PS3 Coupler UP IN SMOKE

You should see it when you find it, but this does happen.  Normally those fet burn up when they short especially with coupler load.  The coupler needs to be replaced too.  G

Thanks G. The coupler is definitely toasted. Could a short "before" the slave board have caused the FET to fry? Like maybe a wire coming from the mother board in the lead A unit that carries the coupler command. If so I'll pull the shells off the other two units and have a look. Also, do you get many PS3 slave units with this issue? It seems odd to me that both these units I have made in 2013 had the coupler go out.

A short on the coupler will most certainly frequently take out the FET.  The design of the MTH couplers invites that to happen with the exposed connections.

Thanks for your input GRJ. I examined the slave board under the scope and the only component that looked to have possible damage was Q220. I had to buff it to remove a charcoal like finish on it and add a film of oil it to see the part number . Searching the number, c8DOC or 8DOC, on Digikey and Mouser I came up empty handed. It may be a bipolar junction transistor. Not sure it has anything to do with the coupler unless its a component used like a switch to only allow a short pulse from the FET to fire open the coupler. It may be causing the constant voltage I'm reading if it's failed. I can't find any where a short could have occurred with the coupler, or the wiring to it, and the FETs on the board look fine. The coupler is very well protected inside the E8's pilot and it was it's first run for about 30 feet when I saw the smoke coming form the front end. I may have to just run another tether like I did with my other PS3 E8 to operate the coupler with the mother board if I can't figure this out.

See if that component goes to the coupler connection, that should give you a clue.  My guess is it's a FET, but hard to say exactly what type.

Not specific to Slave, but there have been a few PS-3 that have burned up couplers and boards. Steam and Diesel.  The Lead unit is communicating to the slave with data.  Not signals like PS-2.  So this is related to the slave in my opinion.  But there may be a mod of slave board.  I would have to go review. 

Example on Lead A PS-3 diesel, you must have a rear coupler applied or else a resistor in the circuit to prevent FET and or damage.  Just Rear coupler is ok.  But Not just fwd coupler for PS-3 diesel.  Hence a jumper mod for some of the products.  G

A low value PTC might be an option here to protect the coupler and the electronics. 

Good idea GRJ, maybe a TVS too. Very interesting info GGG since this is my second E8 set of the same year that this happened to. I did find a possible way though that this coupler may have gotten shorted. Pictured below is the coupler plug and a black metal bridge that is meant to protect the wires from the trucks limiter. The plug gets pulled tight against it when the front truck starts to turn left and causes ground contact.

I did replace the couplers burnt coil with a good one from a coupler that was damaged in a collission. I then coated the solder joints with Frog Snot' which I find more durable then liquid tape. I think I'll start applying it to all my electro couplers especially ones with an axle under them.

Since I had no luck finding the part number for the component that looked chard and couldn't see it's traces and where they went,

I ended up taking the the easy way out and ran a tether, the same as I did with my other failed E8 set, to the lead A unit's rear coupler wires coming off the 40 pin connector.

SLAVE BOARD "A" UNIT male tether end.

LEAD "A" UNIT.  I replaced Molex plug with the female tether end. Today I will add a tether to through the "B" unit. WARNING to anyone who uses this method of repair, always use the female end on the lead "A" unit because the exposed pins on the male tether can touch the tracks, if your running the lead unit by it's self, and short the mother board FET.