[RFI] RFI from an Alpha 87A linear

Tom Rauch w8ji at contesting.com
Sat Jan 20 08:09:49 EST 2007

> 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 

More information about the RFI mailing list