Open capacitor blows fuse.

[John Popelish]

I recently bought a vintage Tektronix scope (a T922) that
was sold as a fixer upper. I found a blown fuse in the 100
volt supply to the high voltage generator. It looked like
someone had been soldering on all the semiconductors in that
area, and failed to solve the problem. The fuse is a 0.062
amp slow blow (normally with hundreds of ohms resistance).
I looked at the board for a while before it dawned on me
that the flyback type circuit would have pretty high peak
and therefore RMS to average current ratio.

There was a 10 uF 150 volt electrolytic bypass capacitor
down stream of the fuse. I pulled that and found it
measured about 500 pF. So I assumed that the fuse was over
heating because it was carrying the high RMS current of the
narrow flyback changing pulses, instead of the average
(bypassed) current. I was very proud of my deduction.

While a replacement was on order (I got a 10 uF 250 volt
film capacitor, so this would never happen, again), I found
a schematic on the web and discovered that the driver
transistor does not operate in normal, saturated switching,
flyback mode, but as a linear sine wave oscillator with
variable bias feedback current from a measurement of one of
the high voltages. The dried out capacitor, evidently
loaded the oscillator Q down, with the resistance of the
series fuse, enough to prevent oscillation, and the bias
circuit just cranked the average current up till the fuse
blew. But the dried out, open circuit capacitor was still
responsible (indirectly) for the blown fuse.

The scope is working fine now.

 

[MassiveProng]

I recently bought a vintage Tektronix scope (a T922) that
was sold as a fixer upper. I found a blown fuse in the 100
volt supply to the high voltage generator. It looked like
someone had been soldering on all the semiconductors in that
area, and failed to solve the problem. The fuse is a 0.062
amp slow blow (normally with hundreds of ohms resistance).
I looked at the board for a while before it dawned on me
that the flyback type circuit would have pretty high peak
and therefore RMS to average current ratio.

There was a 10 uF 150 volt electrolytic bypass capacitor
down stream of the fuse. I pulled that and found it
measured about 500 pF. So I assumed that the fuse was over
heating because it was carrying the high RMS current of the
narrow flyback changing pulses, instead of the average
(bypassed) current. I was very proud of my deduction.

While a replacement was on order (I got a 10 uF 250 volt
film capacitor, so this would never happen, again), I found
a schematic on the web and discovered that the driver
transistor does not operate in normal, saturated switching,
flyback mode, but as a linear sine wave oscillator with
variable bias feedback current from a measurement of one of
the high voltages. The dried out capacitor, evidently
loaded the oscillator Q down, with the resistance of the
series fuse, enough to prevent oscillation, and the bias
circuit just cranked the average current up till the fuse
blew. But the dried out, open circuit capacitor was still
responsible (indirectly) for the blown fuse.

The scope is working fine now.

If an "open" electrolytic is presented with a high enough voltage,
it can appear as a dead short at the moment if there is a breach and
an arc between plates, yet it will appear as "open" under some
testing.

This is especially true for HV Ceramics. They can be pierced by an
overvoltage, and appear as fine under LV testing, but as a dead short
in circuit or when used at the higher voltages they are rated for.

This is why diagnosing an HV multiplier circuit problem can be very
difficult. Thermal imagers help as the bad cap will always be warmer.