SB220 Questions

[Jim Mullen]

OK, I finally bit the bullet and picked up an SB220 at the local auction. It
was a pretty good price, belonged to a silent key and was reported to have
'some' modifications. Some mods were readily apparent; the front panel had
been stripped to bare aluminum, verniers were in place of the load and tune
controls, the band switch is a big pointy type of knob, the relative power
adjustment pot is gone, the meter switch is replaced with a toggle switch
and there are two extra rocker switches along the bottom. It was reportedly
working earlier this year, then replaced with a new Ameritron. Came with the
manual and some other notes. The price was good so I bought it.

The good news:

1.  Looks relatively well constructed
2.  Harbach capacitor bank added
3.  Harbach diode, meter board added
4.  Harbach (Measures>) suppressors added


The bad news:

1.  Extensive rewiring of the primary side of the transformers, new 10
position barrier strip added underneath.
2.  The barrier strip above forced the relocating of the bias supply
terminal strip to the center of the chassis.
3.  ALC disconnected from the back jack (probably no big thing, I doubt if
it's needed)
4.  Standby switch added (one of the new front panel switches, not needed
but OK I guess)
5.  Other front panel switch was supposed to switch between 120/240 (why,
don't ask) never connected
6.  A nice rectangular hole cut in the upper left and a mesh screen added. I
guess so you can observe the tubes.
7.  Besides the RF Sens control being removed from the panel the circuit is
disconnected at both ends.


I was going to test it out on 120V (easily available in the shop area) but
the wiring is not correct when you move the jumpers. I'll have to puzzle it
out and see why. The fan is not wired correctly for 120V. Assuming it was OK
on 240V I'm guessing the original owner just messed up this part.

Was there ever a mod out there that cut the window in the front panel? Why
would anyone remove the RF sens circuit? I did a quick google search on 220
mods and came up blank on these specific ones. If anyone has any knowledge
of such a mod please forward me the info. It will be a lot quicker than
tracing out all the changes. Other than the relocation of the bias circuit
components none of the under chassis mods would seem to have much of an
effect on overall amp performance. The bias circuit is now very close to the
T/R relay, I'm not sure if that would be problematic.

Anyone know where I can find a new front panel and knobs? Knobs probably not
too difficult, I could rob the ones off my SB200 to start while I'm looking.

Anyway, I'm off to figure out more before putting it to the smoke test. If
the tubes are OK I got a good deal, if not - oh well!

Thanks & '73...

Jim, KK1W

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