Many thanks to Randy and the others who replied to my question. I am glad to be able to report that I have finished a successful install of the Tsnaumi TSU-750, following Jane's Dec. 2010 MR article. I am using the NCE Power Cab paddle controller on my small, one-engine L-shaped switching layout. I isolated the motor of my Atlas S4 by replacing the metal machine screw holding the motor to the frame of the engine, with a nylon screw. I inserted a piece of 0.05 inch thick acrylic sheet between the motor and the frame. Note that the lower brush of the motor is still electrically connected to the frame of the motor in this scheme.
The built-in ammeter in the NCE Power Cab seldom reads more than .11 amps, in the slow speed way that I run this engine. (I do not really understand just how NCE measures the current.) I am having a lot of fun running my layout with good sound.
Harold Leinbach
Since I don;t have any of the Atlas units I was going by the photos, and sometimes it's hard to tell what's metal and what's palstic, especially when it's all black and hidden in the shadows. The Protos, it's pretty obvious the end piece is plastic since it's brightly colored. The Atlas S-1 looks the same in pictures I have seen - totally different from the S-2.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
Randy:
Your suppositions are correct. It has been a while since I did the installs on my S2s and I went back to my notes to refresh my memory. The lower motor brush is connected to the motor frame and the screw that holds the weight on serves to connect it to the truck leads via the light board of top of the motor. When you remove the weight and light board as in this case you can use the screw directly into the motor to make the contact. That appears to be what the author did. If the motor is isolated from the frame via tape and the plastic screw the problem of frame shorting should be eliminated (or so we hope).
Joe
That one of themore confusing installs ont he TCS site - it doesn't mention a thing about the orange wire. It appears thatit gets attached to the screw tha tholds the top weight ot the motor? So I assume the bottom motor brush os connected tot he metal frame of the motor. In the magazine install, the weight was removed because he was installing a sound decoder and speaker, which needs a lot more room than the M2 decoder pictured. It also looks liek he conencted the orange wire to the top motor brush and the grey wire to the screw that used to hold the weight on - probably ran backwards then. It would appear that he is trusting that there will never be contact between the track and the frame, or that expensive Micro Tsunami is going to go up in smoke.
Look at this installation: http://www.tcsdcc.com/Customer_Content/Installation_Pictures/HO_Scale/Atlas/S2/Atlas%20S2.htm. It shows the steps you need to follow with this engine including the plastic screw.
Is the brush mount end of the motor all metal? In the pictures in the article it looks like it's plastic, just like the P2K motor. The MOTOR does not have to be isolated from the chassis, just the brushes.
Everything Randy has said is correct but on the Atlas S2 you need to replace the metal motor mounting screw with a plastic one. The metal screw is metric but you can use a nylon 2/56 screw. I've done this on two S2s and it works very well.
The innards of the Atlas models are almost identical to the Proto 2000 versions of the same locos. In the Proto versions, it is entirely possible for a wheel to bride the space between the rail and the frame int he event of a derailment - instantly frying the decoder if the one motor brush is conencted to teh frame, as it is in the Proto model, and the Atlas model. For decoder installs int he Proto models I remove the motor (one screw on the bottom of the chassis), and clip off the 'tail' of the ring around the bototm brush holder, this tab is what contacts the frame. I remove the brush cap (careful not to tip the motor upside down, lose the spring, or let the brush fall out) and solder a new orange wire to the cap - the Proto brush caps are brass. I screw it back inthe motor, and put a layer of electrical tape under the space where the brush holder sits so it cannot short to the frame. From then on, proceed as the article says to connect the wires (although that bus bar idea is interesting, I don;t bother, I just solder the wires and use shrink tubing for insualtion - remember to put the piece of shrink tube on the wire BEFORE soldering them together!).
That, in a nutshell, is the PROPER way to install a decoder in those locos, so there is no chance of a short blowing the decoder.
I purchased my HO Atlas Alco S 2 in about 1995. I am about to convert this engine to DCC, using the SoundTraxx Tsunami TSU-750 decoder, following Don Jane's MR article. The admonition I find in this article and elsewhere "...is that the motor must be isolated from track power..". I note that the upper brush of the motor is completely isolated from the frame of the motor and the frame of the engine. The lower brush is press fit into bottom of the yoke of the motor, which is in turn bolted to the frame of the engine. These items thus become part of the return circuit of the original wiring of the power leads from the trucks. Am I correct in deducing that the two wires from the wipers on the right and left wheels of the trucks should go directly to the input of the TSU-750 decoder, and then one motor output wire from the TSU-750 go to the upper brush, and the other motor output wire go to an accessible point on the frame of the engine?
Thanks for your help.