Over the years I've used this trick numerous times to diagnose problems,
with good success (and no burning a dot in the face of CRT's!). Recently
though I ran into something quite unusual.
The set is the 31" Hitachi I mentioned in an earlier post. After replacing
some leaky caps, the set appeared to work great. But Q752 (pincushion amp)
was still overheating. I replaced every diode, cap, transistor, choke etc in
the horiz. output area, with no luck. In desperation I tried powering the
set with the yoke unplugged - and the HOT fried within 2 seconds. Previous
experience leads me to suspect a flyback problem, but right now I suspect
there are some gaps in my knowledge that are confusing me.
Any ideas?

That model is notorious for the little round blue capacitors in the
horizontal deflection to go OPEN, change value. Causes all kinds of
parts failures, especially the pincushion and HOT.


Chris F. wrote:
Over the years I've used this trick numerous times to diagnose problems,
with good success (and no burning a dot in the face of CRT's!). Recently
though I ran into something quite unusual.
The set is the 31" Hitachi I mentioned in an earlier post. After replacing
some leaky caps, the set appeared to work great. But Q752 (pincushion amp)
was still overheating. I replaced every diode, cap, transistor, choke etc in
the horiz. output area, with no luck. In desperation I tried powering the
set with the yoke unplugged - and the HOT fried within 2 seconds. Previous
experience leads me to suspect a flyback problem, but right now I suspect
there are some gaps in my knowledge that are confusing me.
Any ideas?

If there is a polarized capacitor in the horizontal
drive, replace it. Some units will "blow" the output,
without the yoke, & some can burn the crt badly.
Use caution. Unplug the CRT board, to keep from
burning the CRT. Rono.

Rono wrote:

If there is a polarized capacitor in the horizontal
drive, replace it. Some units will "blow" the output,
without the yoke, & some can burn the crt badly.
Use caution. Unplug the CRT board, to keep from
burning the CRT. Rono.



hmm, unplug the CRT board?, since it looks like there are
some dangerous idea's being past around! what happens to the
voltage on the anode while you have this CRT board unplugged?
i mean, disconnected the yoke and firering it up don't seem
to be a problem here! i can only assume your going to fire up
the TV with the anode voltage on the CRT while the CRT board
is off?,
i wouldn't want to be the one working in the back of that set
at that time!

hmm, unplug the CRT board?, since it looks like there are
some dangerous idea's being past around! what happens to the
voltage on the anode while you have this CRT board unplugged?
i mean, disconnected the yoke and firering it up don't seem
to be a problem here! i can only assume your going to fire up
the TV with the anode voltage on the CRT while the CRT board
is off?,
i wouldn't want to be the one working in the back of that set
at that time!


Nothing at all happens, with the neck board unplugged the CRT cathodes
aren't heated, no current flows, the pins are dead. If you *do* get any
arcing then the tube is up to air.

Agreed: unless the CRT has a direct short internally the HV will be
contained within the Glass Packaged Capacitor called the CRT. However you
will not want to come into close proximity to the CRT Pins while doing this.
If the CRT indeed is shorted the Uthor Voltage will occasionally be
available directly at the pins. This procedure is used when diagnosing
problems within RPTV devices to not induce the CRT Burns, Etc related to
deflection circuit failures.
"James Sweet" wrote in message
news:BEspe.13241$yS2.874@trnddc07...


hmm, unplug the CRT board?, since it looks like there are
some dangerous idea's being past around! what happens to the
voltage on the anode while you have this CRT board unplugged?
i mean, disconnected the yoke and firering it up don't seem
to be a problem here! i can only assume your going to fire up
the TV with the anode voltage on the CRT while the CRT board
is off?,
i wouldn't want to be the one working in the back of that set
at that time!


Nothing at all happens, with the neck board unplugged the CRT cathodes
aren't heated, no current flows, the pins are dead. If you *do* get any
arcing then the tube is up to air.

Art wrote:
Agreed: unless the CRT has a direct short internally the HV will be
contained within the Glass Packaged Capacitor called the CRT. However you
will not want to come into close proximity to the CRT Pins while doing this.
If the CRT indeed is shorted the Uthor Voltage will occasionally be
available directly at the pins. This procedure is used when diagnosing
problems within RPTV devices to not induce the CRT Burns, Etc related to
deflection circuit failures.
"James Sweet" wrote in message
news:BEspe.13241$yS2.874@trnddc07...


Well you keep on doing that pratice, i know far tomuch about CRT theory..
i service irradiation units designed to cross link polymores which
are nothing but huge CRT's with a titanium window and steel cone scan
section.
and one of the golden rules is not to apply Voltage (which reaches in
the 1.5 M range) unless the hot cathode is connected to the path.

i know your not working with irradition units but the theory of
operation is the exact same thing, just in a much larger scale.

Well you keep on doing that pratice, i know far tomuch about CRT
theory..
i service irradiation units designed to cross link polymores which
are nothing but huge CRT's with a titanium window and steel cone scan
section.
and one of the golden rules is not to apply Voltage (which reaches in
the 1.5 M range) unless the hot cathode is connected to the path.

i know your not working with irradition units but the theory of
operation is the exact same thing, just in a much larger scale.



You're comparing apples to oranges, 1.5M? We're talking 15-35 KV here, the
pins on the neck are connected to transistors rated at 250-300V. Comparing a
TV CRT to an irradiation unit is like comparing a hand crank dynamo to a 350
megawatt turbine, the basic principal may be similar but in practice the
difference is night and day.

If you ever have a chance, flip through the factory service manual for any
projection TV they'll almost always tell you to remove the neck boards
before powering up a set with deflection problems, forget to do it while
you've got the protection circuit disabled for testing and you've just
burned up $900 worth of tubes before you realize what happened.

Jamie !! What are you smoking ?



kip

"Jamie" wrote in message
...
Art wrote:
Agreed: unless the CRT has a direct short internally the HV will be
contained within the Glass Packaged Capacitor called the CRT. However you
will not want to come into close proximity to the CRT Pins while doing
this. If the CRT indeed is shorted the Uthor Voltage will occasionally be
available directly at the pins. This procedure is used when diagnosing
problems within RPTV devices to not induce the CRT Burns, Etc related to
deflection circuit failures.
"James Sweet" wrote in message
news:BEspe.13241$yS2.874@trnddc07...


Well you keep on doing that pratice, i know far tomuch about CRT theory..
i service irradiation units designed to cross link polymores which
are nothing but huge CRT's with a titanium window and steel cone scan
section.
and one of the golden rules is not to apply Voltage (which reaches in the
1.5 M range) unless the hot cathode is connected to the path.

i know your not working with irradition units but the theory of
operation is the exact same thing, just in a much larger scale.

Rats: Just give me a nice clean magnetron, about -1.8Kv and a wave guide.
Opps, K-mart will sell me one for about $40.00 US. Just to cook food. Of
course this has little to do with service of teles, however the voltages
present within the microwave ovens can be lethal, even the current stored in
the HV cap with the flippin thing unplugged can hurt.
Safety first: normally there in no one willing or able to give you cpr or
mouth to mouth when things do go wrong while working on these items, Eh?
Thomson requires that you make voltage measurements at pins of the crt
socket with the set turned on, also with the socket removed from the crt:
again with the set turned on, to confirm the tube is not causing the
symptoms. By comparing the voltages with the crt in and out of the circuit
you then may diagnose the video, brightness, focus, problems affecting the
set. Loss of deflection, be it from an open yoke, bad connections, failure
of the vertical or horizontal deflection devices, can cause permanent burns
to the projection tubes, and will also burn direct view crts over time.
Also,defects inthe 'Spot Killer' circuits may cause problems.
"kip" wrote in message
.. .

Jamie !! What are you smoking ?



kip

"Jamie" wrote in message
...
Art wrote:
Agreed: unless the CRT has a direct short internally the HV will be
contained within the Glass Packaged Capacitor called the CRT. However
you will not want to come into close proximity to the CRT Pins while
doing this. If the CRT indeed is shorted the Uthor Voltage will
occasionally be available directly at the pins. This procedure is used
when diagnosing problems within RPTV devices to not induce the CRT
Burns, Etc related to deflection circuit failures.
"James Sweet" wrote in message
news:BEspe.13241$yS2.874@trnddc07...


Well you keep on doing that pratice, i know far tomuch about CRT
theory..
i service irradiation units designed to cross link polymores which
are nothing but huge CRT's with a titanium window and steel cone scan
section.
and one of the golden rules is not to apply Voltage (which reaches in
the 1.5 M range) unless the hot cathode is connected to the path.

i know your not working with irradition units but the theory of
operation is the exact same thing, just in a much larger scale.