[RTTY] Yaesu FT-920 setup - HELP!!

[Kok Chen]

Here is what I found tracing the pins of the RTTY connector of the 
FT-990 schematics here (not FT-920, but possibly similar).

The RTTY connector's (4 Pin DIN) PTT line is wire-OR'ed with the other 
PTT lines (Packet, Mic, etc).

All of these go to a motherboard connector which goes through YET 
another diode before ii is connected through a 1K resistor to an input 
pin of the microprocessor.  That input pin is pulled up to a regulated 
source (I have no idea what the regulator voltage is but the input of 
the regulator is the 12 volt rail, so it is pulling up to +5 or +9 
volts maybe) through a 10kohm resistor.

If the pull-up goes to 5 volts, it means that you need to be able to 
sink better than 1/2 a milliamp or so to be able to engage PTT.

The SHIFT line is much simpler, it does directly to a pin on what looks 
like a selector of a switched-tail register for generating a sine wave 
using 4 resistors (yep, thats the AFSK generator -- Yaesu rigs are 
AFSK.  The FSK pin merely keys the internal AFSK generator).  The 
selector probably engages the divider to gereate either 2125 or 2295 
tones.  Cheap and dirty single function D/A converter :-).

Now, this SHIFT pin is not pulled up, has no diode drop, nothing.

So it is puzzling that Kyle can key the PTT but cannot key the shift.

The two diode drop was probably what caused grief to WA9ALS when he 
used the 4N25 with his ex-1000MP, I think -- Vce(sat) of the 4N25 is 
0.5 volts but only when you push 50(!)mA forward current into the light 
emitting diode.

The fact that Kyle can ground an open SHIFT line and achieves a shift 
means that there is sufficient pull up current internal to that AFSK 
generator, at least for an open wire.

I will write Kyle to ask him to listen to the AFSK tone to see if it 
shifts frequency when he plugs in the Rascal.  If not, it is simply the 
AFSK generator not having enough pull up current, since the Rascal's 
open collector driver probably only pulls down, with no collector load.

73
Chen, W7AY