Greetings to all,
I wrote this for Tom, K0KDG, and then decided is relevant to the
entire group. So, enjoy, or destroy.
73 de K5JV
Tom,
I just ran across your wonderful SB-220 page. You did a great job
with this information. It is, and will continue to be, a great help to the
Ham community for years to come.
However, I want to point out one very important possible oversight
that, I think, is very important. The SB-220 schematic you show has a very
glaring error. It shows the AC circuit breakers in the wrong place, It
shows them to be in the main AC line which would require different values of
CB/fuse when switching between 120 and 240 VAC power. The CB's should be on
the other side of the voltage selector strip, thus placing them in one of
the individual transformer primaries, not the main AC line, which feeds both
primaries. This way, the CB's do not have to be changes when switching
between 120 and 240 VAC power because they are only protecting one of the
transformer primaries each ( always the same whether the primaries are in
series, or parallel.
Heath published this error, back in the 70's, in only one series of
the SB-220 manual. This incorrect schematic is, unfortunately, the one that
is used most often in modern articles. I immediately notified them about
it, along with many other avid SB-220 users and maintainers. They
immediately publish a correction notice and changed all subsequent printings
of the SB-220 manual. However, the harm was done because hundreds of SB-220
were sold with this incorrect schematic.
Nearly every article I have seen about the SB-220 power supply, in
recent years, shows this incorrect schematic. This subject is always great
for a rousing discussion among 75 meter Amplifier Groups. The question
always comes up from new people, "why does Heath and Kenwood (among other
manufacturers) not change the value of fuse/CB protection when changing
between 120 and 240 VAC operation". Kenwood, in their TL-922 manual, gives
the best explanation for this hookup, even with current flow diagrams.
Tom, I do not mean anything I have said here to be, in the least
bit, critical of your great effort. I repair amps all day, every day. Not
a month goes by that I do not get an amp in for major repair because the
owner doubled the size of the fuses when switching from 240 to 120 VAC power
and then cooking them.
73 de K5JV
Lon W. Cottingham
1110 Golden Bear Ln.
Kingwood, TX 77339
281-358-4207
281-358-4234 FAX
281-795-1335 Cell
k5jv@kingwoodcable.com
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