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I just purchased a Williams 718/719 GP9 set off of ebay. As I ran them around my layout, the horn suddenly blasted and the train slowed down. I continued to run them (powered and non powered) and it happened more and more. I was very confused, so I took the dummy unit off the track and ran the powered around, it slowed down in various locations and eventually stopped. It wont change direction or move. I am running it on a PW Zw. I have not touched anything inside besides removing the shell. The dummy works fine because I have tried it with other Electronic E-units. My question is why does my locomotive not work at all? Did a component break in the E-unit board?

E-unit boardWilliams 718


Images (2)
  • Williams 718: Here is the shell and model
  • E-unit board: Seems that there is no blown components
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It is going to be difficult to help. Looking at the board, I don't see anything obvious. Sometimes electronics fail and you can't see any physical signs of failure. As far as the dummy goes, I don't get what you are saying. A dummy doesn't have motors, so how did you go about trying another e-unit in it and what changed? Do you mean that you ran it with other powered units and they ran fine with it in tow? If that is the case, then you should start by checking all of the wires on the powered unit very closely. Sometimes they break or a solder joint will fail. Trying to run it with the shell off and move the wires around to see if it starts to move when you touch one of them. If it does, then it will be a wire issue. Make sure the transformer is about a quarter of the way so it doesn't take off too fast if touching the wires makes it start running. We are going to need a little more info. So after you look over all of the wires, post again and let us know what you find. It may take a while to figure it out long distance.

Yes I did mean the dummy was towed around. The dummy has sound so I thot it could be making the powered unit slow down. This is not the case. The powered unit slows down on its own and I believe it is the reversing unit. Its just very odd for it to run and then just stop I inspected the wires and there all hooked up. Do they run on the same amperage of other PW Lionel ? 

If you start looking for a new board, make sure you get the one that has the relays on it and not the one with transistors. The relays handle more current and are more reliable. I had an old board that would only run in one direction because one of the transistors failed. I replaced it with a board with relays. I have had several derailments that caused shorts and never once did that cause a Williams reverse unit to fail. They are far simpler and more robust than some of the other brands and will handle shorts without issue in every case I have had. This issue may be the reason they were being sold. I have been buying stuff on eBay for almost 20 years and I have had plenty of undisclosed issues to deal with. If you got a good deal, now you know why. At least it is an easy and cheap fix.



Before the powered stopped working, The powered also continuously slowed down for a while, I still think there is something else wrong with the reverse board besides the transistor. The Powered slows down at about half of the speed, and sometimes stops. 

Now, I get no movement. 

Using a rectifier, I may hook up a PW e-unit but this is my last resort. I would also have to go back and test the capacitors and MosFets too, if i decided to buy a relay or new transistor.


Electronic control boards are subjected to many adverse conditions in the operation of  locomotives on the tracks. Its a hostile environment for electronics. You have  RFI, EMI and power surges, any one of which can blow a semi-conductor IC. The boards are supposed to be protected from these but not always. I find it difficult to pinpoint blown devices on a circuit board and its safe to say they were not designed to be easily fixed.  If all of your wiring checks out ( no shorts or disconnects)  I would just replace the board. Williams sells replacement parts as do companies like Dallee Electronics.

Power draw can be all over the map.  The max is usually if for whatever reason, you have a motor stall.  In that case, the max current can go to a few amps.  For instance, the Pittman 9234S004 that is used in quite a few older O-gauge steam models, has a peak stall current of 14.49 amps at 12V!  Even the small Mabuchi 3xx series has stall currents in the 3-4 amp range, if you have two of them stalled, double that.

I don't know for sure, there are many designs of the inexpensive reverse boards.  I know that several common ones use the TIP126/TIP127 combo.

I have a couple of Williams relay based reverse boards if you can't get that one running.


Hey John,

Just some insight on your very correct observations.  The H bridge is like one I've never seen before.  The PNP transistors have their emitters connected to + supply and the NPN's have their emitters connected to - supply.  The bases of the totem pole are connected together with a high power low ohm resistor.  A small resistor pulls the bases to + supply and the lower part of the power resistor is connected to TO92 NPN for control.  The drive transistors have their collectors tied together to provide Motor1 and Motor2 .

The attempt is being made to force the drive transistors into saturation.  Thus the lack of heat sinks.  However with the base pull ups, the H bridge dynamically brakes the motors when it goes to neutral.  Ouch!

This device was from Dave Ott/Ott Machne Services.

And my apologies to those who could care less.


Lou N




Just to verify Ben, can you take a picture of the motors? Most all Williams diesels use two Mabuchi 385s. A few of the early ones only had one motor. Stall current with two motors is under 6 amps. That is worst case. I am not sure anyone here really knows what E unit you have there. Most all Williams units use relays even the ones from the '90s. K-Line Conventional E Units use transistors and its common for those to fail.

Bottom line, get a newer Williams E Unit and you should be fine.


@Lou N posted:


The attempt is being made to force the drive transistors into saturation.  Thus the lack of heat sinks.  However with the base pull ups, the H bridge dynamically brakes the motors when it goes to neutral.  Ouch!

Interesting Lou, I didn't look that closely at the ones that have smoked here, but obviously something smokes the transistors, that's the part I've replaced a few times for these. I guess under some conditions the transistors don't go into saturation and dissipate a lot of heat. I've seen several that have cracked the plastic case, obviously lots of heat there.

@Ben-1261 posted:

That makes sense, a NPN to drive one motor one way, and PNP for the second motor, the opposite way so they don't bind.

 Actually, it's a little more complicated than that. The motors aren't individually controlled, they're in parallel or series, to drive them independently, you'd need two H-Bridge circuits.

There are four transistors (or other switching devices) in the H-Bridge, the object of the exercise is to swap the polarity of the motor drive.  At any one time, two of the semiconductor switches is active providing power to the motor.  To reverse direction, the other two are switched on and those are switched off.  A fairly common failure of an H-Bridge is when a circuit failure causes one of the inactive switching devices to turn on, that creates a short and you get lots of current and heat.

Here's a pretty good description of a simple H-Bridge circuit using transistors.  This one uses low current 2N2222/2n2907 complementary transistors, but the technique is the same for any size motor and semiconductor switches.  This example also illustrates dynamic braking and coast provisions, depending on the state of the inputs.  Note the cautions about conflicting inputs to the H-Bridge control inputs.

H-Bridge Motor Driver Circuit Description and Schematic



Last edited by gunrunnerjohn

I just picked up an older Williams GP9-214 dummy from a recent auction purchase.  I knew that it had lights, but was surprised that it was tooting it's own horn while in tow.  Looking for answers, this thread popped up and it appears you guys know plenty about these.

So... does the horn just blow randomly?  Can it be shut off?  Is there a bell, too?


I have a Williams SD45 that at one point was blowing the horn randomly. One of the wires ran very close to one of the flywheels. The wire was coming in contact with the flywheel when it went around curves, and eventually wore off the   insulation. The contact of the wire on the flywheel was causing the horn to sound.

This appears to be the older board which was replaced by the "newer" Williams "six-amp" reverse board.  Williams was proud enough of this upgrade to list it as a feature on the boxes of newer units.  The older board overheats, causes numerous problems,  and is not worth fixing.  The newer six-amp boards are plentiful, easy to install, and work reliably.

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