> Anyone ever see this or have any idea what is happening in
> the 87A or any amp. . thanks Van
Arcs like this are generally an open circuit or "poor
connection" arc. It could be a HV rectifier diode burnt or
cracked in half, a poor solder joint, or even corona or
arcing from a current limited source (say a high resistance
bleeder or multiplier resistor being open internally).
It is not is an arc from a HV buss to chassis or some other
point capable of sourcing high fault currents. It almost
certainly is not an electrolytic, blocking, or bypass
capacitor although capacitors across diodes in series
strings can sometimes do this.
It could really even be inside the power transformer (an
open or bad solder joint), in a switch (a bad series
connection), a choke, resistor, or anything that when it
faults won't dump real high currents. I've seen them all
over the years!
I hate to tell you how I locate noises like this if a close
visual inspection and component disconnect turns up nothing.
I have an old airplane headset that uses air pressure
through hollow rubber tubes. It connects to a two foot long
Teflon tube "probe". I listen for the arc by pointing it at
different areas of the amp a few inches out of the amp. I
wouldn't tell anyone else to do this because the amp has to
be on and safety interlocks defeated, but on very rare
occasions that's the only way to find corona or arcing.
73 Tom
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