Toshiba 61H70 61HX70 Rear Projection CRTs won't turn on

[stevie0027]

Hello. Tried doing a lot of troubleshooting on my 61H70. Have normal
Sound, but no picture at all. CRTs don't fire. Probed various points.
Have 220v coming into the CRT Drive boards. Have at least 170v at pin K
of each of the R,G,B board's V901, V902, V903.

Here's where I'm suspecting the problem. The nominal voltages shown on
my schematic (if I'm reading the RED voltages next to the pins
correctly) shows that R , G, AND B lines of the 7 pin connector of the
CRT -D/G board should be at around 3.0 volts. I'm reading 1.5V. Traced
these lines back to the SIGNAL/CONV board's Q510 VIDEO CHROMA IC
(56-pin, ta1316n or ta1276an) and pins 41, 42, 43 (B, G, R
respectively) should all read around 2.5 volts. I'm reading 0.7 volts.
Did a couple of readings on other pins on this ic and seems to read
normally (eg: pin 30 reads 4.5v as it should).

Question: is this Q510 ic typically the cause of this type of problems,
or could the other parts around this ic cause it? I would be grateful
if any of you technicians (or handymen) who have worked on 61H70,
61HX70, 50HX70 and similar models would share your experiences.

Thank you!
Steven L.

 

[Art]

Have you tried removing the three convergence yoke connections to the
convergence correction circuit and try to switch the set on? It may come on
with a very distorted, out of registration raster-picture. If it functions
repair the convergence circuitry.

 

[stevie0027]

Thanks Art for your reply. No I haven't tried removing the conv yok
connections. What bothers me is that normally, you hear a steady "buzz"
when the CRTs are on. I'm assuming that's the current going through
coils sound. But there's absolutely no sound except the normal, clear
audio for for any tv station. But I'll try what you stated. I'm
assuming the convergence yoke connections are the (looking at the
schematic) 7-pin connections with Return and Drive lines on each of H
and A side (not sure what H and A stand for). By the way, when you say
"repair the convergence circuitry" are you implying that I change the
convergence ic's (the TWO STK392-110 ICS)? Believe it or not, about 2
years ago, I changed both of those when I was having some bluish/red
convergence problems and it did the trick. I really don't think it's
the same problem though.

Thanks again. I'll report back after I try this.

Steve

 

[[email protected]]

So here's what i've discovered thus far. It appears to have partly been
an overvoltage problem somewhere. Two fuses on the the DEF/POWER bd -
F851 AND F852 were opened. After installing fuses, the GREEN AND BLUE
CRTS are firing bright now. But the Red is not. Also, there are thin
lines not, pictures showing for the green and blue. So looks like there
is some sort of convergence problem. The sockets to the Red crt have
pins reading the same voltages as the blue and green. Tried
disconnecting the connections to the CONVERGENCE Yoke and no change.
Disconnected the DEFLECTION YOKE and of course all CRTs do not fire.
Looking directly into the Red CRT, you can still see a VERY thin red
line so I believe at least that that is the signal coming from the coax
connections.

One question I have: Is the CRT Board that contains the CRT socket the
only thing that makes the CRT fire bright? Or can the SVM board do that
as well.

Will I damage the CRTs if I interchange the Red and with the Green? I
want to trouble shoot to make sure the Red CRT is not bad. Looking at
the clear glass portion of the CRT (near the socket), the copper pins
glisten nicely for the green and blue, but just faint for the Red.

Sound is still good.

Any help would be appreciated.

Steve

 

[[email protected]]

More info. If I disconnect the CRT boards from the Green and Blue
(especially Green) CRTs, the Red CRT is BRIGHT! Appears to be some sort
of a load problem. When all three are connected, the Red brightness
goes down almost to nothing. Already suspected that maybe it was a crt
problem, so installed a brand new factory Red CRT assembly (cost me
$250!). Any help would be appreciated.

Steve
aka stevie0027
aka [email protected]

 

[mark]

You probably need a circuit schematic, service manuals.net would be a good
place to look their 32 dollars usually but their invaluable when doing this
type of repair.

 

[Leonard Caillouet]

Not sure what you mean by "firing" and how that relates to a convergence
problem and I don't see why you replaced a CRT.

First thing to do on this set is to look for bad solder joints on the black
jumper wires on the boards, particularly the one on the signal board near
the connector between the two boards. Very common problem that will cause
blown fuses. Also, convergence outputs go on these quite frequently, along
with output resistors. Be sure to check for excessive dc on the lines from
the convergence generator to the output ICs. If you have a convergence
problem and you replaced the ICs before you could have a setup problem or a
leaky op amp in the convergence generator.

Leonard