I have a MTH Premier F-7 ABA (MTH 20-20264-1 from 2012 Volume 2) where the rear Proto-Coupler (the one on number 332) came undone when pulling a string of passenger cars. Now whenever power is applied, it makes a buzzing sound and I cannot get it to stay closed. It does seem to latch securely when not powered though.
I am not sure where the problem lies. I have tried power cycling everything, resetting via DCS, and reconnecting the tether cables between the units.
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Sounds like you have a short in the coil. They are powered and open when connected to ground.
Actually you may have shorted Fet on board constantly activating coil. Board needs to be tested and same with coupler. G
Is there any harm in testing the coupler first before looking at the board? I can swap the coupler with one from a similar engine pretty easily. I am not set up to do component level repairs to the board: that would require a trip to one of the local hobby stores.
You can ohm out the coupler to see if shorted to ground. The more you apply power with a potential shorted coupler or fet the more chances you kill the board or make the board unrepairable. G
Both couplers read about 2.5 Ohms when disconnected from the boards. Same as a known good coupler from another locomotive. Both worked normally when tested in the other locomotive.
I did not observe any obvious burn marks, loose or damaged wires so I put everything back together to retest. Miraculously, everything seems to be working now. I did switch the coupler positions when putting them back together though.
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