Wednesday, May 24, 2017

Scratch Building a Gas Mechanical Critter - Part 3

As I explained in my two earlier posts, I am building a gas mechanical "critter" to fulfill one of the requirements for the NMRA's AP Certificate in Motive Power.   To qualify, one must scratch build a self propelled engine capable of running under its own power on track of the appropriate gauge. I opted to build my HO gauge engine on a brass underframe with a styrene superstructure.  The model is based on an S gauge critter I found on Rail Line Forums, which appears to have been built by a Japanese modeler.  Here is a photo of the prototype engine that caught my eye:

And here is what my critter looked like at the conclusion of my last installment:

At this point, I was ready to try running the critter on DC to see if everything worked properly. While the little switcher ran, it was so light (less than 3 ounces) that operation was by fits and starts.  I had already decided to install a Tsunami 2 sound decoder in the cab with a Soundtraxx Current Keeper for better running.  But I felt it still needed more weight.

The most obvious place to add weight was the deck.  I ordered a square foot of 1/24" lead sheet from Roto Metals.  After cutting out a piece of lead the same size as the styrene deck, I used the .100 grooves on the Evergreen decking as a guide to mark the boards on the lead sheet.  I used a scriber from Micro Mark to make the lines on the lead.  It was really very easy to do.  Lead that thin is easy to work with.  Here is a comparison with the old styrene decking and the new metal deck.  The lead has yet to be painted.  I weighed the critter on a digital scale and found that I had added more than an ounce.  For additional weight I added strips of lead sheet under the deck as well.

A few external details remained to be added to the critter before painting.  I fashioned a pair of air tanks out of 1/8" styrene rod, adding piping made from brass wire.  Additional wire grab irons and ladders were added to the frame and superstructure,  Commercial brass castings were added for the bell and whistle.  Here it is ready for painting.

I opted to paint the cab and motor housing with Model Master Navy Aggressor Gray.  The lead deck was painted with a Rust-Oleum Roof Accessory Paint called "Weathered Wood", the same color used on the original styrene deck.  The stack was painted grimy black, the bell and whistle were touched up with Floquil brass.  Here is what the critter looked like after decaling the number 7 on the cab and end beams.

With the exterior more or less finished (except for weathering) I turned my attention to the decoder installation.  As mentioned above, I decided to use a Soundtraxx Tsunami 2 decoder.  Their Baldwin Diesel sound decoder includes the motor, bell and horn sounds for an RGS Galloping Goose, which I felt would be appropriate for something as peculiar as my little critter.  Here is the decoder with a Current Keeper.  These new Tsunamis are really small!

 There was just enough space in the cab and motor housing for both.

It's a snug fit, but the cab and motor housing fit neatly over the decoder and keep alive.  Not shown in this photo is the "sugar cube" speaker.  Originally, I was going to place the speaker on the underside of the cab roof, but it was still visible through the windows.  Fortunately, the speaker and its baffle just fit on top of the Current Keeper, next to the decoder.

I used small surface mounted diodes (SMDs) for the front and rear headlights. The SMD wires run through pieces of shrink wrap tubing cemented into the corners of the cab.  The diodes were coated with ACC to insulate them from the brass headlight housing.  Then they were centered and held in place with white putty.  To give the lights a soft yellow color I used a trick I learned from a friend in Australia:  I mixed a small amount of Canopy Glue with a few drops of Tamiya X-24 Clear Yellow paint.  I then used a toothpick to fill the headlight housings around the diodes with the yellow mix. Initially, the mixture looked opaque, but as the glue cures, it gradually becomes clear with a light yellow tint -- perfect for my little critter.

Here is a picture of the electronic "guts" of the critter.  You can see the speaker sitting on the keep alive.  On top of the decoder is a 3K resistor which will be soldered to the anode wires of the two SMDs.  The yellow and white light control wires have yet to be connected to the SMDs.

When I first placed the critter on the dual gauge section of my layout, it started and stopped constantly, even with the additional lead weight underneath the deck.  But installing the Soundtraxx Current Keepter completely cured the problem.  Now my little critter runs slowly and smoothly with no hint of electrical pickup problems.  My hope is that it will earn a merit award of at least 87.5 points toward my MMR.  But whatever happens, I had a fun time designing and building this unique little engine, which will have a special place on my model railroad.  The following link will take you to the video of my first run of Critter #7.