If you don't get a definitive answer,

go after the electrical path with an ohm meter or a battery with an LED and a 1K or 2.2K resistor in line.  You are on the right track I suspect.  I like Kapton tape as an insulator.

Yes

Your plan will work.  The one caveat is you will only be picking up power from two wheels on each truck, so the model will be very sensitive to dirty track, etc.  I strongly recommend adding a keep alive to your decoder.

A note about Sperry cars, since I've watched them work and had to deal with the results, they do move rather slowly when they are testing the rail, and they stop often to double check, but they will easily do track speed when moving between test sites.

Scott Chatfield 

To further Scotts thought

that I agree with.  Maybe a Decoder Buddy Mini and a 21-pin decoder will work well to easily attach the stay alive (at the very least.)

I doubt that you can isolate

I doubt that you can isolate the motor , Looking at the availble information , the shaft and gearing make it impractical if not impossible. To get isolation you would have to have either plastic gears or both brushes isolated.

You really need to isolate the brush on the motor if possible. I can't tell if thats doable from the photo you posted .

The only other alternative is to remove the motor/truck and replace with something like a Stanton drive.

Isolation?

  I'm not sure as all my DCC engines have had can motors but it seems that you could just isolate the motor brushes ( if possible) instead of the whole motor?  From my experience these older brass engines didn't typically run well on DC when new so a motor or drive replacement might be desired anyway....DaveB

keepalive

@Scott:

That's a good point about the keepalive, especially since this unit also does not have a flywheel. Although I suppose a flywheel doesn't help much at typical rail car speeds. I guess I've only ever seen Sperry at work: I've never seen one go faster than a brisk walking pace. But considering that they have more or less a Galloping Goose engine inside and probably do not weigh terribly more, it's reasonable to believe that they could go about as quickly.

this one's not so bad

That's generally true, but this one works pretty well on straight DC now that I've cleaned the wheels. It arrived with good lubrication, likely fairly fresh, so I think it had some work from a previous owner. It definitely wasn't in a box for 50 years, that's for sure. (I have a couple of those, I know what they look like!)

argh

I guess you're probably right. The worm is metal, and so are the other gears. It's on a metal shaft.

How would that be done?

isolating the brush

From this pic it looks like maybe all I need to do is to clean up the solder on this brush contact?

You just unsolder the the

You just un-solder the  brush lug from the frame, and run a wire, ITS ALREADY ISOLATED !

I love simple

That's a whole lot better than the stuff I was planning to do! :yay:

If you have a multimeter I

If you have a multimeter I would verify the isolation and  would measure the stall / lock rotor current on the motor and size my receiver accordingly.

isolation

I guess I want to verify, for example, that after the unsoldering, the brush contact is isolated from the frame? Anything else?

Brush isolation

It's not the motor that has to be isolated, but the brushes themselves. Often they cheated by simply bending the tab over and soldering it to the motor frame instead of using a wire.

As your picture shows, that is exactly what they did during manufacture. So just unsolder it, and verify the connection between it and the frame has been broken with an ohmmeter.

You' have to unsolder the other wire anyway, as it is the connection to the rail that is needed by a decoder. 

isolation?

  I'd check it with a cheap decoder for a while before adding a sound or other more expensive decoder. :> )  Does anyone know how an all metal gear train affects this?   .....DaveB

Does anyone know how an all

You will need no sound decoder. These type of gears shown in the pictures are not the smoothest running ones. Especially the worm and wormgear are notorious for high sliding friction and for this they need a rather high torque input to be moved. High torque also means a certain amount of power output at the motor shaft. And to make things still more worse, the chain of spur gears to the second driven axle is also rather power consuming.

On the other end you have this antique motor with it's over the decades decreased permanent magnet force. The motor will turn and power the car, no question, if you feed it with enough Volts and Amperes; your DCC decoder should be able to deliver this in adequate quantities. But do not run too long a time this to DCC converted model or you will learn to distinguish the differences of smelling between electric smoke and electronic smoke.

No kidding, you have a rather brute amount of electrical power putting into this motor to get the model moving. High power input at the brushes, but low power output at the motor shaft is the effect. And the rest ot the electrical demand? This will be converted, corresponding to the law of conservation of energy, into unwanted and useless heat. In extreme case it will be fascinating to obseve which of the devices will produce more heat, the decoder or the motor? 

Lutz

Brush Isolation:

I did a similar rework of a twin-motor PRR DD1 and isolating the brushes for DCC was as simple as slipping the red fiber tubes over the brush springs as shown:

The insulating sleeves were some kind of resin I found on an old transformer. Don't be tempted to use PVC wire insulation which will wear through. In your case you have fiber washers to act as the insulators.

You can see where I soldered the orange and gray decoder wires to each individual brush.

Good Luck, Ed

more specifically...

@rasselmag

I understand the general warning.

However, this car runs easily at 5v. Despite its ancient gears it is comparable in sound to more modern (1980s) brass, although clearly not of the quality of implementation seen in later (late 80s, 90s) brass or similar to modern plastic. I haven't measured the current but given that it runs at the drop of a hat, and it's supposed to run slowly with no train, I am very skeptical that it's going to pull even 0.8A. That's what physically heavy locos (BLI, Proto1000) with can motors are pulling up steep grades with long trains (25+ cars). We do that all the time, without blowing decoders, so I will be very surprised if this decoder gets cooked. The decoder in question is a TCS MC2, rated for 1A continuous and 2A surge. Given the comments about pickup, I might swap it for a KAM4, which has the same rating but has a keepalive.

Besides, there are a fair number of old brass locos that have been converted to run DCC with their old open frame motors. Since the brass vendors seem to have moved to installing can motors in the late 70s or early 80s, the open frame ones are mostly 50+ years old. Given that most are actually locos and actually pull trains, I would think that they're significantly more likely than my little Sperry to pop a decoder. Yet I don't seem to hear a chorus of "don't convert those, they'll never survive." Maybe they're putting O-scale decoders in them or something.

@ CandOfan

https://abload.de/img/dsc09034kxill.jpg

https://abload.de/img/dsc09040kfiqe.jpg

For me, i have given up to sugarcoat these old open frame motors. These Pittman or Pittman clones were invented in the 1930's as the then dernier cri to replace the clockwork drive in the locos. For the pictured example above, the speed steps for this streetcar (it has to pull nothing of course) were: fast, faster, still faster und even faster. Attempts to run it at slow speeds resulted in jerky, unreliable and often stalling.

After conversion the amperage dropped dramatically about 90% and the noise level reduces still more dramatical about 90% of the initial level with the old motor. The metamorphosis from an careening rattle can to an smooth, nearly silent and slowly running streetcar model was astonishing.

At least it's your decision what you want. For me, i replace these old rattle motors with modern much better running ones. And i use wireless cell phones instead of wall mounted telephones with crank.

Lutz

one thing at a time

I measured the current draw of the rail car last night. It pulls 310ma when it's climbing a 4% grade and 230ma on straight, level track. I think it will be safe to convert and run it. It does run a bit more speedily than ideal, although it's not horrible either.

I will be leaving the open frame motor and gears alone for now, as a matter of prioritization. I have a substantial fleet of older locos to convert to DCC and sound. Most of them have can motors, but a couple have open frame motors and this experience will obviously help me do those. Several of them (some with can motors, some not) have horrible gear trains that are far more urgently needing regearing than this one. One day I'll get around to remotoring and maybe even regearing this one, but in the meantime it'll be happily running and getting in the dispatcher's way...

A successful remotoring/regearing will likely be an even bigger headache for the dispatcher!