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Decoder Installation into a
Proto 2000 HO E7 or Similar

This installation note assumes you have read the sections on Wiring Specific Locomotives, Using Minature Lamps and Sound. If you have read them previously, review for any updates. 

A Soundtraxx DSD-150 decoder is used.  The speaker will go in the back of the locomotive.  You will be building a box of sorts to act as a speaker baffle.  The decoder will occupy the cab.  Spraying it black will make it otherwise not noticable through the cab windows. 

The top light is the Mars light. 

The motor is already isolated from the frame. 

Grain of rice bulbs (1.5V) are used for the number boards. 

 The blue wire is the common of the special Mars light.  It may be replaced with a grain of wheat bulb (12-14V).  Or you may use this bulb by connecting it's red and white wires together and attaching them to the 22 ohm resistor. If yo don't want a real bright Mars light, use only the red or the white wire. and leave the other one unused. Be sure to put heat shrink on any used wires! This includes the yellow wire which you will not use. 

Any value between 47 and 1000 uF can be used for the speaker coupling capacitor. 

The 680 ohm resistor is a standard value.  620, also a standard value, can be used as well.   As series combination totally 660 or in the range of 620 to 680 is ideal. 

Cab Preparation:

Cut the front plastic piece (cab detail support) that holds some of the cab details flush with it's main surface. 

Remove the instrument stand to the left of the engineer who is sitting on the right. 

Decoder will go with two round protrusions down.  Further modify the cab detail support to accomodate these protrusions.  The highest point of the decoder must be essentially flush with the highest point on the frame so that you can get the body back on.  If you carefully hack it up, it will remain a single piece.   You will also need to remove some of the instrument panel. 

Carefully remove the clear bezels that illuminate the number boards as the new grain of rice bulbs will now do a better job of illuminating the number boards.  Leave the portions containing the pins that hold the clear plastic bezel to the cab detail support you hacked up a few moments ago.  You must keep the clear bezel to support the Mars light.  Remove the piece of clear plastic that goes in front of the headlight.  It's function was to capture light for the number boards. 

I wrapped a piece of bare #18 AWG wire around the Mars light and head light to keep the two lights facing forward. 

Rear & Speaker Preparation:

Remove circuit board.  Add it to your collection of things you think you might use someday or discard.  Grind away large screw mount for transistor that was on the circuit board.  Grind away the four smaller circuit board mounts. 

Make speaker enclosure shown below. You will be making a box of sorts. It's size was designed to maximize the size of the "baffle" this box creates. For the size of the speaker used, the volume of this box is almost exactly correct. 

It will have sides and bottoms (1.9" long and the other 0.7"), but no top. You will find that this box fits perfectly into the locomotive. 

Use the speaker in 11-20 on the section on sound. Also use diagram to make speaker openning.  The speaker goes into the 1.9" long bottom.   Make all bottom sand side pieces, except the complex ones shown, the width of the speaker, about 1.2".   Use the speaker as a guide.  Use thick polystyrene sheet to reduce buzzing due to the speaker operation.  The dimensions shown are the total dimensions you will need. You will need to account for sheet thickness or trim when done. 

Glue the pieces together using a strong, latex rubber glue such as Liquid Nails. CA may come undone after time from speaker vibrations.  Rubber glue will contract and will cause pieces to bend towards each other when drying.  So hold firmly to shape while drying.  Glue in sections and allow at least 8 hours or more before continuing box assembly.  I found a toothpick was useful in keeping the lower right portion of the box from folding inward.  After gluing all the bottoms flat side pieces, join with the complex side pieces shown. 

You will notice that the speaker sticks out the top.  The speaker will fit into the curved section of the roof.  Put electrical tape on the inside of the roof.  Put some caulk along the edges of the speaker enclosure, particularly those ends that do not have curves to match the roof.  Fit this into the locomotive. The caulk will take the shape of the roof.  Do this last as you close up your model  and after you have soldered the wires on so you don't have to disturb it or it does end up sticking. The electrical tape is intended to prevent a permanent bonding. 

You have very few places to put the capacitor.  You might be able to place the capacitor to one side of a truck.  You can also place it inside the speaker enclosure.  This will reduce the volume of the speaker baffle somewhat, but you will find this a more convenient location than just about anywhere else.  You will find that you will have to tape things down just to get the body shell on as it is! 


Though not required reading, you may want to read the section on Hexadecimal Demystified.
CV Value Comment
33 41h x1: Headlight on when direction is forward. 

4x: Mars light (via F5 output) on when direction is forward. Use 0x if you want something else to control Mars light.

35 04h F1: Bell
36 08h F2: Horn
37 02h F3: RPM+
38 04h F4: RPM-
39 08h F5: Dim
40 40h F6: Dynamic brake sound effect.
41 02h F7: Number boards (via F6 output)
42 20h F8: Mute   (Use 01h if you want F8 to control Mars F5 output instead of mute function.)
49 01h Fwd output: Rule 17 dimmable headlight.
50 -- Backup light effect.  Not used (no backup light)
51 02h F5    output: Mars light.
52 00h F6    output: Number boards.
55 Not needed.  Therare no ditch lights.
56 03h xxxx xxx1 Wait for packet before making sound.  Works only when auto notching is disabled. 

xxxx xx1x Pop value sound enabled.

58 10h Lock enabled (see Soundtraxx manual for description of lock.) and Auto Notching disabled.  Must used F3 & F4.
59 04h Bell ring rate. 00 fastest.  0F slowest.
60 68h Bell volume: 6x. 

Horn volume x8

61 66h Dynamic break volume: 6x. 

Engine exhaust volume: x6

Reassemble locomotive.  Before vacuuming up, inspect your locomotive and then the floor for the detail parts you broke off!  Reattach. 

Soundtraxx Reminder:  Outputs that are affected by direction, do not turn on until after the throttle is advanced.  So if your throttle is set to forward and you are not getting the headlight on with F0, advance the throttle and it will come on. 

Also, if you elected to use locked throttle/engine speed, your locomotive will not move unless you have pressed RPM+ at least once to start the engine. 

Copyright by Allan Gartner 1996 - 2006 © All rights reserved. You may print this for your own, personal, non-commercial use. Non-commercial, non-personal reproduction may be requested by visiting . All users, commercial and non-commercial, may link only to this site at

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