Re: 453 blows HV fuse

Richard Knoppow
 

Still have to follow up and make more measurements. Its the fuse on the 20V supply to the HV assembly. Tek calls it the HV fuse even though its in a low voltage line. Probably less than an amp flows. I can see the arc. I thought at first it was along the 1meg resistor that feeds the HV to the CRT but its on a lead underneath that, probably on one of the caps on either side of the 1Meg resistor. Its a sort of corrona to the air. Very odd. Its this arc that's drawing all the current. Making measurements here is difficult both because of the narrow range where the arc is not happening and because of the somewhat cramped space.
The base voltage on Q930 is wrong. Supposed to be -4.4 volts but I get about +11 volts. This is with the fuse out so voltages are probably pretty upset. It would turn the transistor on hard. Maybe an illusion. Anyway, I will make some more measurements and make sure the other supplies are working. I know the +75V supply is OK but first thing should always be to check the PS.
Don't think its an insulation failure.
I need to study the handbook and schematic further to see what I want to measure. Very strange problem although I suspect someone else has encountered it before.

On 4/28/2018 9:32 AM, b c via Groups.Io wrote:
Okay yeah looks like the data collected do line up with each other and
your CRT HV seems to be working at least to a point.
This is F937 (2A) that you're seeing burn? I would call this the "HV"
fuse, though there are many other voltages that are quite high. How
many amperes is flowing through the fuse you're working with when the
CRT starts glowing at that circa 90V line voltage you found?
Perhaps it's just insulation failure, these scopes are quite old now.
This could cause higher current drain and burn the fuse. Are you
saying the high tension second anode lead is leaking charge? Can you
stick some plastic in its path to prevent the arcing discharge?
--
Richard Knoppow
dickburk@...
WB6KBL

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