Using the 400 Hz CW filter in narrow
Digital Modes

First look at the IF frequency scheme and notice the following things...

Upper and Lower Sideband voice filters are offset from the 455 Khz
IF.   The center frequency of these filters is not 455 Khz, but a
value somewhat above or below 455 Khz depending on the sideband
selected.

The CW filter as well as all the AFSK filters are centered on the 455 Khz IF.

You cannot simply switch the CW filter into line while in USB for RTTY
without readjusting the BFO and having the display frequency be correct.

CW mode is selected USB or LSB.  This is done by a switch on the filter
board.

The way CW works in the RF350 transceiver is like this.  When you switch
to CW mode the microprocessor offsets the 1st LO and BFO frequencies according
to the chart on Figure 5-1.  You will also see according to the chart, the
frequencies depend on the setting of the LSB or USB switch on the filter
board.  This gives you a 2nd IF centered on 455 Khz.

Look at Exciter Schematic A1A1 sheet 1 and A1A1 sheet 9

The CW signal is generated with a constant 1 KHz tone that arrives at the
exciter board at J-4.  When CW mode is selected, UP0, the CW Audio/ Keyline
select goes high at U-21 pin 4.  This is applied to switch U-3A which
selects the CW audio path (yes audio path).  When you depress the code key,
UP7 goes high and switches U-11A allowing 1 KHz audio into the CW audio
path.  UP0 also sets the VOX time constant, and keyline is done with the
VOX circuit.

What this means is CW mode is just another audio circuit.  If we could
set the transceiver into CW mode so that we could use filters centered on 455
KHz, and de-select the 1 Khz CW audio circuit in favor of the NBSV (audio 2)
audio circuit we can pass RTTY tones generated by a soundcard into the exciter.

All this can be done at U-21 on the exciter board.  U21 pin 4 is UP0 (CW
audio and keyline select).  U21 pin 7 is UP3 (NBSV audio and keyline
select).  By installing a toggle on the front pannel, and running the three
wires from the toggle to U21 the modification can be placed into and removed
from the circuit at will.  The trick here is pin 4, which has to be removed
from the circuit board without breaking it from U21.  Probably the best
method here is to get a new 4094, remove the existing one, and install a IC
plug.  I cut the pin on mine where it entered the circuit board, and bent
it up to get it out of the way.  That worked until I tried to bend it all
the way up and broke it off.  Pin 4 goes to the wire that goes to the
center of the toggle (common).  One end of the switch goes to the board
where pin 4 originally was attached, making the "normal" toggle switch
position.  The other end of the switch goes to pin 7.  This applies
the voltage from pin 4 to UP3, switching the audio path and keyline to the Audio
2 (NBSV) connector.

NOTE: the toggle must be in the NORMAL position to tune the LPA.

I have done this and it works well.  Now don't go taking out that 400 Hz
CW filter for one with a different bandwidth.  Soon I will describe how to
use the logic on the filter board to disable the stock filters, and how to plug
in an external filter board.

Below is sheet 7 and 8 from the exciter.  These sheet show the UP0
setting the VOX time constant and UP3 setting the AUDIO 2 / NBSV keyline select.

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