SB-310 questions.

[wa1vwl]

BACKGROUND:
After searching for a while, I finally got an SB-310. This one is in
real good shape inside and out. On the "W1ID Strokability Scale", it's
an 8+ and will clean up easily to a 9+. Even got the SSB filter. I might
look for a CW filter later. Unfortunately, no manual. No matter, I'll
use my SB-300 manual for now. One of the previous owners (obviously
someone with a weak stomach) had the kit installed to change the 11
meter band over to 15 meters. There is a label on the back indicating it
was done at a Heathkit service center. The kit included a new faceplate
installed over the band switch as described by Penson. Removing the
faceplate reveals the old, intact labeling as one would expect. The
chassis has all the proper silkscreening including a new sticker over
the old 11 meter crystal label to show the new crystal frequency.

QUESTIONS:
There is one glaring anomaly. The front panel mode switch shows a RTTY
position in addition to the usual LSB/USB/CW/AM modes. All the
literature I have only shows 4 modes, no RTTY. Inspecting the mode
switch reveals that it is indeed only 4 positions. In fact there are no
contacts where you'd expect RTTY to be. One thought would be that this
might have been a "field" modified SB-301. I don't think so. RTTY was
one of the changes incorporated in the 301 versus the 300 but the front
panel bandswitch is for the 310. Would I be correct in assuming this
might have been a manufacturing problem/decision? When the change was
made from the SB-300 to the 301, there might not have been an easy
(low-cost) way to manufacture SB-310 front panels without making a new
screen. Also, there might have been a number of SB-310 kits in-house
with the parts already packed. It might have been considered too costly
to repack with SB-301 mode switches, crystal, and socket, write an
addendum, etc? The chassis is punched and silkscreened for an SB-310 so
there is no doubt in my mind as to the pedigree of this receiver. I
suppose if one was so-inclined, an attempt could be made to locate an
SB-301 mode switch and add that mode myself. I don't have much interest
in RTTY but it might be more esthetically pleasing. The second anomaly
is that the filters are of the smaller form factor indicating it must
have been a later production variant. I haven't checked the AM filter
part number yet but I sure hope it's NOT the stinky (much to sharp for
AM) 3.75 KHz filter as used in the later 303/313 series! I really wanted
to get my hands on the 5 KHz filter. Bummer if it isn't. So, I wonder if
any one out there has ever come across this or any other SB-310 variant?
As an aside, I also have a Drake SW-4A variant. The manual that came
with it shows an SW-4A on the cover with a B line dial, tuning knob, and
front panel trim. However, the schematic is for the SW-4 which used a
pair of bipolar transistors in the PTO as opposed to a FET and bipolar
combination. As if that weren't enough, the front panel trim match the C
line and the PTO assembly and tuning knob are from the C line also.
Therefore Heathkit may not have been alone. Maybe I just have a
propensity for picking up strays. Comments anyone?

Gerry/W1ID

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