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Motorola Maxar/Moxy 2M Conversion [message #1476] Sat, 01 September 2012 01:21
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Registered: December 2004
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By Tom Herman, CETma N1BEC/7

I've always wondered how easy it would be to put the Motorola Maxar/Moxy series radios onto the two meter ham band.

My gut feeling was that it shouldn't be too hard, but any interest was strictly academic because I had multichannel synthesized equipment for 2M. I hadn't had a reason to go ahead and try a conversion until my friend John, KC7OQN, came up with an old radio that he wanted to pop onto 146.52 Simplex. Many thanks to John for the "Guinea Pig", and for ponying up the $30.00 for two crystals.

My philosophy with radio conversions is to do the minimum amount of parts substitution that will give best practical performance. The usual approach is to replace capacitors, and not mess with coils, fragile, or hard to get to parts unless I absolutely have to. K.I.S.S!

The victim was a "Mid Split" (150.8-162 MHz) D23TRA1300AK two channel low power unit. The first order of business was to check out the radio to see if it worked at all on its original channel. It did, and the rocks were ordered from West Crystal in British Columbia. They got here three weeks later, right on schedule.

A quick check showed receive sensitivity was pretty good (~ .23 uV for 12 dB SINAD), but the transmit power was a tad low (4.6 watts @13.8 Volts), but I decided to go ahead with the conversion anyways.

I inadvertently left the unit powered up while I got called away from the service bench to attend to some "honey do's". When I got back, there was the unmistakable odor of burned electronics in the air. The unit was still receiving, but had no transmit.

Checking the circuit board showed a burned inductor (L 106). A closer check revealed this was an effect, and not a cause, and if I had installed another part, it would have burned up as well. The ultimate problem was a dead short on the other side of L 106: C 130, which turned out to be a .05 uF/25 volt tantalum capacitor.

It's fairly common for the Tantalum caps in the Moxy/Maxars to go critical, so a good S.O.P. is to leave a potential candidate unit powered up on a current limited power supply in a well ventilated area to see if the caps will hold up or not.

Fortunately, I have a small mountain of UHF Maxar/Moxy's, and since the exciters are practically identical to the VHF??????s, they became a valuable source of parts and quickly got the D23 going again.

Original operating frequencies were about 158 MHz TX, and 152 MHz RX. The old crystals were removed, and the '52 rocks installed. I elected to do the transmitter conversion first. The plan was to try to just align the unit, and if that worked, to go through afterwards and mop up any circuits that needed optimizing.

Keying up, I spotted the fundamental and triple of the oscillator, and zeroed the tripled signal on frequency.

L 102 and L03 are tuned first, while metering M3. L 102 tuning is quite sharp, L 103 a bit broader.

There was no L105 M5 reading, so M3 was tuned for a dip when adjusting L 105. The tuning was broad.

L 107 and L 108 tuned broad, then the PA was adjusted. Power output screamed to 14 watts! (I assume that the lower output prior to conversion was due to a partially shorting C 130 pulling down the B+ going to the tripler stage).

The tuning of L 105, L107, and L 108 were not optimum, the cores being bottomed out on the circuit board or close to it. (Managed to break the L 105 core, parts car to the rescue again!)

Check out the following table for the parts that needed to be changed out:
Inductor AffectedCapacitor ChangedHighMid"Low"
L 105C 127434756(Note: all capacitances in pico-Farads.)
L 107C 1316810
L 108C 13481015

With the new capacitors installed, all three cores tuned well towards the center of the slug form. Power was then backed down to ten watts even. Don't try going to 12 pF for C 131, 10 is ideal.

The receiver was a most pleasant surprise! Without doing anything other than dropping the crystal in the right slot and centering it on frequency, it gave 12 dB SINAD at 1.75 microvolts! Further tuning was done, and the radio dropped to a stunning .25 microvolts 12 dB SINAD with no parts replacement. No conversion was necessary on the receiver.

[Updated on: Sat, 01 September 2012 01:22]

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