Repair Question>: Can a smoke unit failure damage a Receiver/Driver Board?

I don't have these specific models but it appears (from Lionel's parts pages) that they have recent generation Legacy smoke units with 8 ohm resistors and plastic reservoir bowls. The later (6-85215) units also appear to be equipped with a thermistor on the PCB board - not a component shown on the earlier board.

What you are describing reminds me of what happens when an AC regulator fails on earlier generation smoke units, as in full track power to the resistor and it and the PCB are smoked. But I don't think that can be the cause in your case as the components are different.

None of the Legacy engines I have with boards similar to yours have displayed the kind of failure you describe. They've been reliable in operation although the smoke output is not quite what I hoped for. Your experience makes me wonder if there has been some kind of maintenance or power supply issue. Have you kept the smoke units fully topped with fluid? What transformer are you using? 

First off, I'd check the smoke unit over carefully to see if there are any shorts.  Since that has the plastic fluid reservoir, that seems unlikely unless it's a wire.  Any short to frame ground is very bad for the RCMC.

Thanks to you both.  I use Lionel Powermasters to keep 18 volts in the track.  I do my best to not let my smoke units run dry but as most of its know this is sometimes difficult due to evaporation and many locos to remember.  Perhaps a good inventor (are you listening John?)   could invent a dip stick or strip type device that would let us know the status of smoke units.  Like those used to check the status of pool or hot tub chemistry.  

Just so you know.  I removed all the plugs from the driver board one by one and checked each time is the short was eliminated.  I finally removed power from the pickups to the board and the short ended.  So I have concluded the short is in the board itself.  I tested the smoke unit from the 38702 and both fan and resister seem ok.  

Could a bad thermistor cause this kind of problem?  Not sure what this is?

Not likely, that short is typically the drive motor FETs or the bridge rectifier.

Regarding the thermistor, if it's not in the older unit (as the parts diagrams indicate is the case) I can't see that its failure accounts for both sets of similar problems. But for the sake of completeness, and because there were some references to possible failed components GRJ mentions, thermistors were explained in a couple of older threads. In each of them scroll down until you get to the posts by SantaFeFan (Jon Z formerly of Lionel who designed the relevant smoke systems):

https://ogrforum.ogaugerr.com/...52#67736898071546452

https://ogrforum.ogaugerr.com/...95#79846750335720495

This might be more info than you need but you never know when the knowledge base can help.

It seems obvious that smoke unit failure can damage the boards but the issue seems to be, what caused the failure? A short somewhere seems likely - just occurs to me to ask if you have got any internal wiring rubbing on a flywheel or otherwise getting frayed?