The military gets cool toys.
One of those neat gadgets is the MEP (Mobile Electric Power) 362A aviation generator set, circa late 1980’s/early 1990’s. It is an all weather turbine engine generator that provides 28 volts DC at 357 amps, all day, any day, in -65°F (-53.9°C) to 125°F (51.7°C) weather.
The turbine engine itself made by Tiernay Turbines Inc, model TT10-1. The engine alone has a dry weight of 100lbs (45kg), spins at 50,300 rpm, and drives a 4.2:1 reduction gearbox into the generator head. Max output in terms of shaft horsepower is roughly 60 horsepower. It gulps a healthy 8 gallons per hour of fuel, which means it can run for 4 hours on its 32 gallon tank.
If you have small aircraft, military trucks, or anything with a 24-28 volt charging system, this is a really cool way to charge batteries or jump start. Outside of those applications it is little more than a novelty to start up and admire when your gearhead buddies stop by for a beer.
Well, turns out one of my gearhead buddies has two of these; one that runs, and one that doesn’t. The one that doesn’t run has an electrical problem.
When the MEP362A starts up, the turbine engine is cranked and fired by an audible igniter. As long as your engine spins and has fuel + spark, it should start. Once the turbine fires it ramps up to an intermediate rpm for a few seconds, and then continues to climb in speed until the generator head is excited. At that point, you will hear a electrical “whine” suddenly begin and the output voltage gauge will wake up and display the target output volts (somewhere around 28 volts DC). The engine should maintain a constant speed at this point and the generator is normally operating.
Well, the problem generator in this case would keep shutting down right before the second startup stage (generator excitement) would complete. Since most of the electronics are two boxes under the control panel: the EECM – Engine Electronic Control Module) and GECM (Generator Electronic Control Module), we thought why not swap boxes between the working and non-working generator to narrow down the problem? This was an easy place to start, since the boxes are connectorized with your typical circular twist lock mil style connectors.
It turns out that swapping the GECM from the good generator to the bad generator got the bad generator to run. So we can say with confidence that our problem is located somewhere inside the GECM. Time to take this box to the bench, crack it open, and investigate.
Opening up the GECM and taking a close look, a few obvious problems began to stand out.
First, D4 was blown clean off the board.
In addition to D4, a few other diodes (D8, D30, D33) were looking a bit sad. They appeared to all be of the same type, and had this white, corrosion-like substance where the leads met the diode package.
A closer look under the board also revealed a few identical corroded diodes across a relay.
A look at the service manual (below) identifies D4 as a 1N4148 and the remaining diodes as 1N5614’s.
One Digikey purchase later and we are ready to replace the bad/missing diodes:
After dropping in a new D4, D30, D33, and two extra 1N5614’s across the relay, we were ready to see if it works. Sure enough, she lives! Headphones recommended for that sweet turbine engine sound:
Overall, an interesting piece of vintage military gear and a fun repair! Stay tuned.
Kevin Aghaei's Lab 