UPATE: Problem with ABA PS2 5V F3 set

Well, it's a slave board in the trailing unit and there are a dozen ways to damage them.  Think of it as a buffer board. it takes in track power, and gets the exact same electrical signals as the lights and motor and smoke unit function from the lead engine- but uses transistors to "buffer" and repeat the function.

A known failure is the transistors have no heatsinks for the motor drive other than the board itself. They can go into thermal runaway and fail- often shorted on in a direction- hence the runaway condition.

They also have those transistors but you may have more damage than just the motor drive section.

One thought is a bad tether system that caused a short- or worse if you plugged one in upside down- that would probably kill a slave board.

Sounds like motor fet shorted on slave board which is not uncommon on PS-2 5V versions.  Can be repaired or replaced.  G

OK thanks Vernon and GGG. Looks easy enough to change. Does anyone have the item part number so I can see if Digikey or someone else has them? Are those them in the bottom view with their tabs soldered to a big copper pour?

The weird part is everything was running fine the night before and when I fired them up the next morning; POOF! Makes no sense to me.

Hi Rod,

I had the same problem with a PS 2 Alco ABA set. I replaced the FETs on the trailing A slave board and all has been good since.

Ron

Here is a close pic of the defunct slave board. There are two TO-220 devices that are fastened to the aluminum heatsink. Are those the FET's, or are  they the two SMD devices on the back of the board per Vernon's pic?

The 2 SMD devices on the backside. I only can find the MTH part number, I'm sure you likely could find the part elsewhere, they are not that special.

@Vernon Barry posted:

The 2 SMD devices on the backside. I only can find the MTH part number, I'm sure you likely could find the part elsewhere, they are not that special.

BB0000052

MOSFET / SLAVE / FOR SLAVE BOARD AC-0000011 / SMT

$3.00/ea

Yah they look like LR024N from what I can see. I'll check in a while. The TO-220's on the front side are F9724N.

Rod,

I believe I bought the FETs from Digikey. The number was  IRLR024N. My ABA set was a 3 volt PS2.

The cause of the problem was a short caused by worn insulation:

Ron

@Rod Stewart posted:

Yah they look like LR024N from what I can see. I'll check in a while. The TO-220's on the front side are F9724N.

correct, but the other one is 7805 regulator.

@Vernon Barry posted:

correct, but the other one is 7805 regulator.

Vernon, you're cheating, you've got a parts layout haha! Anyway all good, thanks for the clarification.

Hi normally MTH puts heat shrink over the wire on the motor after the wire was soldered to the motor terminal you can also use liquid tape in a can Lowes sells this product or you can also purchase this product from harbor freight! It's generally about $10.00 ! who ever soldered that wire over heated the insulation with left the conductor open to easily short out to any metal which would blow the board you're lucky it didn't blow the PS 2 board also ! also I have replaced the FET's on the slave board on a G scale engine which kept blowing the FET's so I installed a higher current and then the owner was happy as the slave board never failed again ! the higher current FET's will fit on the bottom circuit board !

Alan

@Alan Mancus posted:

Hi normally MTH puts heat shrink over the wire on the motor after the wire was soldered to the motor terminal you can also use liquid tape in a can Lowes sells this product or you can also purchase this product from harbor freight! It's generally about $10.00 ! who ever soldered that wire over heated the insulation with left the conductor open to easily short out to any metal which would blow the board you're lucky it didn't blow the PS 2 board also ! also I have replaced the FET's on the slave board on a G scale engine which kept blowing the FET's so I installed a higher current and then the owner was happy as the slave board never failed again ! the higher current FET's will fit on the bottom circuit board !

Alan

Good point Alan, but I don’t think shorted motor wiring is the issue here. There is nothing nearby to short to and the frront motor is well shrouded by the cab insert. I am at a loss as to what caused it. I only hope new mosfets fixes it. If you have the PN for that higher amp replacement mosfet handy could you share it please. The original is good for 10 amp but a higher value wouldn’t hurt.

@upguy posted:

Rod,

I believe I bought the FETs from Digikey. The number was  IRLR024N. My ABA set was a 3 volt PS2.

The cause of the problem was a short caused by worn insulation:

Ron

Interesting picture showing the frayed wire. I see the rubber band that is used to hold the motor wires close to the motor (and prevent flexing) is absent in your pic. That no doubt led to the problem, but if the rubber band failed due to age you would have no way to know that of course.

@Rod Stewart posted:

Good point Alan, but I don’t think shorted motor wiring is the issue here. There is nothing nearby to short to and the frront motor is well shrouded by the cab insert. I am at a loss as to what caused it. I only hope new mosfets fixes it. If you have the PN for that higher amp replacement mosfet handy could you share it please. The original is good for 10 amp but a higher value wouldn’t hurt.

FWIW, I have seen one or two fail and could not pinpoint a reason. You are not alone. My guess is just some stray current and the tether system- maybe a loose plug, maybe just a differential current from one end of the train to the other.

This was my concern- you also mentioned some other "possible" symptoms or faults

@Rod Stewart posted:

Next morning on startup, it would not move. Seemed like the trailing A unit was locked up. Took the trailing A off the track, opened it up, and found the board was quite warm to the touch. The smoke unit was also very hot, even though smoke had been turned off all the while it was running. Put it back on the track, unattached to the lead. As soon as it was powered up it shot ahead uncontrolled.

The motor FETs is one thing, but again, it could also be you lost ground or had some form of a partial plug in the tether and so the board saw stray signals and voltages.

So, we know the FETs are damaged from the runaway symptom with no input.

I'm worried there could also be more damage in the other buffer ICs causing stray outputs- like smoke and lights. They normally repeat the PWM or pulsed input signal from the lead unit- however, if the buffer ICs are damaged and stuck on- then you have a stuck on output- and example for smoke resistor- that could be bad scenario when normally it's expecting that PWM'd signal.

This is why I often have to have a spare slave board around- a known good one, and then attempt repairs on the bad one, and maybe do some swapping to see what is going on. Here lies some of the love/hate relationship with this setup of tether and slave boards- they can be a royal pain and a little bit costly when they don't work.

I should add my theory why the board fails:

Yes, a shorted output will fail the board- actually most boards

Yes plugging the tether in incorrectly can also cause similar destruction to a short

But the one that causes this scenario- an intermittent tether or some other reason- where the sensing and powerup of the track power and thus the board- the motor drive rapidly toggles between forward and reverse- and the 2 FETs are basically shorting each other. This then rapidly goes into a thermal event. I don't claim to fully understand the dynamic of exactly how and why this happens- could be different track voltage and currents given the distance between lead engine and trailing engine and wheels and pickups on a layout, could be some problem with the tether- intermittent wire, not plugged fully seated.

Again, from what I have seen of a few of these failures- I beleive the mechanism of why the transistors are failing for motor drive is a rapid toggling of forward and reverse so fast they end up shorting to each other. maybe a component fails, maybe just an instability during the power up sequence, maybe that dreaded tether with all those connections and failure points.

One reason that the FET's can fail in an H-Bridge configuration is if the FET's get hot and/or the bridge rectifier gets hot and slightly leaky, the FET's that should be off can have leakage and start to conduct.  As they get warm, they end up conducting more and that's when thermal runaway kicks in.

Jon Z. pointed out to me years ago that one primary failure mechanism of the FET's on the ERR cruise products was actually the bridge rectifier getting hot and leaky and starting the process.  That's why some locomotives and many large scale locomotives have a tethered bridge, they add heatsinking to the bridge to minimize it's overheating under load.

With the higher quality MOSFETs of modern electronics, the FET's are actually surface mounted with no heatsink, but the bridge rectifier is the part that even in the latest boards has heatsinking.  Note that the Lionel RCMC and LCPx boards all have SMT FET's, but require a heatsink for the bridge rectifier.  Also, the MTH PS/2 and PS/3 boards have no heatsinking for the FET's, but they take care to heatsink the bridge rectifiers.

FWIW a previous topic and notes on the analog slave board https://ogrforum.com/topic/mth...unit-constantly-runs

Diagram by Stan2004 from this topic

Good point on the rectifier diodes not being heatsinked as well.

This explains a lot. I know from my own playing with H motor driver circuits that they can be cantankerous and thermal events can happen. Sounds like it would be good to keep a supply of these LR024N mosfets on hand!

This whole sequence started not long after a derailment of the trailing A unit (the one with the slave board) happened, that also took out the TIU and its related handheld somehow. The handheld wouldn't talk to the TIU, or any other TIU thereafter; nor would the TIU send out any DCS command signals thereafter. I can't imagine what might have happened to cause all this trouble. I have had a basically trouble free 15 year run with DCS  up until now. And yes I have had TVS devices strategically spotted at various locations around the layout the whole time. The day all this happened was not a good day!

Given the remote sees strictly RF signals to/from the TIU, I can't imagine how any track event damaged it, that seems to be to be a bridge too far.  It's hard to believe the remote was damaged by any event that happened on the layout as long as it wasn't tethered to the TIU.

Correct me if I am wrong, but this is not a typical H bridge.  The Relay swap PV to motor leads and the fet pulse the return from motor to ground.  Unlike the PS-2 3V board which uses single fet, the slave has one fet for fwd and separate for reverse.  The circuit is certainly complicated with the various opamps used to condition the Lead motor signal to the fet.  Also the return for forward has a complicated circuit around the fet with diodes, zener and fets.

Operationally, if a motor signal is lost the fet can saturate and burn up.  There were issues with the early PS-2 5V slave unit being slow to react to the motor signal of the lead.  If the motors don't turn quickly the truck gears can bind.  There were diode and resistor changes on the board to make sure it was more responsive to lead signal.

Depending on failure can have FET, resistor, opamp go.  But usually just the FET for forward. (Trail moving backwards but forward for train). G

Got it GGG thanks! I’ll do some testing of the FETs and hope thats all it is. Do you happen to know about when they made the referenced part changes for better response? This set is 2001 vintage so I am curious.

I read the referenced earlier posts on this problem and the description matches what this one is doing; takes off in forward when track power applied. If it goes wonky again I think at that point I will put in an upgrade kit and run them as a lashup.

@GGG posted:

Correct me if I am wrong, but this is not a typical H bridge.

You're right, the relay for direction control changes the circuit so they only need a single FET for direction control.

So I measured the slave board mosfets looking for a possible short. The nose one measured 180R while the tail one was 890R. They are quite different, though neither appears shorted. Does this sound like anyone else's findings for these?

@gunrunnerjohn posted:

Given the remote sees strictly RF signals to/from the TIU, I can't imagine how any track event damaged it, that seems to be to be a bridge too far.  It's hard to believe the remote was damaged by any event that happened on the layout as long as it wasn't tethered to the TIU.

I agree John; this makes no sense at all. Shortly after the failure of both the TIU and the remote, I recall that I did tether the remote to the TIU hoping to  establish communication. No joy, but that might be when the TIU somehow scrambled the code in the remote? All I know is the remote won't start up now, and rerpeated attempts to re-flash V5.0 software did not work.

@Rod Stewart posted:

So I measured the slave board mosfets looking for a possible short. The nose one measured 180R while the tail one was 890R. They are quite different, though neither appears shorted. Does this sound like anyone else's findings for these?

What's 180R?  Are you saying 180 ohms?  Truthfully, other than seeing a shorted FET, I think it's pretty hard to get meaningful measurements in circuit for those parts.

@gunrunnerjohn posted:

What's 180R?  Are you saying 180 ohms?  Truthfully, other than seeing a shorted FET, I think it's pretty hard to get meaningful measurements in circuit for those parts.

Yes; ohms. I thought from reading the older posts on this topic that measuring in situ would show. If one was shorted you would think it would show shorted no matter what it might be attached to. A reading of 180 ohms deems a bit inconclusive to me. I'll probably just change them both out anyway though.

Use diode setting and measure base to E/C and E to C.  It would normally be shorted 0 Volts. G

@GGG posted:

Use diode setting and measure base to E/C and E to C.  It would normally be shorted 0 Volts. G

I'll give it a try. Do you mean  "it would normally be shorted" if the fet is shot?

When the FET fails it shorts.  Typically, it is the same one that does.  The one that allows forward motion of the trail engine.  G

UPDATE: Got the new mosfets from Digikey so changed them out. Before changing them I measured every which way between G, S & D on both of the originals and nothing was shorted. Changed them anyway and no change, it still runs away backward (train forward) when track power is present. Hmmmmm. Is it possible that the direction relay is shorted in the backward direction? Could something like this cause the problem?

I do have a new slave board coming from MTH parts so hopefully that will get it going. But it would be nice to get this board repaired as a spare.

My guess is then the damage is in the logic buffer section- those smaller ICs all grouped at the front of the board.

A stuck welded relay still would not cause runaway. At least one of the FETs would be seeing gate drive or be shorted for motor to see power and complete the circuit.

@Vernon Barry posted:

My guess is then the damage is in the logic buffer section- those smaller ICs all grouped at the front of the board.

A stuck welded relay still would not cause runaway. At least one of the FETs would be seeing gate drive or be shorted for motor to see power and complete the circuit.

I see what you mean about one FET has to be shorted for it to runaway. So if the trouble is in the logic IC section, can it/they be re-flashed to correct the issue? Or is the board a paperweight?

@Rod Stewart posted:

I see what you mean about one FET has to be shorted for it to runaway. So if the trouble is in the logic IC section, can it/they be re-flashed to correct the issue? Or is the board a paperweight?

It's an analog slave board- they are analog ICs- just replace them. There is no flashing- there is no firmware.

@Vernon Barry posted:

It's an analog slave board- they are analog ICs- just replace them. There is no flashing- there is no firmware.

Any particular one, or the whole works? Do you happen to have the generic PN's handy? Wish I had the circuit so I could trace the fet gate back to its source sigh.

@Rod Stewart posted:

Any particular one, or the whole works? Do you happen to have the generic PN's handy? Wish I had the circuit so I could trace the fet gate back to its source sigh.

Sorry, have not repaired one to that level or reverse engineered it. When it's not the FETS or something obvious- I just replace the whole board. Time=Money in my world- not yet retired.

Got the new slave board from MTH parts; very fast service! Got it installed and presto everything is happy once more. Got about 3 hours logged since the swap. Keeping my fingers crossed that she will keep rolling now.

Another thing I did at the same time was switch out the BCR for a battery. The BCR did not seem to be healthy as sounds would cut out the instant track power was removed. Even an instantaneous interruption would cause complete shutdown requiring a powerdown and restart to get her going.

@Rod Stewart posted:

Got the new slave board from MTH parts; very fast service! Got it installed and presto everything is happy once more. Got about 3 hours logged since the swap. Keeping my fingers crossed that she will keep rolling now.

Another thing I did at the same time was switch out the BCR for a battery. The BCR did not seem to be healthy as sounds would cut out the instant track power was removed. Even an instantaneous interruption would cause complete shutdown requiring a powerdown and restart to get her going.

I know you blamed the BCR and capacitors can fail so not saying that was wrong. What I am saying is did you then use a meter and check to ensure the charging circuit is also working when the track is powered (you should be seeing voltage rise at the battery terminals)?

I'm saying this because either side of the circuit can be damaged- there is a charging circuit and a discharging circuit and on PS2 5V, there even some known failures an external transistor I have seen "dead bugged" to the board as a repair/fix.

In other words, great, you changed to a chemical battery- and that has more energy storage for a longer period of time compared to super capacitors, but if it's not charging the battery- eventually the battery goes flat.

I have to go with Vernon here, I suspect your new battery is not charging but just running on it's existing charge.  I've run across several boards with that issue.

If you want to check the BCR, use a DC supply with current limiting, that's how I test them.  I set the current limit to around 100ma and the voltage to 9V for one of the BCR's for 5V boards.  Turn on the supply and it will be obvious if they cap is good, the voltage will slowly ramp up with the current pegged at the 100ma limit until it gets to 9V.  That should take 20 seconds or so, then the voltage stops changing and the current starts dropping.