Re: 453 blows HV fuse

bc
 

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?

On Fri, 2018-04-27 at 14:01 -0700, Richard Knoppow wrote:
    Some progress. First to answer your questions; D940 appears 
to be good.
  I do not have either a ring tester or a bulb tester. What I use 
is a metered Variac and put a Triplett 630A across the fuse 
holder to measure the current. Its set on the 12A range and I can 
see clearly when the current  begins to rise with line voltage. I 
tried disconnecting the CRT at its socket and also disconnected 
the HV lead to the helix at the HV compartment. Made no 
difference but thank you for the suggestion.
    So far, here is what I've found. When the line voltage begins 
to get to normal something in the HV compartment begins to arc. I 
found its a lead from the HV transformer to the filter for the 
"helix". Since there was HV I decided to see if I was getting any 
sort of trace. At around 90V line voltage (I have to note this 
again) I could get a trace with the beam finder. The trace was 
distorted and that makes me think I've been chasing my tail on 
the HV. I now think the problem is in one of the lower voltage 
supplies. I have errands to run today so will get to this 
tomorrow. It would be great if I had a second decent scope but I 
don't so will have to trace this down via a voltmeter. In any 
case I think the presence of a trace of sorts indicates the HV is 
OK and the CRT is OK. I now need to study the other power 
supplies to find the problem. I can run the scope at full line 
voltage with the HV fuse removed. The distorted trace should give 
me some clue as to what has failed. My guess is a filter cap 
somewhere since the thing overheated.
     Any suggestions would be welcome. In any case I now have 
some confidence that I can fix the thing.

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