# MTH F3/F7 shell removal issue...



## Krieglok (Sep 7, 2013)

I just bought a ABA set of MTH F3/F7 Diesels. They are mystery engines, as I don't have a set number for them, nor can I find them at the MTH website.

They are PS2 with 3 volt boards. 

Anyway, I opened the "master" A unit to check the battery. Four screws out and the chassis and body separated easily. 

Now the secondary A unit, also powered, has a defective smoke unit and the red markers don't light.

I removed the four screws and the front of the chassis was slightly loose. It seems there is something holding the chassis and shell together. I did some light prying at the rear end, but no go. I didn't want to break anything, so I put the four screws back in for now.

Is there something I am missing? The manual shows only four screws on all the units, but the trailing A unit will not come apart.

Any ideas?

Tom


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## cole226 (Feb 8, 2013)

maybe a stray wire hung on something on shell. :dunno:


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## Krieglok (Sep 7, 2013)

Hmm...it seems much more solid than that. There is no give at all on the back end with all four screws out. I looked in the end with a flash light and couldn't see anything that might be holding it in there, unless there is something up top, above the rear motor...

Thanks for the suggestion though, Randy.

Tom


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## Lehigh74 (Sep 25, 2015)

I have a few engines that don’t want to separate even after all the body mount screws are removed. The shell is too tight on the chassis. I stick a penknife between the shell and chassis and pry a little to free things up.

You might also want to remove the fuel tank. I have a few engines that have half of their fuel tank screws double as body mount screws.


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## Krieglok (Sep 7, 2013)

Thanks Bob. I did remove the fuel tank...all short screws. No luck. 

It does seem the car body sides are tight, but I tried the light prying with no luck. Maybe I will try with something a bit flatter than a screw driver...

Tom


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## Krieglok (Sep 7, 2013)

I put the problem "A" unit away for a while, then I took it back down again and went to my work bench. I pulled all the screws, removed the fuel tank and truck frames to get a better look along the edge of the chassis where it met the car body shell.

Seeing nothing, I took a small flat screw driver and levered against the chassis at the rear of the unit. It slowly started to give and finally popped loose.

Why was it stuck? It seems the smoke unit suffered severe overheating and actually had melted to the shell where the smoke chimney meets the carbody. The chimney looked fine except the hole where the fluid runs down and the smoke comes up, was sealed shut with melted plastic. I can probably drill it out. It almost seems it was a faulty plastic casting that didn't allow the hole to be formed in the molding process.

Here is the unit. The resistors where so charred, they disintegrated when I touched them. I figure I can get the smoke feature working if I can get a new smoke unit top PC board. The tank and hood are salvageable and the motor seems okay...

I removed the whole smoke unit for now...

Tom


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## gunrunnerjohn (Nov 10, 2010)

The smoke control FET has obviously shorted and put full voltage on the board. The top board is available for the smoke unit from MTH, and obviously the full unit as well. Your slave board also needs repair it would seem.


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## Krieglok (Sep 7, 2013)

Due to the seller(and me) not knowing the model number, could I use a smoke unit from any F unit that MTH ? Does MTH sell replacement slave boards? The unit seems to run fine when teathered to the master and B unit....

Tom


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## gunrunnerjohn (Nov 10, 2010)

There are two standard MTH smoke units for O-gauge, one with short posts and one with long posts.

AA-0000070 short cast mounting posts (steam)
AA-2100000 long cast mounting posts (diesel)

There are other less common smoke units, but those are the common ones.


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## Krieglok (Sep 7, 2013)

Okay, thanks John. I am curious to see if I need a new FET board too...

Tom


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## gunrunnerjohn (Nov 10, 2010)

That's the slave board, and it does indeed work in both 3V and 5V systems, no difference in the slave operation. See if fixing the smoke unit solves the problem first.


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## Krieglok (Sep 7, 2013)

Okay. Thanks John. I am hoping it is the case where I will not need the new board.

I have a little rebuilding to complete on the smoke chimney/ funnel. The old brass cup from the burned up smoke unit is melted right into the plastic chimney. That was where the carbody hung up and wouldn't separate. 

When I finally pulled it apart, the threaded collar from the smoke unit PC board came out and stayed on the cup. Pretty solid failure...

Tom


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## gunrunnerjohn (Nov 10, 2010)

Sounds like it got pretty hot.


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## Krieglok (Sep 7, 2013)

I am surprised it didn't melt the shell.

It just melted the plastic chimney (funnel, as MTH calls it) but I may be able to repair that too. MTH is out of stock with those...

Btw, what should the voltage be on the wires running from the FET board to the smoke unit resistors? Is that voltage variable since the DCS allows for adjustment of smoke levels?

Tom


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## gunrunnerjohn (Nov 10, 2010)

Yep, it varies, and it's also a PWM signal, so you won't be able to measure it using conventional meters.


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## Krieglok (Sep 7, 2013)

Not even with my cheapie Chinese meter? Lol. 

We will just see if the unit works without melting down. That will be the ultimate test....

Tom


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## Krieglok (Sep 7, 2013)

New smoke unit arrived today. I had to transfer the wire harness from the old one to the new one. The old unit was slightly different than the new unit, but it looked like it wired up the same.

Well, the fan motor runs, but no smoke. I ran it with the shell off and the new unit didn't even get warm. I am going to recheck connections for continuity but I think the board is bad.

One question, does it make a difference which wire goes to each end of the resistors? I was going to try reversing the resistor leads if that might make a difference...

Tom


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## gunrunnerjohn (Nov 10, 2010)

The polarity of the smoke resistor is not significant, apparently you have an issue with the board.


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## Krieglok (Sep 7, 2013)

Okay. Of course the spare board I should have bought was bought by someone else...lol.

I read up on resistors and found out just what you said. I also check the continuity of the circuits from the resistors to the board and they are physically sound. 

Like you say...a bad board. Know any sources for a board not made anymore or some sort of conversion that would work?

Tom


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## gunrunnerjohn (Nov 10, 2010)

What exact board are we talking about that's not available?

Dumb question, does the smoke on the master work?


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## Krieglok (Sep 7, 2013)

The PS2 3/5v slave unit board. 

The smoke unit on the lead unit does work...

Tom


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## gunrunnerjohn (Nov 10, 2010)

I recently bought a slave board from MTH, I'm surprised they're saying it's not available. I have one here that when I get a chance I'm going to repair, it has motor drive issues. They are fairly simple boards, and since I spent the money for the special test harness for them, I might as well use it.


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## Krieglok (Sep 7, 2013)

I didn't know MTH still made them. Being unfamiliar with the nomenclature of the part makes it somewhat difficult to pick it out of a parts listing. 

I guess knowing the name and part number is something that would help! Lol.

I don't know if it better to have the existing board/locomotive tested for a possible minor defect or just swap out the board all together...

The red marker led lights do not function either. Possibly related to the board issues. I was able to get the other end, green A unit markers working as it was a simple connection issue...

Tom


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## gunrunnerjohn (Nov 10, 2010)

Probably should have the slave board tested. I'm not in a position to do it right now, but other MTH techs should have the proper harness for testing it.


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## Krieglok (Sep 7, 2013)

Okay, thanks John.

Tom


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## Krieglok (Sep 7, 2013)

I replaced the trailing unit board with a new (used one) and the smoke now works as intended. 

Couldn't get the marker lights to illuminate though...

Tom


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## gunrunnerjohn (Nov 10, 2010)

One down and one to go.  Don't know what the deal is with the markers, it's kinda' one of those things that's easy to diagnose in person, but not so easy in ASCII.


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## Krieglok (Sep 7, 2013)

The front (green) markers didn't work either. It was a connection issue with the spring contact between the body and the chassis.

No such luck with the rear, red ones. The LEDs tested fine so it must be the connection to the board, between the spring pad and the connector...

I also noticed that the front motor on the trailing unit is a bit slow to start turning when power is applied, compared to the rear motor. It may be an issue with this "new" board I installed...

Tom


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## gunrunnerjohn (Nov 10, 2010)

The motor "issue" is probably a non-issue if you're observing these on the bench on rollers or suspended. The two motors almost never start at the same time, simply due to small differences in friction and motor characteristics.

I'd break out the meter for the markers, doubtless something not connected.


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## Krieglok (Sep 7, 2013)

Thanks for the input about the motors. It didn't seem bad enough to warrant concern, but your confirmation definitly makes me feel better! 

I noticed too, the number Board lights don't work at either end of the consist(both A locomotives). They seem to be the same type as the headlight. What would be a replacement number for these? It isn't a big deal though, as I never noticed they were dark until I powered up the unit with the body shell removed.

Finally, I added some lifting lugs to the nose of each A unit. It gives them that final Pennsy look....

PS: here is the original problem board with the burned up smoke unit...cooked.

Tom


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## gunrunnerjohn (Nov 10, 2010)

That's WELL done!


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## Krieglok (Sep 7, 2013)

Beyond repair I guess...lol

Tom


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## gunrunnerjohn (Nov 10, 2010)

Once the PCB is charred and the traces lifting, I don't try unless it's a "one of a kind" board that I can't replace.


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## Krieglok (Sep 7, 2013)

Off to the compost heap then.....

Tom


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