
worthless either way. I don't have a rejuvenator but I would like to
give this a shot before pronouncing it DOA. Has anyone had any luck
doing this? Would I have a better chance with say a CR70? Or can I
duplicate what the CR70 would do in this instance? I would hate to
destroy the tube by using a procedure possibly not proper for this
particular problem. With a low ohms meter I can possibly isolate which
side of the filament the short is on if that would help. Thanks for
any advice. Lenny.

They used to, when CRTs were run from 6.3 V AC on a transformer
winding from the 120V power mains, have a special 6.3V transformer 1:1
turns ratio, but with very good high-frequency isolation, that you
used to isolate the heater-cathode from the low impedance of the
regular transformer windings. The smear you describe is due to poor
low-frequcny isolation. You could run the tube with an ordinary 6.3V
power transformer if you could put inductors in each of the secondary
windings and also maintain low capacitive coupling across the
windings.

Now that CRTs are run from a high-frequency source, either the pwer
supply or the horizontal output circuit, things are not so easy
beacause they are running on pulsesthat have the equivalent heating
power, but which are nothing close to sinusoidal and thus impossible
to use an isolation transformer.

H. R. Hofmann

 

[Art]

One procedure is to actually flash the CRT with the focus voltage. Ground
the heater and quickly touch and release the cathode pin(s) with the focus
lead. Of course the set must be powered when doing this, or another source
of similar voltage may be used. Be advised: This is a probably destructive
procedure and may be marginal regarding safety of the technician. Absolute
interest regarding your safety must be considered.
Yes, I have done this with positive results and with a totally flacked CRTs
as results. Good luck.

 

[Dani]

I just posted a TV with the same "common" Phillips CRT. I'm going to
try the "focus, & G2 arc trick"! Here's a technique I have been using,
with good results. I hope it helps you. Dani.

I would like to share this great tip with all you great helpfull
Tech's
out there! This is only for Technicians who really know there way
around the inside of a T.V.! Please don't just anyone try this!! If
you
don't have a proper CRT rejuvinating set, this will be a great fix,
for
an otherwise lost cause.

Sony 1 K problems, that is T.V.'s with the red, or green, or blue
having too low, or high of an emission to get a raster due to the
"blanking circuit", can be solved with an odd, & somewhat dangerous
technique! With the set off, unsolder each cathode on the CRT board,
then momentarily bridge the gap of each gun, one at a time, to see
how
much emission there is. You will have to raise the G 2 pot to get a
bit
of raster in a darkened room! Next solder the two good cathodes
(usually only one is out of gain specs), back, leaving the bad one.
Now
this is where it gets dangerous!!! While the set is off remove the
focus line from the CRT socket ( 5000 volts), & while the set is
running, momentarily make contact with the unsoldered leg of the bad
gun. Do this with caution, & at least five times. It will try to arc,
but just keep a steady hand, & keep the wire with a good insulated
tool
at the unsoldered leg only. Now, while the set is off, unhook the G 2
wire, & set it to about 200 volts DC unloaded, & charge a 100 uf
capacitor @ 250 volts. Do this two, or three times, then discharge it
to the unsoldered gun, while the set is runing. Do this charging, &
discharging to the gun at least five times! At this point, you should
see the bad gun get brighter, & brighter. I have done quite a lot of
experimenting, & this will save a lot of those Sony CRT's, that would
otherwise be toast. Grey scale comes back to within a close enough
spec, to make the customer happy. Just don't give a long warranty.
Hope
this helps you all out. Take care, Dani.

 

[[email protected]]

Len, I can't believe you don't know this.

You fix the set with a piece of wire. Cut the traces going to the
filament pins and wrap the wire through the flyback. Two turns does it
alot. You need either a true RMS meter to read the 6.3 volts or on a
scope on an NTSC set you are looking for 23 volts peak to peak.

Make sure you isolate those pins completely, that there are no
compenents felt from the original filament supply. One might be
grounded on quite a large foil, in this case some type of small
grinder is useful, because you need to grind all the way around the
solder pad in some cases.

You never know what you are going to get, you might not want to
completely on the hot side because you might want to use the resistor.
If emission is strong, you do not want to feed it any more than about
26 peak to peak, that is like already having a brightwener on it and
will kill these new CRTs.

And when you wire this contraption up, don't be too neat. Don't strap
the wires all up nice and neat, that causes capacitance and that is
what will smear that color.

Keep them loose, or go the other route, tighten them up and apply
equalization to that particular output stage.

To do that you must locate some resistor, most likely in the emitter
circuit of the video output for the affected color. Then you need to
find a small capacitor to put across that resistor. The resistor must
be high enough in value to allow for the changed frequency response
desired. What's more, put a 10K resistor between the cathode and the
heater, that simulates the short, and will keep the short from
shorting if it were intermittent.

Last time I did this it was on Panasonics. I had to because it seems
their video outputs were more of a current drive than a voltage drive.
They did niot brute force it like alot of manufacturers, they actually
matched the impedance and equalization to the CRT. As such, when you
start transmitting the red video from the wire wrapped around the
flyback, it causes a capacitive load.

If you do not want to deal with putting in some pre eq on the video
output, you can just reconverge the set, with the 10K resistor in
place.

You DC supply loaded it down alot more because of mass as well as it's
HF coupling to the power source. It will not be as bad with a piece of
wire and even just reconverging it, you have a usable set.

I actually did not invent this process, but I damn well reinvented it.
If you want to know more just RSVP.

JURB

 

[[email protected]]

Len, I can't believe you don't know this.

You fix the set with a piece of wire. Cut the traces going to the
filament pins and wrap the wire through the flyback. Two turns does it
alot. You need either a true RMS meter to read the 6.3 volts or on a
scope on an NTSC set you are looking for 23 volts peak to peak.

Make sure you isolate those pins completely, that there are no
compenents felt from the original filament supply. One might be
grounded on quite a large foil, in this case some type of small
grinder is useful, because you need to grind all the way around the
solder pad in some cases.

You never know what you are going to get, you might not want to
completely on the hot side because you might want to use the resistor.
If emission is strong, you do not want to feed it any more than about
26 peak to peak, that is like already having a brightwener on it and
will kill these new CRTs.

And when you wire this contraption up, don't be too neat. Don't strap
the wires all up nice and neat, that causes capacitance and that is
what will smear that color.

Keep them loose, or go the other route, tighten them up and apply
equalization to that particular output stage.

To do that you must locate some resistor, most likely in the emitter
circuit of the video output for the affected color. Then you need to
find a small capacitor to put across that resistor. The resistor must
be high enough in value to allow for the changed frequency response
desired. What's more, put a 10K resistor between the cathode and the
heater, that simulates the short, and will keep the short from
shorting if it were intermittent.

Last time I did this it was on Panasonics. I had to because it seems
their video outputs were more of a current drive than a voltage drive.
They did niot brute force it like alot of manufacturers, they actually
matched the impedance and equalization to the CRT. As such, when you
start transmitting the red video from the wire wrapped around the
flyback, it causes a capacitive load.

If you do not want to deal with putting in some pre eq on the video
output, you can just reconverge the set, with the 10K resistor in
place.

You DC supply loaded it down alot more because of mass as well as it's
HF coupling to the power source. It will not be as bad with a piece of
wire and even just reconverging it, you have a usable set.

I actually did not invent this process, but I damn well reinvented it.
If you want to know more just RSVP.

JURB

I do know about the flyback trick and have used it many times in the
past, but more successfully on B&W's than on color sets. Its sort of
like winding a filament for a 1B3. On color though I almost always
had smearing. Now I know why. Your explanation was really good. I'll
save this post and try some of what you mentioned next time. On this
32 with the Red cathode to filament short I already did part of what
you suggessted. I wound three turns of wire around the flyback core
and connected this to the CRT filament after isolating it from the
chassis. On startup, the filament lit up like a christmas tree bulb
for the period it took for me to see this and pull the plug. Dunno...I
was certain three turns wouldn't be too much and just as certain that
the tube was history, but I was wrong on both accounts. The short is
gone and the pix is great. Go figure! Must have been some thermal
expansion at work there. Anyway, you can't argue with success. Some
times we actually win a few. I'll run it for a few days and then call
the customer and tell him its fixed, but with no guarantee of course.
This was obviously an accident but I might try it again sometime on an
otherwise dead issue. Thanks for all the great advice guys, Lenny.