QSI "hardwire" install on Generation 4 RS-3

R.J. DeBerg has a RS-3 that had a meltdown:

That is the output transistor for the on board 6 volt regulator.

The other side has a regulator IC.

Looks like the regulator took itself out. RJ put in a spare board he had, but nothing worked. Then he replaces the board on the chassis, with the socket, and the motor came back, but no lights.

Rather than trying to debug all of this, we decided to do a hardwire directly to the QSI and basically bypass this board, only reserving the smoke and lights switch if possible.

The on-board regulator appears to be 6 volts, and we tested all the lights and they all still worked and functioned.

This way of doing it will also allow you to run the motors without the 2 plugs J3 and J4 connected to the board in the shell. (For any of you that have an RS3 and worked on it, it's a real pain that you have to have the upper board plugged in to run the motors. This is because the track pickups go from the main board up to the board in the shell and then back to the motors, unplugging J3 disconnects the motors and track pickups and the battery connections.

First steps, find and isolate motor and track pickups

We used an ohmmeter to verify the connections on the "main board" below.


  • J1 is on the left, 12 pins, pin 1 at the bottom, this is the "main connector" for the QSI, most everything is here.
  • J2 is on the right, 10 pins, pin at the bottom, this is the "optional connector" and Aristo rarely uses it, but if they do, they typically put the speaker on pins 1 & 2
  • J3 is the white 7 pin connector on the upper right, it has most of the power connections, it goes to the board in the shell
  • J4 is below J3, 5 pins, and it has the headlights, common and speaker connections.

So the first step I recommend in any hardwire installation is to isolate and verify the track pickups and motor leads.

By tracing the board, you find that right and left rails connect to J3 pins 2 and 3. They do NOT go to the J1 socket! This is why you cannot run the loco with J3 unplugged.

J3 pin2 is the engineer's/right rail
J3 pin 3 is the fireman's/left rail.

Note: the right and left of the loco assumes long hood forward!!.

Tracing the pins on J3, we find how J3 connects to J1, the "Aristo socket".

J1 pins 1 & 2 are connected to J3 pin 4.
J1 pins 11 & 12 are connected to J3 pin 7.

On this loco, the wires on the J3 plug were (from pin1 to pin7):

7. Black (Aristo socket J1 pins 11&12 - left rail)
6. Red (battery wires)
5. Blue (battery wires)
4. Orange (Aristo socket J1 pins 1&2 - right rail)
3. White (left/fireman's rail)
2. Yellow (right/engineer's rail)
1. Brown (positive rectified track voltage) 

So to "jumper" the track pickups to the correct pins on the J1 socket, you connect:

Right rail J3 pin 2 to 4 (yellow to orange)
Left rail J3 pin 3 to 7 (white to black)

Note: Be SURE that your colors match the pin numbers above before just connecting wires !!!

With this done, you should be able to plug in a QSI or the shorting plug and run.

Lighting issues:

A problem here is that there are only 3 wires to each small board in each "nose" of the loco. This means the led headlights share a common ground with the incandescent number boards (and the cab light). Using the QSI, and any other DCC decoder requires that the common is positive.

On the front (long hood) there are 3 wires to the circuit board in the shell:

  • Red "Lamp +6v"
  • Blue"Headlight (F)"
  • Black "com"

The leds are connected to the red and blue wires. The red wire is positive, and blue is negative.

On the rear (short hood) there are likewise 3 wires:

  • Black "Lamp +6v"
  • Blue "Headlight (R)"
  • White "com"

The leds are connected to the blue and black wires. The black wire is positive, and the blue wire is negative.

In order to proceed, we must somehow either eliminate the negative common between the marker lights and the headlights, or change it to a positive common. 

Isolating the headlights from the marker lights

In order to overcome the fact that the common between the leds and the number boards is negative (and we must have positive common), we have to isolate them. One way is to rewire the small boards by the headlights. This can be done if you want to pull a lot of hot glue out. We have done this on USAT locos.

The other way, which is what we did here, is to clip off the small diameter red and black wires from the headlight boards, and extend those wires separately. Now you have isolated the leds from the number board and cab lights.

We measured about 20 milliamps for the leds and 40 milliamps each for the incandescent bulbs.

Calculating for about 19-20v DCC, a 680 ohm 1/2 watt resistor in series with the leds will drop track voltage down correctly.

Take 2 more of these 680 ohm resistors, and put them in parallel, now you have a 340 ohm 1 watt resistor, which will drop track voltage for the incandescent bulbs.

Put one of the "appropriate" resistors in one wire from each of the leds, and the incansescent bulbs. You will be using 5 dropping resistors (of course the resistors for the incandescent bulbs are really 2 in parallel).

Now you have all the lights and leds ready to take track voltage.

J4 has the connections for the headlights on it.

The wires to J4 are:

5. Orange (speaker)
4. Orange (speaker)
3. Blue (front headlight)
2. Red (rear headlight)
1. Black (ground)

  • J4 Pin 1 (Black) is the ground or minus.
  • J4 Pin 2 (Red) connects to J1 pin 9, the rear light. (short hood) (blue wire)
  • J4 Pin 3 (Blue) connects to J1 pin 4, the front light. (long hood) (blue wire)
  • J4 Pins 4 & 5 go to the speaker pins on J2 (pins 1 & 2)

We need the lights (positive) common now, which is J1 pin 6, which comes out on J3 pin 1 (brown)

So to connect up the headlights:

Common: The brown wire from J3 pin 1 goes to the positive of the leds (remember positive common on decoders), this will be the red wire on the long hood, and the black wire on the short hood.

Front (long hood): J4 pin 3 (Blue) to the negative from the headlight (blue).

Rear (short hood): J4 pin 2 (Red) to the negative from the headlight (blue).

Test the loco on DCC now. I recommend if your locomotive runs backwards, you swap the motor connections on the main board, not just set the direction backwards in the DCC configuration.

Note: proper operation will give you dim and bright of the lights.

If the lights are wrong, swap the wires between J4 pins 2 and 3.

Once it's right on DCC, then test on DC, if you have a convention for forward or backwards, and the loco runs in the reverse direction, swap the  wires you connected for the track pickups (J3 pins 2 and 3).

Now everything should be fine for DC and DCC.

Lastly, we need to to connect the marker and cab lights and the smoke unit to track power.

We need to supply track power to the small board with the switches, and then use the switches to control the smoke unit and the marker and cab lights.

Re-power the switch board to control the other lights and the smoke unit.


We removed the small regulator board from the main board.  That was done just to keep power flowing everywhere else. It has a PIC processor to act as a voltage regulator, just like many other Aristo products, including the smoke generator.

The black wire from xxx is still connected to the switch board.

We connected the brown wire from xxx to the 3rd land from the top on the back side of the board, this would normally be the regulator output. You can see that this trace "Feeds" the center pins of the smoke and lights switches.

100 3889


PIC 16v6  20a  


Weather Underground PWS KCACARLS78