[Swan] Re: 700CX HUM!!!

[Jim]

One the 117XC, I'd suggest replacing all the caps, all the diodes, and the 
carbon comp. resistors. Aging caps are common knowledge. On diodes, it is 
possible for one or two to fail open and have the power supply still limp 
along but with probably some additional ripple when it's loaded. Also the old 
resistors are very likely to float out of spec. I know the resistance on mine 
when I checked it was at something like 150% or 200% of the rated value. This 
higher resistance along with the current the radio needs to pull through it 
was likely making the resistor operate above it's rated power dissappation.

The replacement board is probably the best way to go. You might save a bit on 
the parts by shopping around, but rewiring the power supply salvaging nothing 
but the transformer and the choke isn't necessarily a great project for the 
faint of heart recognizing the voltages that will be floating around in your 
work when you're done.

I replaced all the diodes in the HV supplies with 1 kV diodes. I don't 
recommend replacing capacitors with hugely higher values (it greatly 
increases the RMS current drawn from the transformer). Since a lot of HV is 
in the power supply, I used spaghetti insulation on most of the exposed leads 
I had.

73 de
Jim W4JBM

http://www.geocities.com/w4jbm

"With a soldering iron in one hand, a schematic in the
other, and a puzzled look on his face..."

Soon in the new QTH at New Dog Iron Ranch!