From: billp@niagara.Tymnet.COM (Bill Putney)
Newsgroups: rec.radio.amateur.misc
Subject: G.E. MVP to a Repeater (Instructions)
Date: 29 Jul 92 17:29:57 GMT
Reply-To: billp@niagara.Tymnet.COM (Bill Putney)
Distribution: usa
Organization: BT North America (Tymnet)
Originator: billp@niagara

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. :)

----------------------- Cut ? -------------------------------

                   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!
Xref: sparky rec.radio.amateur.misc:12750 rec.radio.cb:729