Anyone seen this? Toshiba HDTV CRT dead?

[John-Del]

I did a house call on a Toshiba 36HF12 that the complain was no
picture. I had repaired this about 6 months ago for vertical
shrinkage, and it's been running fine till now.

It develops 32KV HV, filament, 250 kine supply, G2, and all secondary
diodes sources off the fly are normal. The cathode voltages on all
three cathodes are high rail. I didn't have a scope with me, but the
60hz vertical "growl" from the yoke is unmistakable, so I know it has
vertical deflection. The G2 voltage varies from 200 to 850VDC.
Carefully raising the G2 to maximum induces no light whatsoever on the
CRT.

I brought in my old B&K 467 and hooked it up to the CRT with clipleads,
and the tube reads DEAD, like maybe the G2 or G1 pin is disconnected.
The TV is just over 2 years old, but the customer is going to try to
see if Toshiba will cough up a CRT.

Has anyone seen one of these do this?


John

 

[Jeff Rigby]

I did a house call on a Toshiba 36HF12 that the complain was no
picture. I had repaired this about 6 months ago for vertical
shrinkage, and it's been running fine till now.

It develops 32KV HV, filament, 250 kine supply, G2, and all secondary
diodes sources off the fly are normal. The cathode voltages on all
three cathodes are high rail. I didn't have a scope with me, but the
60hz vertical "growl" from the yoke is unmistakable, so I know it has
vertical deflection. The G2 voltage varies from 200 to 850VDC.
Carefully raising the G2 to maximum induces no light whatsoever on the
CRT.

I brought in my old B&K 467 and hooked it up to the CRT with clipleads,
and the tube reads DEAD, like maybe the G2 or G1 pin is disconnected.
The TV is just over 2 years old, but the customer is going to try to
see if Toshiba will cough up a CRT.

Has anyone seen one of these do this?


John

Cathode voltage rail high means the tube is turned off. It's an electonic
problem in the video out circuitry. Kine bias circuit?

 

[[email protected]]

John:
Check the G1 voltage. If it is very negative, eg: -90V to -120V,
the tube is biased off. Try connecting a resistor box from G1 to
ground, and gradually reducing the G1 voltage. If picture comes
back, you have a bias problem.
I also try a 1K ohm resistor on a jumper wire to ground. Touching the
other end of the resistor to a R,G, or B cathode should cause that
color to appear on the screen.
Hope this helps...
John

 

[John-Del]

An update, but first things first:

Jeff, let me clarify that. The voltages right at the cathodes are
exactly the same as the collectors of the kine outputs. In other words
there is no voltage drop across the 560 ohm resistors that connects the
kine outputs to the cathode. The voltage drop across the limiting
resistor is typically a few tenths of a volt when the tube is drawing
current. The collectors themselves show normal voltage drop across
the 15K resistors. And remember, even if it was a video problem, the
tube could easily be driven to raster using the G2 control (and this
one wouldn't). Sorry for the confusion.

Jdg, The G1 on most Toshibas (including this one) is actually grounded
to the aquadag, so the voltage is zero, which I didn't mention but
should have......


Okay, I have a Panasonic 34 HD in the shop that has a propblem in the
DG board, so I ran the Toshiba chassis on the Panasonic, and it runs
perfectly ( slight geometry overscan issues with the Panasonic yoke
, so the tube is most definitely bad. I've been doing this for over 35
years, and this is the first one I've seen go cold dead despite having
good filament.

Thanks for the replies.
John