Heathkit Crystal Filters.......SB-300, HW-101, etc.

[Herman N4Ch]

Just a couple of comments on the crystal filters used in these
Heathkits.....saw a couple of notes here from a couple of guys earlier today:

ALL SB series radios, from the original SB-300 receiver up thru the
solid-state HW and SB 104 radios used the same IF, 3.395 MHz, so any SSB filter from
one of these radios will work in the others, TO A DEGREE.    Changing filters
from one radio to another is possible, but there may be mechanical issues.
Some changes were made early on, which you need to be aware of.   The SB-300 (the
first piece of gear released in the "SB" line) used a filter which was a
little wider than what was used in later products.   I'm not sure exactly when,
but later filters became a little narrower, and the "footprint" (fitting 4 holes
in the chassis, 2 for the connection pins and 2 for the mounting studs)
changed, so if U have a "newer" filter and want to use it in an "older" radio, you
may have to get out your drill.   It may also be possible that you could have
a mechanical squeeze job to contend with, if U try to put an older (wider)
filter in a newer radio, such as trying to stack both an older SSB and a CW
filter under the chassis in an SB-102 transceiver, which I am pretty sure used the
narrower filters.   I am not sure when (I think it was pretty early, perhaps
around 1967-8) that Heath changed to the new style.   I built my SB-300 in
1966, and when I ordered a CW filter for it (I think around 1968), I found that I
had to drill out the chassis to get it mounted.

There is yet another issue, an electrical one, which also needs to be kept in
mind.   The "SB" radios used superior filters, when compared to the ones used
in the "HW" radios.   Being that the SB radios were more expensive, they had
filters which had more poles (better ultimate selectivity) than those used in
the HW products.   You can readily notice the difference......tune around the
band with a HW and SB 100/101 pair, and you will notice the difference.
Listen to the crystal calibrator......the heterodyne disappears much faster as U
tune away from center frequency on an SB versus a HW radio.   I'm not sure
(I've never opened up one of these filters), but I'm guessing all the HW radios
had 4 poles, and the SB radios had 6 (or possibly even 8).   It could also be
that the terminal impedances of SB versus HW filters are slightly different from
each other as well, meaning that slight mismatches could occur if a HW filter
is put in an SB radio, and vice versa.   This could result in more ripple in
the desired passband response of the filter.   I've never tried such a swap,
but this is something that should also be kept in mind.

Summary:   It's probably acceptable to replace a "SB" filter with another "SB
filter (as long as it mechanically will fit); same goes for a "HW" to "HW"
swap.   Putting an SB filter in a HW radio may be OK.....I've never tried it.
Putting a HW filter in an SB radio will probably work well enough to get U on
the air, but you will definitely notice a degradation in ultimate selectivity.

73, Herman, N4CH.

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