F3 slave unit always running

Sure sounds like the slave board went bad and will need replacing.

Is there a reason they go bad? Something I should check so I don’t blow a new board?

It's like a lot of other electronic boards, they just go bad for no apparent reason, doubtful anything you did.

Are the 3v and 5v slave boards the same? There’s a vendor on eBay selling new MTH boards and the ad states they replace ps2 &3, but thought I’d ask to be sure.

Yes, the slave boards are the same for 3V and 5V PS/2.

@2032 posted:

Hi,

I’ve got the DAP B&O F3 set, part 20-80004. Both A units are powered. Today I was running them and went to use the PFA. I hit the button and let it run halfway around the track with the bell ringing, all of a sudden the slave A unit starting running wide open. I shut the entire system down and restarted. The slave A will try to run even if it is not teathered to the B unit. I tried feature and factory resets with no luck. The main A does work perfectly fine. It continues to try to run. Any ideas? Is the board hosed?

Thanks,

John

Typically, those boards can be repaired, don't panic yet.  Sounds like the motor drive FET is shorted.

I’ve got the board out. Do you know which part is the drive FET?

The big ones on the back.   Note the indicated components.  You can also see that I replaced the left hand one, that's the one that usually goes.  You can use an ohmmeter to measure between all the pins.  If you see a short with any two of the pins, the FET it probably toast.

Thanks GRJ, your knowledge is invaluable

I pulled the left one and it is definitely shorted. Ordered a new one. Thanks again

I measure them on the board, the short is usually obvious.  Check the other one as well.

I checked it on the board and it showed a short so I removed it to double check.
The other does not show a short, but I bought a second for good measure.

That should do it.

Thanks for all your help. I really appreciate it.

Most times just the fet, but don't be surprised if you still have issue with fwd motion.  Other chips sometimes go with the fet.  Also make sure no motor is stalling or slow to start with motion on the slave unit.  When you test run it, have the shell off to watch response of motors. G

Ok thanks. I’ll try to run it with the shell off once the FET is replaced

Update

i soldered in the new FET and all is good. Runs like new now. I ran it with the shell off and both motors are turning freely.
I think what happened was a traction tire came off and locked one motor for a slip second. I’ve got new tires on order. I won’t run it again until the tires come in.
Thanks again for all the help and suggestions.
john.

We love a success story. 

FWIW, I cooked a 5V board years ago when an engine threw a traction tire and it only took a few seconds before the board was toast!

Yikes

Were you able to repair it or was it junk?

@gunrunnerjohn posted:

FWIW, I cooked a 5V board years ago when an engine threw a traction tire and it only took a few seconds before the board was toast!

@2032 posted:

Yikes

Were you able to repair it or was it junk?

It was junk, couldn't revive it.  "stuff" happens, nobody is immune.