Assistance with early Max Gray K4 motor

That is a bridge rectifier, hooked up so you can run DC.  Sure, try it.  It will probably run quite well.   Then replace it with a good can motor.  Be sure the mechanism is free before trying to run it with a motor.

I have no experience with an MG motor that old.    The newer permag motors that max grey used have a reputation for being slow, ie low rpm at 12 v.    Or at least lower than many people like.   I have had this experience with a few MG locos and I have replaced the motors with newer ones.  

However, if this runs at a speed you like, I would say stick with it.  

I put 12 vdc on the contacts and got nothing. My only power source is a 4 amp battery charger. The ammeter gave a slight flicker when I applied the voltage but remained at zero. There was no measurable voltage across either the brushes or field coil.

Can anyone direct me to a schematic or explanation as to how this should work? My preference is to keep the locomotive original - I don't have a layout, but would like it to run. A replacement motor would be a last resort.

Jim

Sure.  Later.  Might want to check solder joints - and never hook a battery up without a fuse or circuit breaker. You may have burned something up.

I found a wiring diagram for a Scale Craft universal motor on an OO site. My motor is wired the same way. I did check all the connections; they are secure and I have continuity through all the wires. There is continuity through the field coil as well.

The motor ran for an instant when I connected the field wires to the brush wires then applied power. The rectifier wiring was left in place. I think the rectifier is bad. I've scoured the internet to find another but can't even find a picture. The OO site shows a modern substitute but gives no information as to sizing. The original was called a "stack rectifier".

Here is the substitute rectifier and the supplied wiring diagram.

I just did a sketch too.  Find a Radio Shack bridge rectifier - from memory 50v1A will do it.  It will be marked +/- and AC or squiggles, like my sketch.  Under $5.

Also forgot - battery chargers make lousy train transformers.  You may have blown out the regulator circuit in the charger.

Thanks for your help Bob. A replacement rectifier should fit in the boiler above the original, which I will leave in place for future generations to wonder about.

The other drawback of the original motor is it will draw up to 3 times the amperage (around 5 amps) of a can motor.  The reason for this is there are no magnets in the original motor. That's the reason a rectifier is needed.

I went through this with my first Lobaugh Berkshire. The  stock motor overheated a 350 watt bridge rectifier!

350 watts at 12 volts is almost 30 amps.  I have a number of these motors using the smaller bridge rectifiers.  Six amps at 12 volts is 60 Watts.

But yeah - they are dirt cheap.  Buy the biggest one that fits.