QRZ! Ham Radio 3/QRZ Ham Radio Callsign Database - Volume 3.iso/world/mods/ge/ge_mvpex.mod

Text File | 1994-06-04 | 6KB | 113 lines

Newsgroups: rec.radio.amateur.digital.misc
From: [e-mail] (Gary Coffman)
Subject: Re: 2-m homebrew packet rig?
Message-ID: <[e-mail]>
Reply-To: [e-mail] (Gary Coffman)
Organization: Destructive Testing Systems
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Thu, 16 Sep 1993 03:21:50 GMT
Lines: 99

In article <[e-mail]> [e-mail] (Steve Brown) writes:
>In article <[e-mail]> Chris Terwilliger <[e-mail]> writes:
>> In article <[e-mail]> Gary Coffman,
>> [e-mail] writes:
>>
>> >For 9600 baud, the Micor is fairly trivial to convert,
>>
>> please elaborate...
>
>Yes, please elaborate _on the net_.  I tried to send you a message every
>way I could think of and got kickbacks.

Well (aaarrrgg) my Motorola manual is out on loan so I can't give
you a part number by part number mod sheet. It's basically simple
though. You just disconnect the audio amp from the modulator and
connect your TNC output instead, and you disconnect the audio
amp from the discriminator and connect your TNC instead. It can't
get much simpler than that.

Since I *do* have my GE manual handy, I'll give you the method for
making a UHF Exec II or MVP work on 9600 baud. It isn't so simple.
On the limiter detector board, U602 is the limiter detector IC.
Pin 1 is the audio output, but we don't want to take our signal
directly from there, we'd load it too much. Instead we take it
at J601 pin 2. That's isolated by a 180k resistor from U602.

Now we'll only have 0.2 volts of signal here, and that isn't
enough to drive our TNC. We could try to take the signal from
the emitter of Q603 but we'd have bandlimited de-emphasized audio,
not good. So we outboard an OPamp and set it for a gain of 5, just
use a 900k feedback resistor in conjunction with the 180k resistor
already in the radio. This will give us a volt to the TNC which will
make it happy. Note that the signal at J601-2 is offset +0.35 volt
DC so we have to null that voltage with a bias pot in the non-inverting
input to our OPamp.

All is not well yet, however. The radio uses a 4 pole crystal filter
in the IF which is too narrow for our purposes at +/- 7 kHz.  We've
got to widen this a bit. There are several ways to approach this
problem, but the easiest is to solder a gimmick capacitor across
pins 1 and 3 of FL502 and twist for the desired response. FL502
is the filter nearest J502, the +10 volt power connection. Solder
two 1 inch insulated hookup wires, one to pin 1 and the other to
pin 3. Now twist these tightly together for the desired response,
about +/- 10 kHz response. This is easiest done with a sweeper.
You may need to touch up C504 and L503 for flattest response.
Cut off any excess lead length on the gimmick and put a touch
of glypt on it to hold it in place.

Now the easy part is over. We have to modulate this thing with
our 9600 baud signal. The modulator is a classic variable reactance
phase modulator. That won't do for our purposes so we're going to
turn the radio into an FM modulated transmitter. First we're going
to operate on our Icom (channel element) by removing C1. That's
the square green bypass capacitor just below the piston trimmer.
The varactor in the Icom is too small to give us the swing we
need, so we rob the varactor from the phase modulator, CR180,
and parallel it across the varactor in the Icom. Now we have to
cut the trace going to XY101-4, the Icom socket on the exciter
board. Be sure when you cut the trace that you cut *after* the
connection to P902-3 or you'll lose your receiver temperature
compensation.

Ok, now we need about 2 volts of signal to drive the varactor
in the Icom, and we want to keep our compensation voltage too
which will be in the 4.0-6.0 volt range depending on temperature.
To do that, we're going to use an OPamp as a voltage summer.
Feed the TNC audio, 1 volt, and the compensation voltage to the
inverting input of the OPamp. Take the audio through a 100k resistor
and take the compensation voltage via a 400k resistor connected
before the trace cut. Use a 50k feedback resistor. Use another
section of the OPamp as an inverting unity gain amp with a 2.5 volt
positive offset in the non-inverting input. This gets the compensation
voltage going back in the right direction and biases the varactors
properly. You should now have a 4.5-5.5 volt positive DC output with a
2 volt symetrical audio signal riding on it. Note that we've halved
the compensation voltage swing because we've doubled the amount of
varactor. Now connect the output of the OPamp to the XY101 side of
the trace cut. You should be able to get 4 kHz deviation.

There *is* another way, but it puts pre-emphasis on the signal.
That's to just accept the phase modulator and put the audio in
on the Channel Guard input, P902-9. It takes a *lot* of signal
though to get 4 kHz deviation. You'll still need the OPamp.
If you do this, you'll want to add a de-emphasis network in
the receive section. You can do this by adding a capacitor
in the feedback section of the receive OPamp to give a 6 db
per octave roll off. Don't try to use the speaker output. It's
bandlimited as well as de-emphasized. That won't work well at
all. The modem operation will be compromised, but may work if
others in your area are using pre-emphasis/de-emphasis in their
radios. I don't recomend it, but it is easy to do.

Gary
--
Gary Coffman KE4ZV          |"If 10% is good enough | gatech!wa4mei!ke4zv!gary
Destructive Testing Systems | for Jesus, it's good  | uunet!rsiatl!ke4zv!gary
534 Shannon Way             | enough for Uncle Sam."| emory!kd4nc!ke4zv!gary
Lawrenceville, GA 30244     | -Ray Stevens          |