Author Topic: Best Of DCC decoder installation in a Samhongsa brass PRR BS-10 (DS-4-4-10) switcher  (Read 733 times)

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Last year I took on the task of tuning up this model, and installing a DCC decoder.  It is finally finished! :)

I first removed the couplers. Next, there were two screws on the rear, and one up front holding the shell and frame together. Those were also removed. As it is typical in brass models, there were rather stiff wires connecting the truck pickups to the motor. This model had no headlights.  It also features a bit unusual (at least to me) construction.  Most brass models utilize electrically live chassis, passing the electricity from one side of the trucks to the motor.  In this locomotive the trucks still have a metal gear housing which is electrically live, but then each truck is insulated from the frame. So the frame (and shell) are electrically neutral.

I proceeded to fully disassemble the chassis. First thing I noticed was that there were small pieces of  brass soldered to the pickup wipers on the trucks.

Then another wiper also had a brass pad soldered to it, and it was also installed incorrectly, because it wasn't fully contacting the wheel tread.

The problem here was that the wipers are made from stainless steel and the solder didn't actually wet the surface of the wiper. They were just mechanically tacked onto the wipers.  I'm not even sure why those were added as the wipers weren't showing much wear on them. If the wipers weren't making good contact with the wheels, they should have been bent down until positive contact with the wheel tread was achieved.

The other problem was that the factory-soldered wires were were poorly attached. The solder joints were very large and sloppy, and the wires were thick and stiff.

I replaced them with much thinner and more flexible wire, and made much smaller and neater solder joint.

I fully disassembled the trucks, cleaned all the parts in ultrasonic cleaner, then lubricated them sparingly and reassembled the trucks. Here is a view of all the cleaned parts for a single truck.

It has interesting construction: the worm is steel, and the gears are mixture of nylon and brass.

Upon reassembly I noticed that the rounded axle ends were contacting the inside of the sideframes. I decided to use a countersink bit to slightly shave down the area where the axle ends were touching. The modified sideframe is the top one.

When reattaching the trucks to the frame I also noticed that the insulated kingpin very easily popped out of the socket in the loco's frame. If both trucks ended up popped out that would create a short across the frame.  I machined new washers, with a taller center section,  from a piece of acrylic rod (shown left in the photo).

After I reassembled the chassis I started contemplating the decoder install. I also wanted to add the headlights without needing any wires between the chassis and the shell.  There wasn't much clearance anywhere inside the shell, but after taking some measurements and experimentation I determined that there was enough clearance over the motor for a 0.020" thick circuit board. I could then attach the decoder to the circuit board, suspending it over the front truck's gear tower.  I also hate to permanently glue any parts of the model which might have to be disassembled at some point for servicing.  So I decided to attach the circuit board to the motor using some carpet tape and a 0.005"  brass bracket.  Carpet tape is very thin and has a very strong adhesive on both sides, but if needed it can be removed.

I drew the circuit layout on a piece of paper, cut a copper-clad board to size, then using an old drafting pen and Testors green hobby paint I drew the traces and pads on the copper.  The smaller circuit board will be used as a contact area glued inside of the shell for the headlight contacts.

The board was the etched, drilled, and the components along with the brass bracket were soldered onto it.  The 1/8" piece of acrylic rod visible in the photos is a tool I made to twist the strands of stripped wire before tinning them. Each end of the rod has a tiny cone rolled from a piece of stainless steel wire glued in.  I got this idea by looking inside wire nuts (used for household wiring). To use it I first strip the insulation from the end of the wire, then carefully put the stripped end in the end of the tool and then twirl it. It does a really good job twisting the small length of exposes strands. I couldn't do that using my fingers or tweezers.

Here is the TCS Z2 decoder soldered to the circuit board. I also used carpet tape to attache the decoder to the board.

This photo shows the phosphor-bronze contacts for the headlights.

The  2 pairs of tinned pads between the contacts and the resistors (marked 102) are for the rear truck pickup leads and for the motor leads. That will minimize the wire clutter inside the model.

This is the circuit board test-fit onto the motor.  Even though there is a bit of clearance, I covered the top of it with Kapton tape to prevent any possible shorts with the top of the brass shell.

I then made sure that the headlight contacts were properly contacting the small circuit board glued into the shell (rear truck removed to make that area visible).

These photos show both sides of the fully assembled chassis.  The short lengths of wires are neatly routed and soldered to the pads on the circuit board.

And top view of the circuit board.

To illuminate the headlights I used relatively large SMD LEDs I had in my stash, and short pieces of fiber optics to pass the light to the headlight housings.  This photo shows the inside of the shell with the LEDs installed and wired to the contact board at the rear of the hood.  The LEDs are the white rectangles.  For wiring I used enameled magnet wire and Kapton tape to retain the wires.  The LEDs are aligned to glow into the ends of the fiber optics and attached to the shell with 5-minute epoxy.  After making sure they work, I covered both LEDs with epoxy and then painted the area with several coats of black paint to prevent any light from shining inside the shell.

Also worth noting is that I polished the ends of the contacts and the copper on the contact board using metal polish. That will make the contact between more reliable.

Closeups of the front and rear headlights showing the ends of the fiber optic. I used warm-white LEDs, but the light was still a bit too bluish. To give them a warmer glow I dabbed some transparent orange paint on the end of the fiber optics.

The model was fully reassembled and it is now it the hands of its happy owner.

« Last Edit: July 17, 2017, 04:11:18 AM by peteski »
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Re: DCC decoder installation in a Samhongsa brass PRR DS-4-4-10 switcher
« Reply #1 on: February 17, 2016, 01:54:51 PM »
WOW -- I am impressed.
Nice write-up.