I sure wish Good Guy Ham would sign his emails... (That is just common
courtesy). It sure would be nice to know who he is.
The main issue with SB-220 band switches is because of the front panel
layout, the band switch gets turned instead of the loading control when
in tune-up mode.
The very late SB-220s and HL-2200s had longer tune caps. Or Heathkit
made them available on a replacement basis. I have seen several SB-220s
with them.
73
Jim W7RY
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
On Behalf Of Gudguyham@aol.com
Sent: Saturday, June 03, 2006 10:42 AM
To: r@somis.org; n4zr@contesting.com
Cc: craxd1@verizon.net; amps@contesting.com
Subject: Re: [Amps] R. MEASURES PRAYERS ANSWERED
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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