Retrace lines appared after arcing

[Charon]

I have an older crt projection big screen and I now have retrace lines
that appeared after turnning the set on a few times and hearing a arc
immediatly when turnning on. I waited too long to fix the arc and it
has affected something else.

The arcing was comming from the Red CRT from under the node and arcing
to the chassis, I have since fixed this but have been unable to track
down what is causing the retrace lines. The B+ voltage appears correct
but I do not have an oscilloscope to test ripple in any of the parts.
Any one know what the arcing may have effected ? The blanking circut or
perhaps from the flyback backwards ?

 

[[email protected]]

Post the rest of the troubleshooting information so someone can point
you in the right direction. Things like which crts have the retrace
lines, crt voltage measurements, etc..

 

[James Sweet]

I have an older crt projection big screen and I now have retrace lines
that appeared after turnning the set on a few times and hearing a arc
immediatly when turnning on. I waited too long to fix the arc and it
has affected something else.

The arcing was comming from the Red CRT from under the node and arcing
to the chassis, I have since fixed this but have been unable to track
down what is causing the retrace lines. The B+ voltage appears correct
but I do not have an oscilloscope to test ripple in any of the parts.
Any one know what the arcing may have effected ? The blanking circut or
perhaps from the flyback backwards ?

Are the retrace lines on all three CRTs?

 

[Leonard Caillouet]

Post the brand, model, and chassis. Post the info that others have asked
for, too. More info yields better advice. Less info yields guesses, except
from the true psychics on this group. Actually, I guess a true psychic
would not need to read the group at all. Maybe phychos is more likely on
usenet...

Most likely thing would be an open resistor, but with HV arcing things can
get unpredictable.

Could be unrelated to the arcing, if the arcing was caused by a coolant leak
like we see on some of the RCA sets. Could also be that the CRT was
damaged.

Leonard

 

[[email protected]]

I have an older crt projection big screen and I now have retrace lines
that appeared after turnning the set on a few times and hearing a arc
immediatly when turnning on. I waited too long to fix the arc and it
has affected something else.

The arcing was comming from the Red CRT from under the node and arcing
to the chassis, I have since fixed this but have been unable to track
down what is causing the retrace lines. The B+ voltage appears correct
but I do not have an oscilloscope to test ripple in any of the parts.
Any one know what the arcing may have effected ? The blanking circut or
perhaps from the flyback backwards ?

It's usually a driver or output filter circuit resistor/capacitor open
or change in value,
guess you will have to test by substitution. Could also be crt did you
try swapping the
crt drive boards? If the lines go away its in the drive circuit if
they stay then it probably the
red crt.

 

[Charon]

The retrace lines are all white, so it should be comming from all of
the crt's. The model is Hitachi CT4532, I have a pdf service manual for
it. And can make this available to anyone interested.
www.bauld.com/~eric/ct4532.zip

B+ was at 111 V Should be 110
Unsure of the crt voltage, or how to test that yet.

I am trying to get my hands on a ocsilloscope for further tests.

Im rather new to electronics repair, so I learn as much as I can as I
go. I have wanted to do electronics repair for some time as a hobby. I
have been reading, reading and more reading but know far from
everything

 

[[email protected]]

You don't need an o'scope yet. Simply measure all the voltages
(carefully) that are on the crt circuit boards. Pay special attention
to the approximately +200Vdc going to the video output and the
G2/screen voltage. If there is a G1 voltage that could be important
also.

Those are the three "common" voltages for all three crts.

 

[James Sweet]

The retrace lines are all white, so it should be comming from all of
the crt's. The model is Hitachi CT4532, I have a pdf service manual for
it. And can make this available to anyone interested.
www.bauld.com/~eric/ct4532.zip

B+ was at 111 V Should be 110
Unsure of the crt voltage, or how to test that yet.

I am trying to get my hands on a ocsilloscope for further tests.

Im rather new to electronics repair, so I learn as much as I can as I
go. I have wanted to do electronics repair for some time as a hobby. I
have been reading, reading and more reading but know far from
everything

Well it sounds like you're off to a decent start, be real careful using
a scope on a TV, it's absolutely imperative that you use an isolation
transformer to power the TV for that. Good luck, projection sets can be
a real pain.

 

[Charon]

I tested the voltages on the three CRT boards.... and provided I did it
correctly. (I followed the servic manual schematics)
I only read approx 70V - 82V(once warmed up) dc on each one of the CRT
circuit boards, Service manual says it should be 200V

The screen voltage read around 776 give or take a few volts, there was
nothing on G2 Voltage found in the service manual, I am guessing that
screen is G2 voltage. And I could not find out what it should be in the
schematic, it just listed it as screen.

Looks like the the +200V is way off....
Have to figure out where to go from here now.

 

[James Sweet]

I tested the voltages on the three CRT boards.... and provided I did it
correctly. (I followed the servic manual schematics)
I only read approx 70V - 82V(once warmed up) dc on each one of the CRT
circuit boards, Service manual says it should be 200V

The screen voltage read around 776 give or take a few volts, there was
nothing on G2 Voltage found in the service manual, I am guessing that
screen is G2 voltage. And I could not find out what it should be in the
schematic, it just listed it as screen.

Looks like the the +200V is way off....
Have to figure out where to go from here now.

Yeah Screen = G2, IIRC it's a few thousand volts on most sets.


Trace the 200V back to wherever it comes from, might be as simple as a
resistor that's gone high.

 

[[email protected]]

Where was the ground lead of your meter during your testing?

If you had the ground lead of the meter on the ground that was going to
the crt pcbs then yes the +200v is way too low.

The G2/screen voltage typically reads on a meter between 350 and 900
volts on most sets depending on where it is adjusted and the specific
crts in use.

At this point you could have an open 200v line fusible resistor, and/or
a damaged video output device that is loading down the 200v line.

 

[Charon]

I used the ground lead on both the chassis and the GND pin on the crt
pcb, I read the same voltages each time.

Ill try tracking the 200v backwards and see what I get. Although I will
have to upgrade myself on how to test some components while still on
the pcb.

 

[Charon]

Ok
Ive gone backwards and tested various points I have the following
circut

R707 D702
FBT-------/\/\/\/-------->|--------------------------200V to screen
| |
R742 C713
| |
GND GND

The voltage comming from the FBT was 110, the voltage on the other side
of R707 was about .5 of a V.

I pulled the resistor off the board and tested it. I cannot get a
reading. It should be a 2.2 Ohm resistor.
I think the problem has been found. Is there anything else I should
watch out for ? Something that may have caused the resistor to go high
?

 

[[email protected]]

R707 is there as a fuse, so if it opened up there is a hard load on the
200v line somewhere.

The diode simply rectifies the ac pulses coming off the flyback, make
sure it measures like a diode and is not shorted.

Odds are very high if you simply replace the fuse, er resistor, that it
will go up in smoke as soon as the set is powered on.

You can at least narrow down the problem to a specific crt pcb by
disconnecting the 200v line from each of the crt circuit boards and
then making resistance measurements of each crt circuit board between
the 200V in and the ground to the crt pcb. I am betting that one of
those will be very low. Then it is a matter of finding the video
output device that has shorted on the crt pcb.

 

[Charon]

I dissconnected the 200V line from each of the crt's and did a
resistance measurement on each of the boards. Between the GND pin and
the 200V on the crt pcb I had infinate resistance measurement with 200v
dissconnected on each crt pcb.
I also did a resistance measurement with the 200v connected to each crt
pcb with the 200v disconnected on the other crt's. And they each read
equal at 234 k Ohm. My guess is because it was reading resistance the
other way around with the 200v connected.

I also tested the transistor on each of the crt pcb's in circuit they
all read exactly the same.

The diode measures just fine at .5 V from the anode to the cathode and
nothing the other way around.

 

[Charon]

I was talking to my father about the resistor, and the resistor did not
turn black or burn out. And he seems to figure it was just a faulty
resistor.

I also just found this in the service manual, which I did not
understand untill talking on here.
And it lists the exact problem i had and tells me to check the voltage
if it is low then(should the voltage be low) test and replace D702 and
R707

I have a picture of that problem from the service manual

http://www.bauld.com/~eric/ct4532error.jpg

 

[[email protected]]

You still need to check the load on the 200v line after making sure the
diode is in fact good.
Given the history of the arcing I am still betting on a shorted video
output on the red crt.

 

[Charon]

By checking the load, do you mean once I replace the resistor check to
make sure the voltage is +200Vdc ?
Or the resistance measurement ?

 

[[email protected]]

After verifying that the diode is in fact good.
You can either:
A. Replace the resistor and wait for the magic smoke to come out of it,
with a chance of causing other damage.
B. Before replacing the resistor. Measure the resistance on the 200v
line to ground where it goes to the crt pcbs and make sure you are not
reading some really low resistance. The resistance reading should be
way more than the 2watt or so resistor on the collector of the video
output. That resistor is usually around 1.5K - 2K as a rough guide.
If one of the transistors is shorted you will have a reading roughly
equal to the value of that resistor. Take a look at the schematic on
the crt pcbs.

For your information, the so called troubleshooting guide is done by
engineers who are guessing as to the typical failures, not technicians
with real world failure experience in the set. Yes it is possible that
the diode or resistor is the only problem, but given the history of the
set you need to confirm the other possibility.

 

[Charon]

I am trying to make sure I understand you correctly, after all I am
still learning
I had disconnected the 200Vdc on each of the crt pcb's previously. On
each of the crt pcb's the resistance between the +200Vdc in on the crt
and the GND pin was infinate on each of the crt boards.

I also measured the resistance between 200Vdc and GND when it was
attached separatly to each crt and then read a resistance of about
234k.

Is the first resistance measurement what I am looking for or am I
checking the wrong spot ?

This is really appreciated by the way, I have learnt more in the last
month about electronics then I have in years.