Subject: FT470 Packet Radio Mod
In article <1991Sep27.033057.22510@agate.berkeley.edu> ajk@garnet.berkeley.edu (Adam Jacobs N2LAW) writes:
>
>1.  Does anyone have the exact wiring specifications for the Yaesu
>side of the cable to the TNC?  Is "where do I plug it in" a stupid
>question?  Presumably the MIC and EAR jacks, but how is the MIC jack
>configured -- if it's (as the manual specifies) a 2-conductor
>micro-mini phone jack, then what do I do with the PTT signal?  Or is
>there something else I should know?  I can wire the cable all right,
>but I have no appetite for blindly trying configurations on an
>expensive piece of equipment.  Do I have to open the unit up?

The FT470 is wired like an Icom. You connect the audio from the TNC
to the tip of the mike plug through a capacitor and connect the PTT
to the tip with a resistor. The Icoms and Yaesus use a "leaky ground"
to generate PTT. The problem with this approach is there is a tradeoff
between rapid PTT and audio level and response. Typical values are
.1 ufd and 2.2 k ohms. The RC time constant limits TR turnaround.

A better scheme is to use a tiny audio transformer sideways like so,


TNC PTT----------))))))))))))------------> radio tip (audio)
                 ============
TNC AF OUT-------))))))))))))----X--------> radio sleeve (gnd)
                                 |
TNC GND--------------------------|

You can rip a suitable transformer out of an old transistor radio or
buy one from Radio Shack.

>2.  Suppose I want to run the HT off a 12V external power supply.
>Where do I feed the power?  Not the battery terminals, I hope.  I
>would have expected a DC power jack somewhere on the unit, but again I
>don't see anything except a mysterious looking rubber plug-which-might-
>hide-a-jack-but-I'm-afraid-to-pull-it-and-look.  The manual, again,
>says nothing about this.

The 2 meter only model does have a power jack under the rubber plug, but
the 470 doesn't. There's a place on the board for one, but Yaesu recomends
that you use a PA-6 module instead. This is a module that mounts in place
of the battery and contains regulators for running the radio and charging
a battery connected to the bottom of the PA-6. This is a really nice
accessory and well worth the price.

>3.  Anything else (useful modifications, hints, caveats) I should know
>about the FT-470 (or PK-88?)

Just the standard remarks that you should carefully set the audio level
so you wind up with a 3 khz deviation for your tones. Don't exceed that
level or many units will have trouble decoding your packets. Make sure
you have the power saver on the 470 turned off when you run packet or
you'll miss the first part of every packet. This can drive you nuts
because everything seems to be working but nothing prints.

One last note. Use a separate antenna and use shielded cables on your
TNC. Otherwise the RFI and RF feedback will ruin your packet operation.

Gary KE4ZV