® MPA UHF portable
There are three versions of this radio (not counting the 'splits'). The Select, Scan, and System. The Select radio has no keys on the front panel under the display and is capable of only 16 channels and is incapable of scanning. The Scan model has four keys on the front panel, can scan and can be programmed with up to 192 channels (12 modes of 16 channels or 16 modes of 12 channels). The System model has the same four keys the Scan model has and also a 3x4 "DTMF" keypad. The System model can scan and can be programmed with up to 192 channels (12 modes of 16 channels or 16 modes of 12 channels).
All UHF models of MPA meet the MIL-810C and D specifications.
For a rough guideline on the software side of the conversion, see the MPD conversion document, the steps are similar. You will have to re-lock the VCO in this radio (a "screwdriver mod") to bring it down on the Ham bands. Program the radio with 440.025 MHz simplex in channel 1, 445.000 MHz in channel 2 and 449.975 MHz in channel 3.
In order to relock the VCOs (yes there are two of them) you will need to separate the radio halves (remove the battery and loosen the 4 TORX screws at each corner of the back of the radio). Inside the radio you will see many adjustment and test points. The ones you are interested in are on the bottom of the radio on the left side. They are the TX VCO coil (bottom left corner) and the RX VCO coil (to the right of the TX VCO coil near the center of the radio). The adjustment tool is a square PLASTIC tool which can be made out of a cheap tuning tool of you don't have a square one already.
Use a fine tip permanent marker to make an index mark on both cores and on a convenient surface nearby. Turn the RX VCO coil clockwise 1/8 turn. Put the radio back together without screwing the TORX screws in. Turn the radio on, set it on Channel-1 (440.025 MHz) and send a fairly strong on frequency signal from a Service Monitor or another radio into the antenna. If the VCO is locked, you will hear the signal on the MPA's speaker. If not, open the radio and give the RX VCO coil another 1/8 turn and put the radio back together. Repeat as necessary. If after 1 complete turn you have still not found the lock point, return the core to the original setting and start going the other way 1/8 turn at a time. You are looking for the VCO to lock just outside of the Ham FM band. Test on Channel-2 (445.000 MHz) and Channel-3 (449.975 MHz) to confirm the RX VCO lock throughout the Ham band. Do not try to get below 440.0 MHz as the 450-470 MHz radio RF Board will not go that low cleanly (the RF board will start generating NASTY spurs if you go lower than 440.0 MHz).
For the TX VCO, the steps are similar except you will be transmitting with the MPA under test and listening on the Service Monitor, scanner, or other Ham band radio. Use Channel-1 for the adjustment. At one point you may hear a squealing sound from the receiving radio. At that point, give the core another 1/16 turn in the same direction and say "IT's FINISHED"!
In did not tune the receiver in my radio and am satisfied with it's performance (around .2 microvolts for a 'usable' signal). Remember that this is a commercial radio and is not intended for "weak signal" work.
For a peek at the factory tuning procedure, click here... To follow the factory procedures, you will need to get (or make) ribbon cables with the correct connectors to tie the halves of the radio together while it is open.
The battery pack for the MPD also fits the MPA radio. New batteries are available from TESSCO Inc and W&W. The long battery uses "Sub-C" cells with solder tabs and the short battery uses cells, with solder tabs, which are slightly larger than AA batteries. I have some guaranteed DOA, but rebuildable, batteries, contact me via E-Mail for details.
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Copyright © 1999, 2004, Sterrett J. Carter - Last Revised 06 May, 2004