In a message dated 6/2/2006 5:47:24 PM Eastern Daylight Time, r@somis.org
writes:
Finally, there were some changes made during the SB-220 production
> run that may have affected stability; one I have heard of is that
> the spacing of the tuning capacitor was increased to prevent
> arcing,
It is NOT my experience that this was ever done. Every SB-220 I have ever
seen from one of the earliest to the latest always had the same plate tune
variable cap, however they did change the loading cap somewhere along the
line.
Perhaps this is what you meant?
Speaking of the SB-220 bandswitch. It always seemed to me that the contacts
on these switches were marginal at best for a 1 KW output amplifier, however
I do believe that Heath designed it as 1KW DC input amplifier (legal limit
at the time). That said, the manual tune up instructions only called for a
maximum of about 500 mills of plate current. So at best the output would be
about 500-600watts. At this rating the contacts may be OK, however we all
know
and the reports from others here on the reflector tout their SB-220's going
1200-1500 watts output. That is 2 times the amount Heath called for
(according to their loading instructions). Since I have autopsied dozens of
SB-220's
I have found at least 80% of them had one or more badly burned contacts.
That's 8 out of 10. Since the SB-220 has redundant contacts, many amps still
worked on the bands that had a bad contacts, but the slightest mishap in
loading (ie too much drive out of resonance) would result in a nice arc at the
contact since the current capacity of the contact was now 1/2 what it was.
It is very difficult to examine both sets of contacts on an SB-220 to see if
they are 100%. One really needs a small mirror like a dentist uses to see
the contacts and check them for burns. I'll bet that alot of you out there
who have SB-220's that were bought used probably have a contact that is
somewhat compromised and does not really know it. In the same time period
there
where other brands of amps using 3-500's which had MUCH heavier bandswitch
contacts. I have worked with many of those amps too. The likelyhood of a
burned
bandswitch contact was near ziltch on them. It is obvious that many run the
Sb-220 to 800 to 1000 mils of plate current. In order to report outputs of
1200-1500 watts this is surely the case. The contacts are simply too wimpy
for that kind of current, especially on the higher bands, which by the way
were
the contacts that were burned. Most of the time it was the 10 meter
contacts that were burned (or totally gone!) then the 15 meter or both.
Rarely did
I ever see a bad 80-40-20 meter contact.
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