GE MVP Radio Mods by Bill Putney
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!