[Collins] Boat Anchor Fans

[Dr. Gerald N. Johnson, electrical engineer]

There are a few parts in a PA that ARE sensitive to dirt accumulations.
Those with quite a bit of voltage across them. Those with DC tend to
accumulate dirt like an electrostatic precipitator, especially the DC
bypass at the bottom of the RF choke, the stand off insulator for the RF
choke and the RF coupling (DC blocking) capacitor between the top of the
RF choke and the plate tuning capacitor. Then the insulators of the PA
tuning capacitor cause trouble when they get dirty. Any of these paths
can arc through the dirt creating a carbon path. It seems to have been a
common mode of failure in some amplifiers, probably worse in the bypass
and coupling capacitors when they have small lead spacings typical of
disk ceramics. Less of a problem with Centralab 858s in those positions,
or home brew with TV horizontal output capacitors for those circuit
positions. Many ham amplifiers are prone to mysterious flashovers, some
come from parasitic resonances and oscillations (one caused insulation
breakdwon in the middle of the plate transformer winding), or from dirt
accumulations on those coupling capacitors and standoff insulators.
Cleanliness is a virtue in achieving long amplifier life accompanied by
reliable operation.

I've found a 1/2" flat artists brush (a box arrived here in MARS junk
once) is excellent for loosening dust while sucking it up with a vacuum
with a small diameter hose. Long before computers, I acquired a milker
inflation from a farm supply store. The large end fits the end of the
standard vacuum cleaner hose and the small end is just under a half inch
ID and its very flexible. Now for several times the price the computer
cleaning kit would work well.

It doesn't hurt to dust and vacuum the cd rom and floppy drives in the
compute either and all visible accumulations of dust and dirt. The power
supply and CPU fins are the most critical though.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.