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Anyone seen this? Toshiba HDTV CRT dead?

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

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

Reply to
Jeff Rigby

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

Reply to
jdgill

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

Reply to
John-Del

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