On Nov 3, 2004, at 2:04 PM, Vic Rosenthal wrote:
One reason that an SB-220 can develop VHF parasitics is the following:
the original suppressors use 2-watt carbon composition resistors.
These resistors tend to increase in value when they age, and
especially when they are heated. If an SB-220 has been used heavily,
especially on 10 meters or -- worst case -- on 10 (or 11!) meter AM,
the resistors will have gotten quite hot and may have changed
significantly in value, or even opened up.
If the resistors are discolored, they should definitely be replaced.
Vic -- The suppressor resistors in my SB-220 toasted during operation
on 7MHz. I replaced them with new resistors and they got toasted too.
I did the math and there was no way that 7MHz energy should have been
able to do that with a 60 or so nH L across each R. I observed that my
SB-220 had a slight second-dip in plate-anode current as I tuned
through resonance on 40m. These things started me thinking. Several
years later I concluded that Murphy's theorem that things are not as
simple as they first look is usually true.
...
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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