Toshiba TV29C90 problem; Image fades to black...

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.
 
May sound simple, but I had one in from another shop with one of the
signal transistors
in the PWM power supply in backwards...worked ok, but B + went up &
down with screen
changes. Dani.
 
Charon wrote:
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 :)
Click to expand...

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.
 
ITS 8 yrs old a 60" ,all of a sudden, it has " color like
rainbows" if you turn off the set [tv] and then turn it back on,
the picture is perfect, for several seconds, then, thecolor rainbows
reappear, { "rainbows" for a lack of a better
description}like blurred vision, heavy in colors, I'm guessing it
needs some adjustment, but i lost owners manual. THIS is the first
problem I've had since new, thanks,jiw
 
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.
 
Charon wrote:
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.
Click to expand...

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.
 
Agreed. Usually, stand-off is only for better heat dissipation and
protecting the pcb from getting to hot ("burns brown" otherwise).
Multiple resistors: Well, several reasons exist:

One is voltage splitting due to safety recommendations. Another one is
heat. It depend on budget and the cleverness of the enegeneer
developing the circuit.

For startup: TDS 320/340A/380 supply used 27 k 2 watts. Well, is a
custom buyied part (not Tek designed). Tektronix scopes of 2xxx and
similar: As Jim already mentioned, 150 k 1W. clever design ,low startup
dissipation. I like it.

Transient protection: hmm, usually not a reason for multiple resistors
in Tek supplies. Maybea reason and in use for other manaufacturers.
The Tek 2xxx series scopes use a voltage dependent resistor seperately
(VDR, directly between hot an neutral wire, right after EMI filter/fuse
combination. fuse blows when transient is lasting long enough -
"permanently").


hth,
Andreas
 
Adjustments don't fix defective\failing components.

The smps for the convergence amp board is shutting down. It could be
the smps itself, or it's detecting excessive current draw from a defect
on the amp board (which could be drawing too much current because of
bad inputs from the correction generator section.

Have it checked professionally.

John
 
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.
 
John-Del wrote:
Adjustments don't fix defective\failing components.

The smps for the convergence amp board is shutting down. It could be
the smps itself, or it's detecting excessive current draw from a defect
on the amp board (which could be drawing too much current because of
bad inputs from the correction generator section.

Have it checked professionally.

John
Click to expand...
Couldn't it also be loss of color sync? In the olden days (vacuum
tubes, etc) this was much more common.

H. R.(Bob) Hofmann
 
Many thanks for the input,
...... but the signal transistors are good and installed correctly....
also, the smps is regulating OK, but at too high of a voltage, about
155 volts... it HOLDS good regulation with or without flyback load
and with a AC line input from 90 to 130 volts.
How do I reduce the regulation voltage? .... or is there another
problem I am overlooking?
electricitym
..
..
 
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.
 
Hi B,

Just to update you on my progress getting the combo hooked up....
Today I found and bought the RF modulator, a dinky little box with
Audio visual, scart and coaxial ports. Despite me trying all the ways
possible of arranging the cables, I'm STILL flummoxed.
This is how I'm currently connected:
1. Main aerial into modulator at 'Antenna in'.
2. RF cable from 'to TV' on modulator to Antenna in on freeview
3. RF cable from TV OUT on freeview to Antenna in on DVDR
4. Audio visual cable (Red/white/yellow) on modulator to audio visual
on DVDR.
5. 'TV' scart on freeview to TV combo scart.

At this arrangement, all my freeview channels are fine, coming through
E1.
But I'm back to no signal at all from the DVDR. I cant get the start up
screen or play a disc. And whats more I'm meant to be able to get a test
signal from the modulator but I dont seem to have been able to get that
either. Boo hoo. ;o))

Floss.


--
floss
 
Thank you for the advice. I'll pass it along.


"Mark D. Zacharias" <spammenot@nonsense.net> wrote in message
news:ic7xf.17596$oW.15206@newssvr11.news.prodigy.com...
Samsung's quality and customer service have been improving - but your
friend
should not expect any of the newer high-tech type displays to be
absolutely
trouble-free. LCD, DLP, Plasma - they all have issues and potential
issues.

Mark Z.
Click to expand...
 
On Wed, 11 Jan 2006 03:35:46 GMT, "Clif Holland" <clif@avvid.com>
wrote:

Sure it does Ray. Just like the two hams on 40 today. One had installed RG8
in place of 58 and wanted to know how much better his signal sounded. I
almost fell off my stool....
Click to expand...
What? With the improvement in signal strength? ;-)

--

"What is now proved was once only imagin'd" - William Blake
 
hrhofmann@att.net wrote:

Couldn't it also be loss of color sync? In the olden days (vacuum
tubes, etc) this was much more common.

H. R.(Bob) Hofmann
Click to expand...
Ah, loss of burst or a drifting oscillator.......... I wouldn't mind
spending a day working on that stuff again.

I guess it could be anything from the OP's description, but it still
sounds like a convergence shutdown, not uncommon for these chassis.

John

Hey, where the hell are my 6GH8s!!!!!
 
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
?
 
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.
 
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.
 
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
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top