"Rick " ) writes:
Typically, can ham-bands-only receivers such as the Drake R4 and Collins
75A or 75S series, transmitters such as the Collins 32S and Drake T4
series, and transceivers such as Drake TR-4 and Collins KWM-2, be modified
to work on CAP and MARS frequencies?

I know that some radios have provisions for installing crystals so that
they'll work on off-band frequencies.

How can I determine which radios are modifiable that way, and also where
is a good place to get crystals these days?

CAP and MARS frequencies were (I don't know if it's the case now) generally
chosen to be adjacent to ham bands so ham equipment could be used. There's
little sense of having an auxiliary if the operators have to buy
extra equipment or the organization has to do so.

Remember, they date from before the new fangled equipment came along.
Transmitters were easy, just pop in a suitable crystal and the rest of
the unit would have enough leeway to tune the adjacent frequency. Receivers
were more of a problem, since retuning generally meant retuning the
local oscillator, there weren't always crystals to select the range. But
a receiver that had a bit of overlap was usually okay.

Hence, most equipment would be modifiable, if there wasn't enough overlap.

Michael VE2BVW