I've got a Heathkit SB-102 that I'm working on making operational (problems with transmit, receiver works just fine). This model radio has a 9-pin Molex connector in the back, intended to connect an external auxiliary VFO.

By design, this feature is intended to support "split" operation -- listening on a different frequency from the transmit setting (much more different than the usual 1 KHz offset of the CW setting in this model family). It occurred to me to wonder, however, whether this external frequency input could be used to operate on bands outside the radio's normal complement of 80, 40, 20, 15, and 10m amateur bands. There are a number of other amateur bands available within this freuqency range (30m, for instance, as well as 17m and 12m) as well as bands outside this range that might be within the capability of the hardware (160m and 6m come to mind).

Is it possible to use the external VFO connector on an SB family radio to transmit and receive on amateur bands outside the 80-10m range, or unsupported bands within that range? Would it become possible if a bypass for the internal VFO (aka LMO) were installed, so the "primary" operation was controlled by the external VFO?

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    $\begingroup$ You don't happen to have some block diagram to show where in the double-conversion architecture of the radios of that era the SB-102 uses the VFO? $\endgroup$ – Marcus Müller Oct 25 '19 at 18:24
  • $\begingroup$ @MarcusMüller 's question is an important one. However, to make it work on other bands, you're looking at many other modifications besides --and likely instead of-- the VFO. The crystals and tuned circuits are some of the factors. $\endgroup$ – Mike Waters Oct 25 '19 at 18:31
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    $\begingroup$ huh, the SB-102 manual I can find shows no 9-pin connector on the back. Would you have a photo of yours? Also, if that is a modification, that'd be cool – but then we'd need to look inside and see where that taps into the existing circuits. $\endgroup$ – Marcus Müller Oct 25 '19 at 18:31
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    $\begingroup$ What @MikeWaters says is on-spot: There's basically but one point where in a IF receiver, we have "freedom" in frequencies, and that's typically the first mixer's LO, because all other frequencies are fixed to what the fixed-frequency filters can operate at. A simple consideration by the designers: mix to a fixed IF, and only build ONE good filter for all bands you want to operate. $\endgroup$ – Marcus Müller Oct 25 '19 at 18:33
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    $\begingroup$ Many Google searches for terms such as heathkit sb-102 "30 meters", warc bands, and modifications turn up nothing. This does not prove that it cannot be done or that nothing can be found (you could also try sb-101 and hw-101), of course. My suggestion is just enjoy the bands that it came with. :-) But you've asked a very good question. +1'ed. $\endgroup$ – Mike Waters Oct 25 '19 at 18:45

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