The MRC-83...

  ...amounts to a TRC-75 installed in an M38A1 jeep, along with an appropriate high-amperage alternator, a PP-2352 3-phase 400~ inverter, and an AT-1011/U 32 foot whip antenna. My 2006 birthday present to myself was a 1952 M38A1 jeep and a matching M100 trailer. This page will follow the construction of an MRC-83, and will change as I make progress on the install.

  I got the TRC-75 from the Barstow Marine Corps base in 1991. It had seen a recent rebuild at the Albany Marine Corps base, but had a lot of Mojave Desert in it and needed a complete cleanup before even applying power. After a lot of vacuum cleaner & compressed air work, I applied power to it and then the real work began. The receiver/exciter was functional as was the RF PA, but the antenna coupler had problems - it would not always tune correctly. Sometimes the coupler would tune correctly and the radio set put out good power, sometimes the coupler would never finish it's tune cycle and the radio set would fault. The problem turned out to be light corrosion on the antenna coupler's inductor shorting drum - disassembly, then going over the shorting drum with a palm sander & cleaning the silver ribbon with acetone took care of the problem.
  The radio set has given me little trouble since then. I have had it on the air at several surplus radio & military vehicle events, running it from a 3KW 400~ MIL-SPEC gasoline GENSET which weighs more than the TRC-75. The radio set & GENSET usually made the trip in the back of my pickup truck - not a convenient way to carry the big beast around. It was becoming clear to me that the TRC-75 wanted to go back to it's natural lifestyle & habitat - living as an MRC-83 in the rear of an M38A1 jeep.

  Over the years, I have accumulated almost all of the parts necessary for the MRC-83 install. Some are difficult to find, particularly the radio mounts and the power supply case as they are close to 100% aluminum and the scrappers love 'em. The alternator is a favorite of the scrappers too, as it is just full of copper & aluminum. Thanks to another TRC-75 fanatic I was able to come up with a 150 Amp alternator from a late M38A1 install, the radio mounts for the M38A1 install, a complete MRC-83 power supply box, and even the correct voltmeter/tachometer & tachometer drive electronics box. Because of the scrapper's love for copper, I am without most of the correct MRC-83 wiring harnesses so will have to build some from scratch - I'll make them from wiring stripped from TRC-136 shelters so the pedigree is OK there.



The 150 Amp alternator...

  The TRC-75 manual has three appendices, covering:
 Appendix 1.  100 Ampere alternator installation in either an M38A1 or an M170.
 Appendix 2.  MRC-87 installation (a TRC-75 & an ARC-55 in an M170).
 Appendix 3.  MRC-83 installation (a TRC-75 in an M38A1).

  Appendix 1 is only of casual interest, since the alternator I have is a newer version than the one covered in the TRC-75 manual. The original MRC-83 alternator used selenium rectifiers in the main rectifier stack, and had a small carbon pile & solenoid setup to regulate field current in the rotor - those selenium rectifiers got so hot that the whole rectifier package was mounted in front of the M38A1 radiator, separate from the alternator & regulator package. I am using a newer alternator from a late M38A1 install - it is a PU-656/M, a brushless unit with silicon rectifiers and a transistorized regulator.

  Appendix 2 covers the MRC-87, again not of much interest to me as I am allergic to ARC-55s & ARC-27s and just can't see dragging one around inside the power supply box...

  Appendix 3 covers the MRC-83 install and will be the bible for this setup.

  First order of business is to pull apart the alternator and look for worn or smoked parts. The pic shows the bare rotor - on the right end of the rotor are the reluctor windings & to their right is the small fibreglas drum containing the reluctor diodes. The reluctor provides DC current to excite the main rotor field coils on the left end of the rotor. The drive end of the rotor is on the right and uses a caged roller bearing, while the non-drive end has a ball bearing to handle thrust loads. There is a grease cup on both ends of the alternator.

  After cleanup & reassembly, it's time to figure out just what this thing is. Opening up the regulator box reveals a stack of 12 large diodes - six for each of the two stator windings. Also in there is a subchassis carrying three transistors and some other miscellany - this is the regulator circuitry. Power for the regulator circuit comes directly from the +28V output of the alternator. Fiddling around on the bench, I find that the regulator draws 4.5 Amps at 26VDC, and also that the diode stack reverse current at +26VDC is less than 1 microamp.

  In the old MRC-83 alternator setup, neither the selenium diode stack nor the carbon pile regulator circuit are connected to the battery unless the engine is running. A _large_ relay connects the selenium stack & the regulator circuit to the battery when the engine develops oil pressure. This new alternator's diode stack draws negligible reverse current so it is a good candidate for direct connection to the battery - that way there is no need for a large relay capable of handling the 150 Amp output of the alternator. A smaller relay is still necessary to switch on the 4.5 Amp regulator load when the engine is running (4.5 Amps amounts to about 115 watts, and most of that is dissipated by the reluctor's stator winding - probably best to power up the regulator only when the alternator fan is spinning, rather then whenever the ignition switch is on...).

  You ask 'why not just power the regulator circuitry directly from the ignition switch?'.   Doing that would eliminate the small relay but then,due to IR drops in the jeep wiring, the alternator output voltage would change whenever the headlights were turned on or whenever the horn sounds because the regulator's sense line for battery voltage is also the line that supplies power to the regulator.

  The relay unit that I have also differs from that shown in the TRC-75 manual's appendices - mine has an extra relay. There is an emergency switch (BATTLE OVERRIDE?) which allows the TRC-75 to be operated from batteries alone - I'm not sure I need that option so I probably will not drill a hole in the M38A1 for that switch... The relay box bolts to the firewall, just to the rear of the alternator and right next to the battery box - it will serve as a convenient tiepoint for high-current connections, but my wiring will differ considerably from the circuitry Uncle Sam used.



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