Help With Legacy RCMC Motherboard

I do not have the picture, but it is near the smoke plug and a larger square FET.  Should see the damage, or short with meter.  G

Rod, a picture of your RCMC would be useful.  There is more than one version of the RCMC, so it would be nice to know which one we're dealing with.

I took a random RCMC out and here's the FET's for the smoke unit outputs.  They're right next to the two smoke unit connectors.  It's an F7341 FET in the SOP package.

Thanks for taking the trouble to take that pic John; that's most useful. Here is a pic of the actual board in the engine.

So are the FET's in question the 8 pin packages just to the right of the 4 pin smoke unit connectors, at the far left? If so, they look easy enough for even a hacker like me to hopefully change. I better do some testing as suggested by GGG beforehand though, just to make sure it's actually the problem. Thanks for the info John and GGG!

Rod

Yep, verify they are the same part, note that the actual part number was the bottom row on these particular parts.  Armed with the part number, you can check the datasheet for the pinouts.  if it's open, it may or may not show damage, but you can at least ring out the leads and make sure you're replacing the FET associated with the connector that isn't working.

I am coming up with quite a few options for these in dual N-channel mosfets. Mostly 55V 4.7A or 5.1A, in 8SOIC packages. One candidate that looks promising is Digikey PN IRF7341TRPBF-ND, which are about a buck each in small quantities. John you had said they are an SOP package, but I don't see anything in that style. I see SO-8 packages. Can you elaborate more?

The lower of the two smoke headers is the main, the upper is aux. So I am thinking it would be the lower mosfet, but I would check the pins against the header pins to make sure. No sense trying to fix it if it ain't broke!

Thanks, Rod

Sorry, it's SO-8, that's "Small Outline 8-pin".

You can also measure the dimensions and pin spacing of the existing chip to make sure you get the proper package.

So SO-8 is different than 8SOIC?

Rod

I believe those are two names for the same package.

SOIC-8

SO-8

Rod,

I found that if the smoke fan motors are not working. You need to replace these. Sids Trains can confirm on that fixed the problem on his RCMC.

These ones next to the smoke unit plugs are for the Heating element and also drive the couplers.

OK Bruk thanks for this information. I get the 3 flashes of the cab light, so I think it's the fan motor FET that is unhappy. I already changed the fan motor a while back to no avail. The symbols engraved on those FET's in your picture don't seem to turn anything meaningful up on Digikey. Might you happen to have the actual PN for those?

Thanks, Rod

Larger chip heat, smaller chip fan motor.  Was your fan running?  DId you replace it with a Lionel motor?  How old was the part?  Try an MTH motor.  Typically when those parts are bad, they show sign of damage.  Short.  Granted and open can occur too, but....  G

@GGG posted:

Larger chip heat, smaller chip fan motor.  Was your fan running?  DId you replace it with a Lionel motor?  How old was the part?  Try an MTH motor.  Typically when those parts are bad, they show sign of damage.  Short.  Granted and open can occur too, but....  G

Thanks George, I replaced the fan motor with a new MTH one. No difference, still the 3 blinks. My next plan is the small fet. Do you happen to know the pn for these fets? I can't get a match on Digikey using the data etched on the part in Bruk's picture. Thanks for any help,

Rod

I couldn't find the small FET either based on the numbers on the chip.  If I had to guess, I'd probably replace it with a SOT23 N-Chan FET.  I also notice that Bruk's RCMC picture is a later board version than the one I posted.

Here is a better picture of the suspect FETs on the #3751 mobo; fan motor FETs in the upper circles. These look way different than those in the picture Bruk posted earlier; I think Bruk's is a newer board. I don't know whether the motor FETs are the big ones to the right or the way smaller ones to the left of the big ones. Picture doesn't show the case markings very well, but the large ones are marked 1K9, and the tiny ones are marked KJG. None of which makes any sense in the real world, so I don't know what they are. The big ones are only 4.5 mm wide. If anyone can shed any light on these I would be extremely grateful.

I also checked the fan motor that I already replaced. It works well at 5vdc and draws about 40 ma, pretty much as expected.

Thanks, Rod

The fan motors would be small FET's, the large ones are for the heaters, they're the ones I posted previously.

Gee John; not sure about this. I spent some time tracing connections just to see which component does what. Here is what I got.

The red connections are the smoke fan hots. The blue are the heater element hots. And the green are the coupler hots. So it kinda looks like the large upper FETs are the fan controls. The dual 8SOIC packages (F7341's) look like they control smoke heaters and couplers. Then I measured across the leads of both fan FETs; the left one (aux smoke) measures .6 Kohm, but the right one (main smoke) is a dead short between pins 1 & 3. So I am inclined to conclude that the right FET is our smoking gun. Now if only I could find a replacement part number?!? Drat. No luck there so far.

Rod

Forgot to mention; I shamelessly lifted the basic outline drawing above from Bruk's wiring collection for the RCMC boards. Thanks Bruk!

Rod, I hate to break it to you, but the 8'pin parts are also FET's, and they're higher rating I would imagine.  I posted the part number of those, and I knew they were more than sufficient to handle the smoke element.

Oh sorry John, I had it confused, duh. When you referred to larger FET's I thought you meant the large 3 pin guys up near the 8 & 7 pin headers; not the lower 8SOIC packages. Anyway, all good now, we are on the same page. And it looks like I found my smoking gun.

Rod

@Rod Stewart posted:

These look way different than those in the picture Bruk posted earlier; I think Bruk's is a newer board.

You are correct, I forgot that loco had the earlier revision without temp sensors

Wow, right from the horse's mouth, thanks Jon!

@SantaFeFan posted:

I remember this design, 1st Gen RCMC with fan stall feedback sensing!   I remember it was a NPN transistor, used as an emitter follower.  This allowed the transistor to control the fan, as well as being a voltage regulator for the RAW DC - setting the fan voltage to ~4.3v!  I searched Digikey for a package that would fit the RCMC (SOT89), so I am posting what I found - it should work, at least worth a try.  Good luck.

ZXTN25100DZTA

TRANS NPN 100V 2.5A SOT89

Thanks very much Jon; much appreciated. And here I thought all along it was some kind of a mosfet! OK I'll add a few of those to my Digikey order and give it a whirl. I figured it was an SOT-89 package; that was the only pattern that had the correct dimensions. Glad to see I got that part right at least. The 2.5 amps looks like plenty of beef.

Rod

Good thing you did not try a mosfet, about 26v would have been put on the fan motor.  On the upside, you would have had good smoke, albeit for a sort time heh

Haha Jon, good point. That would not have been fun.

I skimmed through the datasheet for that transistor and I saw that the case marking is 1K9. This just happens to be exactly what is etched on these on the RCMC.

Rod

Might just be the right part.

UPDATE; Parts finally received from Digikey. Repairs being attempted. Progress (hopefully) TBA.

Rod

Any smoke yet?   How about Magic Smoke?

No smoke of any kind just yet. The wife had other plans for my day.

That happens a lot here.

Well after a 2 day hiatus (due to conditions outside my control haha); SUCCESS! Yahoo, thar she blows! Nice to have the old girl working again, hopefully for a good long time. Checkout the video clip.

Happy endings are good. I want to thank the entire cast including GGG, Bruk, grj, and Jon Z. for all the help. Couldn't have done it without you guys! Oh, and FedEx for finally delivering the Digikey parts haha! All good.

Rod

That's great, always nice when you can rescue one of these boards that would otherwise go to a landfill.

This thread has come in very handy!  I have a Centipede on the bench with a bad smoke motor drive.  I think I tracked it down to some sort of short in the cable to the smoke unit.  Since the Centipede has two identical smoke units, I swapped them and the problem didn't move with the smoke unit, so I figured it wasn't the smoke fan.  I checked the 4-pin cable from the problem smoke unit and no grounds and the wires go through end to end.  So, I figured I'd just swap the two smoke connections at the RCMC, BAD IDEA!  ZAP, it took out the other fan drive!  Apparently, there is some flaw in the long cable to the front, looks ugly to dig it out and replace it, but apparently that's going to have to happen!  I'm not sure exactly what it could be since it has continuity end to end and at least it's not shorted to ground.  However, knowing what part I need to fix the RCMC is a good thing, especially since Lionel parts is closed and I couldn't get a new RCMC if I wanted to!