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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.

Background
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.

Procedure
Preparing the MVP for repeater service is a 3 step process:

1. Retune radio to the amateur band
2. Convert to duplex operation
3. Bring the required I/O connections out to the rear of the radio: the receive antenna, and the signals that go to to the repeater controller.

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).

Transmitter:

Metering Connector J103- Adjustment Tune For 1 T101 peak 1 T102 dip 1 T103 peak 3 T104 peak 3 T105 dip 4 T106 peak 4 T107 dip 7 T108 peak 7 C155 dip 6 C157 peak 6 C167 dip 2 C171, C175 peak 1 T101 peak 3 T102, T103, T104 peak 4 T105, T106 peak 7 T107, T108 peak 6 C155, C157 peak 2 C167, C171, C175 peak

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.

Receiver:

Metering Connector J601- Adjustment Tune For 3 C406 peak   C411, C416 same pos. at C046   C306, C307, C308 full CCW 4 C411, C416, C411 peak (twice on C411) 4 C406, C411, C416 peak 4 C306 any change 7 C307, C306 peak twice 7 C308 dip 7 C306 peak 1 C301 thru C305 peak or best quieting 1 A303-C2 peak or best quieting (P903-1) L603 peak (P903-1) R603 1 V RMS

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 control line to the +10vDC bus to keep the receiver on while the transmitter is keyed.
2. Cut the receiver MUTE line to keep the receiver audio working while the radio is transmitting.
3. Bring the CTCSS decode signal out of the MVP's "Channel Guard" (CG) board

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 picture:

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:

Connector J1-	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

Pitfalls
Like any other radio, the MVP has its weak points.   Fortunately, these are easy to deal with.

1. Overheating of the transmitter power amplifier. The MVP's transmitter is not designed for continuous duty, so it will overheat unless it is run at reduced power or supplemental cooling is provided. I use a muffin fan positioned to blow directly on the back area of the 20 watt radio or heatsink of the 35 watt model. You can also simply add a heatsink to the 20 watt radio & not use a fan, but I recommend the fan in either case. If you're happy running the radio at 10 watts or less, then you can probably get away with not adding a fan or heatsink.
   
   
2. Intermodulation between the transmitter and receiver local oscillator.
   A strange problem was discovered a few years ago by someone on the repeater owners mailing list when he tried to duplex a G.E. radio that was set up for 448.625 receive and 443.625 transmit. This problem appears to affect a range of frequencies, as another ham had trouble duplexing his G.E. radio at 448.500 receive and 443.500 transmit. To this date the specific cause of this problem has yet to be found; those who have been affected by this problem have resolved it by using two separate radios for transmit and receive.
   
   
3. Another problem was discovered a few years ago when one MVP repeater received an unusually high amount of IMD that included the output transmitter as one of the mix products. It was discovered that the "howling" would occur whenever a strong signal (~-60 to -70 dBm) at one of two specific frequencies was input to the receiver while the transmitter was enabled. The receivers "soft spots" depended on the TX & RX frequencies in the following manner:
   F_SS = F_RX ±  [(F_RXOSC X 28) - F_TX]
   
   Where F_SS = receiver's "soft spot", the frequency of spurious reception when the TX is on (2 frequencies due to the ± term), F_RX = nominal RX frequency, F_RXOSC = RX LO oscillator base frequency (equal to (F_RX - 11.2) / 27 if using low-side LO, or (F_RX + 11.2) / 27 if using high-side LO), and F_TX = the TX frequency.
   
   As an example, for an MVP repeater operating on 442.000 MHz receive and 447.000 MHz transmit, the spurious receiver responses would be at 441.756 and 442.244 MHz (rounded to the nearest kHz).
   
   The easiest solution to this problem is to switch to high-side injection on the local oscillator.   In the above example, this would push the "soft spots" out to almost 23 MHz away from the nominal receive frequency!   Since the root of the problem lies in intermodulation with undesired harmonics of the fundamental local oscillator, another check is still required by replacing 28 with 26 in the "soft spot" formula.  However, this still yields spurious receiver frequencies that are over 10 MHz away, so the problem is effectively cured.
   
   Since discovering this problem, I've always specified "high-side injection" when ordering receive crystals for all my G.E. radios (I've found this problem in MASTR IIs MASTR Exec IIs as well). Before discovering the cause of the problem, a friend of mine removed his receiver & put it in another mobile case. Strangely, this had NO EFFECT on the severity of the spurious responses.  He ended up installing the receiver in an RF-tight box with EMI feedthroughs; this of course eliminated the problem completely.

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.