Actually I wondered that myself Geoff. The filter is stamped "282" and "48054" on the top of the crystals.
Actually I am trying to reconcile the differences between the center
frequency of the 500 Hz filter and the 250 Hz filter, both in the
6.3 mHz PB IF.
Yes it would be much nicer if all of your 6.3 MHz IF filters had the
same center frequency. It sounds like you have determined that they do
not. So then when switching between filters you will need to make an
adjustment to the PBT knob, or if you could figure out which one is the
furthest out of nominal you could replace it and hope you get luckier
with the next one.
There are not "CW" and "RTTY" specific filters for the 6.3 MHz IF. Those
are only in the 9 MHz IF. Since the 6.3 MHz IF can be shifted using the
PBT control, there is no need to have different 6.3 MHz filter center
frequencies to get different audio frequency tones out of the rig. They
"should" as you say, all have the same center frequency though, but if
you can stand turning the PBT knob they don't have to.
The 15.3 +/- VCXO oscillator in the PBT system is used both to convert
the 9 MHz IF to 6.3 MHz and to convert the 6.3 MHz IF back to 9 MHz, the
net result is no frequency shift at all, just a passband shift. Whatever
gets through the 9 MHz IF filter will have the same audio tone
regardless of how the PBT is set. The PBT setting does not affect the
relation between your sidetone and where your transmit signal is
relative to received signals.
The BFO frequencies do effect the relation between your sidetone and
where your transmit signal is relative to received signals, so it needs
to be set right.
DE N6KB
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