MicroTrains F7

Overview:

A great workhorse, albiet of very early mechanical design. The locomotive is nice and heavy for good pulling power, although the newer ones are now no longer lead but zinc and are lighter.

The basic system is an open frame motor in a split chassis. There is a "cup gear" reduction from a small gear on each end of the motor, and then worms to drive the truck assemblies, which have a worm gear and then spur gears to drive the axles.

They can be a bit touchy to rebuild, but most of it is getting the parts in place properly and monkeying with the tightness of the 4 screws that hold the unit together.

Power pickup is just ok when everything is clean and adjusted properly, but can be improved with "wheel wipers".

Chassis Variations / Versions:

Note, there are several versions of the main castings. I've counted 3 so far.

Oldest are lead, 2 nakes chassis halves weigh 45 grams. The thickness of the "fuel tank" area (below the motor) is 0.203 to 0.207"

Newer are lead, 2 chassis halves weight xx grams. The thickness of the "fuel tank" area is 0.177 to 0.178". These are of course lighter.

The latest are apparently zinc, and are much lighter. They are all blackened, even inside, and weigh 30 grams. I suspect they are zinc.

After machining for DCC, the lead chassis weigh 40 grams.

Some MT F7 tips from Glen Chenier:

The F7 loco mechanism should be lubricated and free running (worm gears trimmed), motors must be in good working order.  After long use brush debris collects in the commutator slots and can make the motors draw excess current and run hot.  There is an access hole in the later versions of motors which allows removal of this debris with a needle.  Have seen this make a huge difference in motor heating.

The MTL motor is about 12 ohms resistance. If low frequency pulse is used the motor current is no more than 0.55 amp peak, but the DZ123 runs at 15 KHz pulse so peak motor current is even lower, motor inductance averages the current and prevents peak current from being reached.  Decoders with low frequency pulse drive will source peak motor current according to (track voltage - 1.4) divided by 12 ohms and will run hotter.

The MTL F-7 will handle 145mm radius curves but suggest some things to help - trim the corners of the trucks so they do not snag in the
gap between the chassis frames at extremes of travel, trim the inside of the front apron to clear the truck swing in extreme positions of
forward and reverse front/rear movement, and with a SLIGHTLY WARM soldering iron soften the plastic of the body shell steps (ladders)
and bend them outwards a little bit to avoid interefeence with the truck swing.  The best way to do this is to line up the trucks on a
single section of Marklin 145mmm radius track held in your hand and inspect all these points from underneath for possible interference,
adjust as required.

More information from Geoff & Bill & Jeffrey MacHan on cleaning and improving MT F Units:

If you take a MT apart, one strong suggestion: don't mix and match gears on a loco that's been run. The gears take a "set" very quickly, and mixing them up is not a good thing to do. Be certain the axles go back in the same truck and in the same left/right orientation.

Most common problems found?

1. Dirty wheels
2. Poor connection between axles and truck frame
3. If the loco has ever been disassembled by the owner, problems with the cat's whiskers and the truck
4. If the truck has ever been disassembled by the owner, the challenge of finding which axle goes in which truck, and left/right orientation

Part of the power pickup chain are the 4 "swords" that are made of copper (I believe). They press into the chassis halves and touch the sides of the gear tower on the trucks. One of the most common problems, and it has been known to happen with units that were assembled at the factory, not just the units that have been opened by customers, is that the "sword" will actually work its way under the gear tower contact. This can cause the "sword" to get somewhat bent out of shape and generally wreak havoc with the electrical conductivity. A normal symptom of this is if the truck will conduct power when it's turned one way but not the other and frequently not when it's just straight.

When cleaning MT F7 wheels using an MTL speedidriver brass brush, if you notice that the brass screws are conducting but that the wheels are not, then this is a sure sign of poor electrical conductivity between the wheel collar and the truck sideframe. Remove the wheels and give them and the truck well a good cleaning with rubbing alcohol to remove the crud they have collected.

The number one way to kill any engine is over oiling and general poor maintenance. Modern loco offerings now use a high performance "grease" that stays put for maximum coverage and performance. It should be known that there is a period where out of the box an MTL GP-35/GP-9/SD-40 will run perfectly, haul your 25-60 car freight train and then suddenly have slipping issues. This basically happens at about 20 hours of operation and can vary of course somewhat from loco to loco. Kinda twilight zone anomalies that come out of nowhere. Inspect your trucks, clean all of the wheels on the loco including the flange where it makes rail contact. Remove the two brass screws that hold the truck frame in and inspect the gearing for dirt/debris/proper lubrication. Also, Inspect your wheels gauging and put back on the track and it will be "broken in". There is nothing wrong with the loco, it is just a break in and cleaning issue.

Many times I get a loco that is handed to me with the it runs slow syndrome. If you mix oil types and get debris in your trucks or internally to the worm drives........usually a super fine piece of lint/hair.........it slows things down. Usually the owner decides to take apart the chassis to inspect and or clean. Here is where everybody goes wrong. One, the pins for each bushing that supports the worm axle have to fit into the chassis halve and not inadvertently get slipped as the spacer between chassis halves. An easy oversight. The chassis halves snap fit clean and snug when all parts are in the correct alignment. A big problem I see when I get a loco from someone who has attempted to clean or DCC themselves and then run into operational problems is that they over tighten the chassis halves screws and strip out the plastic. The plastic spacers which prevent shorting of the two chassis halves are going to fit cleanly or not at all. Same goes for the worm axle pin guides. I put a red "dot" of paint on the black screw heads for each of the six chassis halves screws so I know how many times it gets turned. Still, by simple "feel", you know when to stop before stripping out the screws spacer.

All of that said, over tightening of the chassis halves including improper seating of the worm axle pins causes the undue pressure on the internal drive components to slow your engine/motor down.

Maintenance / overhaul:

An F7 overhaul and DCC decoder installation procedure is on the yahoo Z_scale forum under Files / DCC / DCC_and_Overhaul_MTL_F7.doc, courtesy of Glen Chenier. The decoder install is for old decoders that are rarely used today, but the instructions on working on the unit are valuable. This article also describes how to prepare B unit shells to fit onto the powered mechanism from an A unit. 

There's an older copy of this document/procedure here: http://www.wiringfordcc.com/zmtf7.htm

Hard to improve on, but some notes here:

First, on removing the shell, I found a simple way is to take 2 X-Acto blades (not in the handles) and push them in between the fuel tank and the shell. Once in, then the shell is pushed away just enough to release the lugs on the side of the fuel tank from the shell. The chassis just drops right out, and you don't run the risk of breaking anything.

When pulling apart, notice the brushes are towards the rear of the loco, the end with the power pickup wires.

The back of the chassis can be identified by the fact that there is a "notch" in the very lower edge of the chassis at that end. That is clearance the legs of the "noise" capacitor and spacer.

There are 2 screws on each side that hold the chassis together. The 4 screws attach to plastic spacers that hold the 2 chassis halves separate, physically and electrically. Pull the screws on one side off and remove that chassis half.

The trucks can now be lifted from the chassis. You can lift the motor and the gears out. Notice on the motor that there are 2 plastic end pieces, and the beveled part points down. It's easy for these to spin around and you won't be able to reassemble the loco if they are wrong.

Also notice the plastic bushings on the ends of the shafts with the cup gears and worms. Notice that the tab on this bushing is towards the outside end of the loco.

There are 3 plastic spacers, in the chassis, normally on the fireman's side, to keep the chassis halves from touching.

Improving power pickup:

Instructions for installation of Wheelwipers and LED Lights used to be at: http://www.teetertottertreestuff.com/ unfortunately, that site is offline.

I'm working on publishing this somewhere, but need permission first.

Spare / replacement part numbers:

Part number for MT F unit brushes and springs, 140002

Installing DCC in a MT F7

A copy of Glen Chenier's article on the wiring for DCC web site:  http://www.wiringfordcc.com/zmtf7.htm

Glen comments on this:

A more recent version with better chassis cut description and better decoder test procedure is on the yahoo z_scale list under Files > DCC.  This is still outdated, the newer DZ143 pcb layout and DZ125 require the right chassis cutting to slope deeper downward around the worm cavity to clear the tallest component.  Will have to look into the TCS Z2.  Anything that raises the shell even slightly can cause the front coupler housing to touch the edge of the shell opening.

A gotcha is if one cuts too deep into the bellgear cavity the wires will rub on the exposed bellgear.

Another way to isolate the motor terminals from the chassis is to leave them uncut but slip a rectangle of overhead projector transparency film between the chassis and terminals AFTER soldering.  Does the same thing, but now the motor terminals remain intact in case you ever want to go back to the original.

Some notes from Lindley Ruddick on his installation:

Isolation of the motor from the frame is no big deal when you trim the brush tabs. I used a jeweler's saw/razor saw/Zona saw to notch the frame on the end away from the cab and did the finish work with a file.

I used a Digitrax DZ123/125 as that is what I had and that is what Glen used. If I were to do it again, I would seriously look at the TCS Z2 which was not available when I did mine.
I have done 2 with no problems and plan to do a third. I would say it took me 3 hours to do the work (including bathroom breaks, coffee breaks and TV breaks). Certainly achievable in one evening or afternoon.

My MTL F7's all run better in one direction than the other so I attempted to orient the frame to run forward in the leading unit and backwards in the trailing unit. If you are not careful when you solder the wires to the brushes and reinstall the motor you might end up with "reversed" rotations. I am pretty sure you can "cure" this by changing the CV's for "forward direction" and lighting effects.

My take on decoder installation:

There's a certain level of difficulty doing this, you need to be confident you can cut away carefully and not chop into the journal areas for the worm gears and the motor. The chassis halves will be very thin there.

You also need to have a very fine soldering iron, and be familiar with not shorting motor brushes to the chassis, a sure way to kill a decoder.

Reassembling the chassis and getting the bearings for the worms aligned right is often the biggest problem for newcomers.

There are also a few other things to be worked out, like clearance for wiring between the shell and the chassis, how to wire up the headlight, and optionally number boards.

Setting up the "second headlight" and a backup light would be cool.

I'm trying the TCS decoder here, so I have the extra functions.

Advanced F7 tips