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465 trace brightness is uncontrollable

 

I have a 465 that, upon powerup, gets brighter and brighter until it is lighting up the
whole room. The intensity and focus controls have an effect on the brightness, but it
cannot be brought anywhere near a reasonable level. I checked the CRT grid bias
circuit (there's a test point on the underside) and the bias varies between 15 and 60 V
as I believe it should. I need a suggestion as to where I should check next, though. I
have a service manual but I'm not familiar with testing scopes, and most of the
instructions in the manual deal with calibration, not troubleshooting.

I heard that sometimes scope CRT's can short internally causing this problem, and I
checked continuity at the back of the tube. Only the filament pins appear to be
conducting between each other.

Thanks in advance!
 

It sounds like your HV tripler is defective. If you can secure a NTE538 (same as ECG538) it will work in your 465. You have to do a little adapting and drill a couple of holes but it does work. It is necessary to remove the termination from your original tripler and solder it to the end of the cord from the NTE538. I slip a thick nylon sleeve over the splice and epoxy the sleeve in place over the solder joint. Round over the solder joint to reduce the chances of a corona arc.

Reed Dickinson

I have a 465 that, upon powerup, gets brighter and brighter until it is lighting up the
whole room. The intensity and focus controls have an effect on the brightness, but it
cannot be brought anywhere near a reasonable level. I checked the CRT grid bias
circuit (there's a test point on the underside) and the bias varies between 15 and 60 V
as I believe it should. I need a suggestion as to where I should check next, though. I
have a service manual but I'm not familiar with testing scopes, and most of the
instructions in the manual deal with calibration, not troubleshooting.

I heard that sometimes scope CRT's can short internally causing this problem, and I
checked continuity at the back of the tube. Only the filament pins appear to be
conducting between each other.

Thanks in advance!

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