post war 736 runs in one direction only

[bossman]

My 736 (early model) was running well forward and reverse. all of a sudden it would run forward. I cleaned motor; commutator , brushes and holder. The e-unit is working. I cleaned it also. All wiring is ok with good connections. The fingers are in contact with the drum and the drum is clean. The drum has to rotate one full turn before the engine runs. This was my Dads Berk and I would appreciate any help you can provide.

Robert

[cwburfle]

Based on your comment that the e-unit drum has to rotate one full turn in order for the engine to run indicates to me that likely cause is the e-unit. Odds are that some of the fingers are no longer making contact with the drum. I would guess that it is either the center two in the four contact group, or the two contacts that are in their own group.
The cause could be dirt or oxidation. The fingers do go bad occasionally. Drums too.

[Dave632]

 I had an e unit that the contacts on the drum had worn enough so that it no longer made contact.

The easiest repair is to replace the e unit. There are generally quite a few on e bay to purchase, just find the one for your engine from a reputable source. Check the sellers ratings for reliability.

 I have bought many a good product thru e bay but there were a few duds in the bunch.

[lionelsoni]

It's odd that the drum has to rotate a full turn, since the direction cycle repeats every half turn.  Are you sure about that?

It can be almost impossible to see whether the fingers are making contact with the drum.  Put the locomotive in what should be reverse and then poke each finger gently toward the drum with a toothpick, to see if the motor starts.  There is a way to restore the finger pressure without disassembling the e-unit.  If you're interested, I'll describe it for you.

[bossman]

Yeh, I thought it was odd too. But it takes eight on/off cycles before the

motor turns. I'll try a closer look at the fingers and try the toothpick.

The motor only runs in reverse. When I first noticed this problem it took

more and more voltage for the engine to run forward. But it ran fine in

reverse. Then it just quit running forward all together. I sprayed the

e-unit with contact cleaner and checked the finger pressure. They all seemed

to have good pressure on the drum. I know electronic problems can be hard to

find at times. I'd like to fix this e-unit because that's what I like to do,

fix things.  Guess I could just replace the e-unit, but that would be too

easy. Thanks for the info, Robert

[TrainLarry]

Although the drum may be clean, the copper contacts may be oxidized and causing problems. To properly clean the drum and contact fingers requires disassembling the e-unit. A spreader tool, part #ST-303 is needed. It would probably be better to replace the drum and contacts once you have the e-unit apart. Be forewarned, reassembling an e-unit is not for the faint of heart or mind. It takes a lot of patience and dexterity to say the least. The drum is part #259E-1 and the contacts are parts #100-45 and #100-46 available from online parts dealers such as The Train Tender or Brasseur.

Larry

[cwburfle]

There is a way to restore the finger pressure without disassembling the e-unit.  If you're interested, I'll describe it for you.

 

I am. There are always new tips and tricks to pick up.

[lionelsoni]

Make an L-shaped hook from a paperclip.  The bent part should be about as long as the width of an e-unit finger, about 1/16 inch.  Hook it behind the finger of interest, right up against the board that it is riveted to.  Push on the finger with a small screwdriver while pulling on the hook as needed to keep the finger flat against the board.  Push close to where the hook is, but a little farther away from the board and not right on top of the hook, to make a little bend in the finger, toward the drum.

For the board with two fingers, you can remove the protective cover by straightening one of the taps that hold it on.  You may also be able to work with the cover on by using a wider hook that can reach around the cover and by wedging the screwdriver between the finger and the cover, but I haven't tried that.

There is also a trick for getting a new life out of worn fingers.  The end of the finger is a semicircle, with a 90-degree bend connecting it to the finger.  Eventually, the metal wears through and half the semicircle falls off.  You can flatten that 90-degree bend to 45 degrees, thus rotating a new part of the remaining quarter-circle into contact with the drum and not too far from the position where the semicircle was.  I like to compare this with machining train wheels to restore the tread profile for a second life--the process indicated by the "2 W" ("2 wear") marking common on the ends of cars.

[cwburfle]

Thanks! I will give the paper clip trick a try.

[bossman]

After all the great input from the forum I ended up replacing the e-unit. Runs great now. There was arcing between the fingers and drum and I could not stop it. I had a spare e-unit but the on/off lever was 180 out. Repositioned it, soldered it in and all is good. I want to thank one and all for the great info. I learned some new info that I can use in the future. Bossman