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Converting the UHF G.E. Custom MVP Radio for Repeater Operation
By Bob Dengler NO6B
Photos by the author
As you can see, the MVP is not much bigger than a 1 1/2 month-old kitten.
The G.E. Custom MVP is a crystal-controlled commercial two-way transceiver closely related to the Mastr II and Mastr Executive II series. Compared to the other two models, the MVP is the smallest of the 3 measuring only 3.5" H x 8.4" W x 11" D, including all projections & heatsink on the 35 watt model. The UHF MVP typically comes in a 20 or 35 watt model. Most of mine are the 20 watt variety, which actually put out 25 to 30 watts. While there are a few low-split models floating around, you're most likely to find the 450-470 MHz model at your favorite hamfest or surplus (last 2 numbers of the model # are 88). Although this article focuses on the UHF model, the duplexing instructions also apply to the VHF model as well; the 66 suffix model is spec'd for 150.8-174 MHz but will tune down to the 2 meter band without modification.
Preparing the MVP for repeater service is a 3 step process:
Retuning the radio
An MVP manual comes in handy at this point. In case you don't have one, here is the basic tuneup procedure as paraphrased from the G.E. manual along with my own comments based on experience in tuning up dozens of these radios. Make sure you key the transmitter, tune the indicated coil or capacitor quickly, then unkey (don't leave the transmitter keyed for the entire procedure).
Exciter board tuning locations
At this point the manual says to basically repeat the steps above shaded in green. I find this unnecessary, but if you really want every stage peaked to the last millivolt, go ahead.
Steps shaded in green are performed with a 50 ohm signal generator connected to the radio's antenna input. If you aren't sure about the output impedance of your signal source, just use a 6 dB or greater 50 ohm pad on the output. Due to the tremendous selectivity of the helical resonators, you will probably need a lot of signal at first (millivolts). If you have trouble detecting a signal to tune up on, first make sure your crystal isn't way off frequency by moving the sig. gen. +/- 30 kHz or so off of your center frequency. Then try presetting C301-5 to within about 2 turns of bottom, as this is near the resonance point for the 440-450 MHz range. If that doesn't work you can try injecting signal through the little holes in the helical assembly provided for just this purpose (the manual recommends using them but I find them unnecessary). Steps shaded in blue are done with a full-quieting 1 kHz modulated signal at 3 kHz deviation. If you don't have access to an FM sig. gen. you can adjust L603 by transmitting a DTMF tone with another radio. When you're finished tuning you should get 0.35 microvolts or better for 12 dB SINAD, or 0.5 microvolts for 20 dB quieting. These instructions assume your radio does not have the UHS bipolar preamp. If it does have it I recommend you bypass it & get yourself a good stable GaAsFET or MOSFET amplifier from Angle Linear unless you're REALLY trying to pinch pennies. The UHS preamp does work, but it's noise figure is going to be around 4.5 dB plus I've noticed problems with RF coupling to the outside of the coax leading into the preamp. This can lead to internal desense problems. Also bipolar preamps are known for having a lower 1 dB compression point than the FET amps, meaning you'll be more susceptable to IMD.
Duplex conversion: There are three steps to perform to convert the MVP to duplex operation. Duplex operation means the radio will transmit and receive simultaneously. Unlike ham transceivers, the MVP is able to do this all on the same band; this is part of what makes it such a nice radio for repeater use.
The three steps are as follows.
1) Strap the receiver oscillator line to the +10vDC bus to keep the receiver on while the transmitter is keyed. To do this you will need to solder a wire on the bottom of the system audio squelch board as seen in the following pictures:
The system audio squelch board is the larger of the two boards on the right side of the radio front. Remove the board and add the jumper.
This orange jumper connects H12 to U2-7 to short the receiver's oscillator control line to the +10vDC bus.
2) Cut the receiver MUTE line to keep the receiver squelch working
while the radio is transmitting. The cut trace is shown in the following
3) Bring the CTCSS decode signal out of the MVP's "Channel Guard" (CG) board.
This requires a little work as it normally doesn't provide the radio with a
discrete decode logic signal. Instead, it pulls the "RX MUTE" line low whenever
no CTCSS tone is being decoded. If your repeater controller doesn't have an
input for CTCSS decode, you can leave the receiver MUTE connected to the receiver
and not bother bringing any logic out. But since 99% of the controllers
currently available do have a CTCSS decode input, you will probably want to
cut the receiver MUTE line here as well (see the already-cut wire soldered to the
collector of Q4 in the picture). The wire just below and to the left of the
receiver MUTE wire also needs to be cut. This is the PTT line, which needs to be
disconnected from the CG board in order to keep the decoder in decode mode
while the transmitter is on. Also solder a shorting strap across CR3; this
will effectively turn the "CG DISABLE" line into a "CG DECODE" output, going
to 0 V when decoding and rising to 10 V when inactive. In my MVP, the
CG DISABLE was already wired to pin 8 of the system connector. Now how easy
can that be!
Bring required I/O signals out to the rear of the radio
Now that the radio can receive and transmit at the same time, we need to bring a second RF connection out for either the receiver or transmitter. The way the MVP is designed, it turns out that it's easier to bring the receiver input out on the added coaxial connection. There are several possible approaches; mine is shown in the pictures. I simply drilled a hole in the back of the radio & ran a piece of RG-142 from the back over to the RCA jack on the helical resonator assembly (or UHS preamp, if you're using one). I actually prefer to use RG-223 for this cable because it's more flexible. Be sure to keep the coax away from the center pin of the SO-239 connector to prevent excessive TX RF from coupling to the shield of the new receiver feedline.
The final step in converting the MVP for repeater use is to bring out
the control and audio lines your controller needs from the radio: the audio input,
the PTT lead, the receiver audio, the COR / squelch logic and the CTCSS tone
decode line. You could use the microphone connector on the side of the
radio, but the transmitter connections are the only ones available there. Besides,
this connector is more difficult to access the back side of than the main system
connector. So I used the 10-pin connector on the back.
You'll probably find that your MVP doesn't have enough pins in this connector to carry out all the signals, so you'll need to get more pins or find another connector with pins. This connector is an AMP #1-480339-0 shell & #60620-1 pin contacts (mates with #1-480285-0 shell & #60619-1 socket contacts), and is available from Digikey, Newark and Mouser. I've also found this connector as a complete paired set (Tyco part #618-10) at some local electronics stores.
The following table shows the pinouts for this connector as defined by G.E., and my revised pinouts to accommodate the added I/O lines:
|G.E. definition||Revised definition|
|1||+12vDC (16 ga. red)||+12vDC (16 ga. red AND the
22 ga. red from pin 5)
|2||gnd. (thin solid wire to chassis)||gnd. (no change)|
|3||spkr. hi||spkr. hi (no change)|
|4||spkr. low (ground)||CG hi|
|5||+12 V (22 ga. red)||PTT|
|6||gnd. (thin solid wire to chassis)||vol./sq. hi (discriminator)|
|7||spare; NC||mic. hi|
|8||CG disable (green)||RUS (squelch logic)|
|9||spare; NC||NC (no change)|
|10||spare; NC||CTCSS decode|
Like any other radio, the MVP has its weak points. Fortunately, these are easy to deal with.
Thanks to Mike Steiner KD6LVP & Bill Wood W6FXJ for their research in discovering & solving the G.E. mobile TX-LO IMD problem. So there you have it: one of the most compact, rugged & cleanest duplex NBFM radios you can find for repeater usage.
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Article and photos by Bob Dengler NO6B © Copyright October 2001.
HTML © Copyright October 14, 2001 Kevin K. Custer, W3KKC All Rights Reserved.
Content updated 11/26/2006 by Bob Dengler NO6B
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.