MTH 20-2619-1 rear A unit problem

I have been reading some of the forums and and I have tried a factory reset which did not help. I removed the 4 pin motor harness off the slave board and all of the function are working. Anything else I can check?

I also removed the 4 pin harness for the smoke unit on the slave board since that's what I tinker with and no luck, the rear A takes off when any voltage is applied.

The rear unit takes off even with it unplugged to the master A?  In which direction?

You mean 5 pin motor connector?  Sounds like a shorted motor get if you did not pinch a motor lead to chassis. G

Thank you for the help. Yes, the A unit takes off unplugged from the front A unit and with the front A off the tracks. It takes off in reverse, and yes it is a 5 pin connector with 4 wires, I missed that one. Both motors run and in the same direction. What should the resistance be for the motor?

Thanks again,

Blake

This is a typical failure for a slave board.  The motor FET is shorted.  This can be repaired at component level.  Motor resistance can be all over the place.  But as long as not a direct short or a full open, it can vary.  You also could have a bad pole not measured.  But typically between 6 and 12 ohms.  Smoke fan motors usually always about 12 ohms.  Better to test on a dc power source monitoring current flow.  Looking for start up voltage and current at no load.  G

Start up voltage: motors starts to hum at 5.0 volts and and 0.7amps any more and they will start to run.

So, you can find and replace the failed FET on the slave board?

Blake

Sounds like you did the work on a non ESD bench.  Static electricity fried something on the slave board. Unlikely it's a defective motor if it worked before servicing the smoke units.  Send it to GGG for repair 

wings posted:

...you can find and replace the failed FET on the slave board?

There are 2 FETs that do the heavy-lifting for motor drive.  One for fwd, one for rev.

They are clearly marked with some subset of the part number "IRLR024N" which are about 75 cents from DigiKey, Mouser, etc.

However, based on what you've described I think there may be other issues with the electronics (other than or in addition to the motor FET(s) .  I concur that contacting GGG for repair is the path of least resistance. 

OTOH if you are determined to make a DIY repair do an Advanced Search on OGR using terms like Slave, FET, repair, etc. and there have been threads describing successful DIY repair of this board.

IIRC a replacement slave board can't be ordered by the general public.

Thanks for the advice. I have contacted G.

Blake

Replaced the shorted FET and the F-3 works great. Thanks for the parts and advice G.

Blake

So for the record (to help the next guy), which FET (left or right in photo above) did you end up changing?  Did you diagnose the bad FET by simple meter measurement?

I think it was the one on the right, pictured above. I removed the board and measured for a short between the 2 legs and replaced the shorted one.

Nice.  I'm sure this will come up again!

Stan, Usually it is a short and easily detected with meter.  May even have visual signs.  But there are cases where it is beyond the fet or other components go too.  But in most cases just the FET.  G

What restores my faith in humanity is that Blake took the time to report back to the forum on his results.   

I wish more guys would do this!   So often I wonder "what happened?"

Love Stan2004's comment! As Paul Harvey would say.......now you know the rest of the story. So many times we don't!

Thank you for this thread.  I belong to the Paradise and Pacific club in Scottsdale and I do most of the maintenance on our MTH engines.  The Double AA MTH engines are very valuable to us because we have a 2.8% grade with 1080 degrees of curves per lap. We can only pull 3-4 cars per MTH motor on the layout (for hours at a time)  and the double AA's let us pull 12 to 16 car trains around.   As we are open to the public 363 days a year we put a lot of time on our engines.  We currently have a slave board with exactly this failure mode and I have seen this before on other MTH engines.  After reading this thread, I think we may try to bring it back to life. 

I doubt that there is a problem with the motors, but here is a very simple and safe way to check them.  We use a DC power supply with a digital readout on the volts and amps.  Take the 5 pin motor plug with the two sets of yellow and white wires off the board (be it a master or a slave).  Verify that the motor spins freely by hand while it is in the trucks in both directions.  If it does not, take the motor off the truck and remove all the grease from the gears and motor shaft.  Inspect the side gears as sometimes they will pick up a tiny rock. A tiny screw drive or a dental pick (which we like to use) works well for that.  Put in fresh grease.  Connect the power leads to the motor.  Turn the amps up on the power supply but leave the voltage at 0.  Then slowly turn up the volts and see where the motor starts.  I normally use 5.0 volts to test them.  Milliamps should be around 200 to 250 at 5 volts when running freely. If it is 300 or more, you may have a problem.  With the motor in the truck, I check them at 5.0 volts and briefly at 10 volts.  With no load, they are usually around 300 mills or so at 10 volts. Check them in both directions. If higher than 400 you may have a problem.  If you don't have a fancy power supply, you can check the current draw of the suspect motor against a known good motor at some voltage between 5 and 10 volts.  They should be within about 20% of the good motor.

The output of an MTH Transformer is AC so DO NOT connect the motor to it directly.  A 25 amp full wave bridge works great for converting the AC power to DC.  We use those all the time to resurrect or convert engines to Forward only engines for our Public Loops.  They replace the electronic E units.  

Again, thanks for the great tip on the FET!