Re: 453 blows HV fuse

bc
 

Sounds like you have something to investigate. Be careful not to
electrocute yourself when finding the source of the arc, it's not
supposed to arc! Try sticking plastic, move things apart, etc. to see
if you can get the arcing to stop.

The voltage on the base is fine when the HV isn't actually running -
it's both a symptom and a result but not a cause. I would suspect the
voltage is around -4.4V (or much lower than 11V) when you have the line
voltage turned down. At that point it's actually running and you're
getting HV. When the HV is not operating, the only way to get things
going is to turn that transistor on, and at -4.4V it may not be enough
to get things going.

The presence/operation of HV is what causes the voltage to go down to
-4.4V, if that would help explain what's going on here.

On Sat, 2018-04-28 at 14:24 -0700, Richard Knoppow wrote:
    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.

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