Originally Posted by Dave Gostelow

Hi,
I have a JVC widescreen TV AV-28WR2EK which has a fault with the blue gun,
or its driver. I searched the NG and found some good posts about similar
problems which suggested it was probably a short in the tube (and to try
floating the heater).
Well, I tried a few diagnostics:
swapped the blue and red cathode drives at the final resistor, fault still
on the blue gun.
lifted the drive transistor collector on blue so that the cathode voltage
rises to the supply, fault still there on blue.
'floated' the heater using 3 turns of wire around the HT transformer core,
and fault is still there!

So I begin to think there isn't a heater to blue cathode short, but maybe a
blue to something else short.

Then I tried disconnecting the blue cathode completely, by lifting the final
resistor, and the blue gun turns off! So is it shorting to something or not?
If I stick my DVM on the blue cathode and ground to measure its 'floating'
voltage it turns on strongly, so its being pulled down by the DVM. If I
measure voltage between blue cathode and supply I get the blue gun very
faintly on and a voltage reading of about 15-20 volts (blue gun more
positive than supply).

The supply voltage is 201V and before I started fiddling I saw at all 3
cathodes I was seeing voltages of 150-170 volts depending on the picture.

I would have decided the tube had a short except for the behaviour when the
cathode is left floating, which suggests to me that the blue gun would
probably turn off if it was driven to a higher voltage than 201V.

Has anyone seen a similar problem? And if so, was there a fix? Or, does
anyone have any further ideas for things to try?

Dave.