The OEM filter in the Collins 32S-3 has a balanced output which is isolated from ground. The
newer Collins filters which Inrad sells have a common ground between input and output and
cannot be directly substituted for the OEM. To change to the new style filter, a transformer must be constructed to balance the filter output.
If the original OEM filter fails, it may be replaced with an Inrad #720 2.0 kHz filter along with
some additional modifications.
The OEM filter has a 2.1 kHz bandwidth and some people would like to widen the
transmit audio. Inrad has #706 2.5 kHz and #726 2.9 kHz filters available which may be
installed with the same additional modifications plus an audio modification.
The OEM BFO crystals are 456.35 kHz USB and 453.65 LSB.
Note that when updating the transmitter to the wider filters, the appropriate carrier oscillator crystals must also be changed in the companion 75S-3B/C receiver. The 2500 Hz (#706) and 2900 Hz (#726) kits for the 32S-3 include two sets of crystals, one for the TX and one the RX.
Full installation instructions are available here: