Question on MTH Slave board.

It gets motor signal, Positive voltage, PCB Ground, and various other signals for smoke, couplers and Rev lights.  Make sure you have continuity through harness on the 10 pin back to there sources for the motors (White and Yellow) and PV purple and PCB Ground Blue. Should be easy to trace wires and colors with both shells off.

After that, it is FETs, and micro controllers, maybe a resistor or such.  If no motion in either direction, might be best to send the board to me, or just get a replacement board.  New boards are $70.   G

Thanks G just what I needed. 

Pete

I had my ATSF Alco PAs lost motor control on the slave and I had burned out a logic gate on the slave board. Everything else worked fine except it wouldn't move. Fix was like $0.46 from DIGIKEY

Norton posted:

...Is it the signal from the tach or the DC motor voltage which is then buffered? 

 

Not clear if you answered the question of the loss of motion in BOTH directions, but if you hear the relay clicking with direction change in the slave board, it could be a relatively inexpensive repair if you're determined to DIY.  But you need to be comfortable working with surface mount parts.  It could be as "simple" as a 50 cent logic gate as in Matt's case or the $1 motor driver FET (the motor signal is buffered as you say).

It doesn't move in either direction. No problem working on surface mount boards. This ain't my first rodeo. Not having a schematic is the biggest hurdle. This is a train show weekend so won't get to proceed further until next week.

Pete

Worked on this some last night. Sounds like the relay is clicking. Yellow, white, purple, and blue wires all ring out from the lead engine to the slave board ten pin inline connector.

Poking around with my ohmmeter I find a short across the 1N4933 diode just to the left of the 5 pin connector that has the slave motor wires. This as you are looking at the component side of the diode. Removing the diode, it is OK but the short remains across the pads. Motor connector is removed. I see the anode side of the diode goes to the motor leads but the cathode pad has no visible trace. I assume its a multilayer board.

Any idea were the other side of this diode goes to? Square pad to the right of "copyright".

BTW both motors work when powered from my DC supply.

Pete

That is a normal reading for the diode.  Measure the 2 Square Motor FETs?  Are they  shorted?  Though engine would move on power up normally if one is shorted, or you would see the short on the transformer.  Either both FETs went open, or you need to look at the logic chips.  Frankly, the FETs look good visually,  G

Since MTH does not publish schematics, perhaps the following might help.  The generic 2-FET with relay DC motor controller is quite simple.  There are other parts of course for current limiting, clamping, etc. but the basic idea is an SPDT relay applies positive voltage to one of the motor leads.  Then one of two transistors is pulsed on to "ground" the other motor lead.  If your relay is indeed toggling properly, then either the FET is "open" and not turning on, or the logic circuitry is not providing the buffered control pulses from the head engine tether to turn on the FET. 

The red lines show the current path for each direction showing how the relay and FETs work together to reverse the polarity of the current through the motor.

I'd think at this point, to go further you need to power up the system and make measurements which I realize can be logistically difficult.  I suppose you could disconnect the head-engines motor so that nothing moves when you command the head-engine forward or backward.  Then you can look at the slave FETs to see if they get control voltage slaved to the head direction.  The Vgs voltage pulses should go from 0V to probably 5V since you have logic chips driving them.

As G says, if a FET is "shorted" on, then something would happen in at least one of the directions.  If the FET is "open" off, then BOTH would have to have failed for nothing to happen in either direction.  I'm starting to like the logic circuit.  Perhaps Matt can step in and identify the exact logic component he changed as it sounds like his symptoms were identical.

Just had an easy fix! Sharknose AB with the b unit dead, no functions whatsoever. went to perform a continuity test on the harnesses and found the a unit's harness barely plugged in. Reconnected and ran perfectly! Don't know what caused it though, possibly while being shipped.

See this recent thread.  Same symptoms.   Simple fix by changing the ~$1 FET...you'll pay more for the shipping than the part!

https://ogrforum.com/t...-rear-a-unit-problem

Could someone please give me the Digikey or Mouser part number for the proper FET?

Thanks,

Ron

FET is: IRLR024N

Thanks gunrunnerjohn, I appreciate that.

Ron

Just noticed this thread has been revived.

The engine has been repaired thanks to the efforts of the ebay seller I got this from. He agreed to have his MTH guy look at it. It turned out the slave board was in fact bad (no details as to what specifically was the problem). But also two problems in the Lead Power unit that included a bad diode and bad solder joint in one of the tether connectors.

A big shoutout to ebay seller kr-trains. Anyone seeing something they are looking for that he has don't hesitate. This is a first class seller.

Pete

I have the same exact issue, good main unit and partially burnt slave on an MTH E8 AA PS2 3V slave board.  The slave will go forward no issues but does not move in reverse.  I opened it up to find a burned LR024N FET.  Looks to be this part.  I unsoldered it and I also took out an SMD resistor and it’s pad.  This will be an interesting repair.  

Slave board

Burnt pad

Burnt chip

Michael, I had a second MTH engine arrive with a bad slave board. It too had a blown FET which I replaced but the problem persisted. One motor would not turn and I assumed it was the motor. Replacing the motor did not help. I finally sent the board to George (ggg) and he found a bad processor and one other bad component (??? CRS). When I got it back I started it up very slowly with the shell off and found it still would not drive the motor. 

I disconnected the motors from the slave board and powered then directly from a DC supply. It was overloading my 2 amp supply. Then removed the motors and they ran together drawing only about 600 ma. 

It turned out the mount was slightly off. When I tightened the motor mount screw it was binding on the worm gear. I placed a .020" shim under the mount, tightened the screw and now it runs light at 600 ma. 

All the time it was a mechanical problem with the motor. 

After you replace the FET run it slowly with the shell off to make sure both motors turn. If not you still have an electronic or mechanical problem.

Pete

Hi @Norton The pair have been running extensively for years but I’ll check that out.  It’s odd that, the FET for forward is fine, the area around it is clean, reverse as a slave A.

I may have to send it out for repair.