From: billp
Subject: G.E. MVP to a Repeater (Instructions)
Date: 29 Jul 92 17:29:57 GMT
I know nobody asked but I did this for someone else and I thought
it would be nice to share the information. A lot of clubs these
days have a need for a new repeater and what with the economic
thing don't have a lot of money to spend. There is a lot of junk
being marketed as ham repeaters (i.e. Spectrum Communications)
for horrendous prices. I thought it would be nice for people to
have the choice to do the Ham thing and save a bunch of money.
These radios are available all over the place for very modest
prices and make great repeaters. They work on our California
mountain tops without being fried by the other R.F. around.
Better yet they are a good neighbor. They put out power on
just one frequency at a time and the receivers are selective
enough to work on 12.5 Khz split channels (though most Japanise
portable radios wont). The transmitters are very clean if
tuned up according to G.E. instructions. Transmitter tuning
only requires a wattmeter, dummy load and a VOM. I have tuned
the receivers up with just a weak signal on channel but I do
recommend a signal generator. The MVP's will put out ~60% of
it's rated power (there are several models ranging from 5-35
watts) continuous duty. This isn't a lot of power but at
least when you add an power amp to it the amp won't be
multiplying garbage.
P.S. No I don't work for a company that sells MVP's nor am
I interested in duplexing any more for other people. I really
love these little puppies and I have them running all over the
place but I've done 20 or so and I'm not as young as I use
to be. :)
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Instructions for Duplexing a G.E. MVP
by Bill Putney
WB6RFW
These instructions will work for MVP's on any band.
Before you start, test the radio completely to establish a
benchmark for post conversion testing. It is better to find any
problems that exist in the basic radio and fix them now. After
the conversion you only need to find the problems you created.
MVP C.G. encoder/decoders are pretty useless after the radio has
been duplexed so now is a good time to get rid of it. Don't
forget to replace the resistor between H1 and H2 on the System-
Audio-Squelch board (The SAS Board). Most of the duplexing is
done on the SAS board.
1) Remove the SAS board from the radio.
2) Cut the printed circuit land that connects J904 pin 2
to U902 pin 7. The best place I've found to do this
is right near U902 pin 7 on the solder side of the
board. There is a land that goes from U902 pin 7 to a
via pad. This trace is only about 1/8" long and is
shown in gray in the MVP manual Outline Diagram. I
use an Exacto knife to make these cuts. Make two cuts
perpendicular to the run of the land about 1/16" apart
then remove the land in between by prying up one end
and pealing the copper away. This disconnects the
receiver oscillator control line so that the U902 no
longer can turn the oscillator off during transmit.
3) Place a jumper between J904 pin 1 and pin 2. I do
that right at J904 on the solder side of the board.
This puts regulated 10V back to the receiver osc.
control line so that the oscillator is always enabled.
4) The next cut is a little harder to find. The line on
the schematic that connects the junction of CR901,
U902 pin 6, and J906 pin 5 to the junction of R905,
R906 CR905, C906 and U901 pin 7. On the schematic
it's a nice straight vertical line just above U902 but
on the board it runs all over the place in and out of
via's, from the solder side to the component side and
back again. The place I cut it is on the component
side of the board near J905. Set the SAS board on the
bench with the component side up and the board
oriented as shown in the MVP Manual Outline Diagram.
On the Outline diagram you can see two component side
lands that run parallel along the bottom edge of the
board the whole length of J905. The bottom most land
connects H16 to J906 pin 4. Don't cut this one...
The one you want is the next one up. It kind of looks
like and S on the board and runs between J906 pin 5
and a via just below U902 (next to H5). I make the
cut near the end (pin 7 end) of J905 before it makes a
turn and goes up toward U902. Make the cut as
described in 2) above.
5) All of the I/O lines to the radio can be found on the
SAS board and should be connected before you put the
board back in the radio.
PTT is at H17. This is a ground to transmit line.
COS is at J912. This line is ~0.1V when the radio is
squelched and ~9.0V unsquelched. Don't try to run a
relay from this COS line. I'd use a CMOS gate to buffer
it. If you're useing a modern controller chances are
that it uses COMS logic and the buffer isn't required.
Mike in is at J913. This line has mic bias on it and
should be capacitor isolated. It takes about 2V P-P
to drive the radio to 4.5 Khz deviation.
Receiver audio is found at H16. This is unprocessed
audio. It is not squelch gated and is not de-
emphasized. There is ~4V P-P with 4 Khz deviation on
an on frequency signal applied to the receiver. This
should not be loaded by less than a 10K input.
6) Put the SAS board back into the radio.
7) Unscrew and unsolder the SO-239 antenna connector and
remove it. (If this is a UHF radio I'd throw it away
and get an N connector but it's up to you). After the
conversion this will be the transmit antenna
connector.
8) Unsolder and disconnect the coaxial jumper at H2 on
the Filter board. Disconnect P1 (this is the antenna
relay control and can be cut off and pulled out of the
harness or left alone. It won't do much when you're
through).
9) Remove the filter board from the radio.
10) Using solder wick remove as much solder from around
the antenna relay can on the component side of the
board as possible. Also use solder wick to remove as
much solder as possible from the relay pins on the
solder side of the board. If you can remove enough
solder from around the pins you can break them lose
with a small screwdriver of knife when the solder is
cold. This will make the relay easier to remove.
With a small pair of diagonal cutters grab a corner of
the relay can. Using the soldering iron, heat
whatever is stuck and remove the relay.
11) Now you need to remake the connection between the
output filter and the antenna connector. I use a
piece of center conductor from a piece of RG-58 to do
this in the hopes that the dielectric and the
conductor diameter will keep the impedance about
right. This jumper should be placed as close as
possible to the circuit board in the holes left by pin
4 and pin 7 of the relay.
12) Now you need to rig up an antenna connector for the
receiver. THE MOST IMPORTANT THING HERE IS TO HAVE A
CABLE WITH A CONTINUOUS SHIELD from the receiver
connector on the front end casting to the antenna
connector. This means no voids in the shield around
the connectors at each end. If there are unshielded
parts of this assembly the receiver will hear the
transmitter and the resulting desense will be
unacceptable. Use a good quality connector like a BCN
or TNC (or type N if you can make it fit...). I like
all of the connectors to come out the back of the
radio. This is real handy from the point of view of
working on the radio later and looks real nice but is
a pain to do.
If you chose to do this (or put an accessory connector
on the back panel of the radio) you need to take all
the boards out of the radio and mask off the back of
the radio from the rest with newspaper and masking
tape to keep drill filings from getting into every
nook in the sheet metal and causing problems later.
Mount the connector as close to the edge of the cover
opening as you can or it will hit the filter board
when you put it back in. Make sure whatever connector
and coaxial you use for this will make the turn to
miss the filter can. You could mount all this stuff
on the front (plastic panel) and save yourself a lot
of work but it wouldn't look as nice and it only takes
time to do it right.
13) If you choose to put an accessory connector on the
back panel I suggest you put it near all the other
connectors. To make room for this you will need to
remove the little sheet metal doodad G.E. decided to
put on the back of the radio. Don't forget to mask
off the power connector especially around the base
where it meets the back panel of the radio. Metal
filings like this way of getting into the radio.
The doodad is held by two spot welds.If you look
carefully you can see where they are. Drill these
down to the point where the drill is just getting into
the back panel. Be careful not to screw up the power
connector in the process.
I put the accessory connector next to the power
connector. I like 9 pin D type connectors. They are
widely available and of good quality. The D shape
makes them a pain to mount without a punch (the
Greenlee punch for this is >$280.00). Get the
connector as close to the edge of the cover cutout as
you can. This keeps it up high in the exciter area
and out of the way. You are working through two
layers of sheet metal so be careful when (if) you tap
the mounting holes as the tap tends to bind between
the layers. Put the radio all back together now.
14) Put the output filter board back in and solder the
transmitter antenna connector back to the tab on the
filter board.
15) Retest the radio to make sure it survived the
operation.
That's all it takes to make a $100-$200 MVP nearly as good a
duplexed radio as a $1,000-$2,000 MSTR II base station!
SOURCE: The QRZ Windows Ham Radio CDROM |
