HO Bowser U25B Decoder Install


SP 6744 is a custom made HO scale GE U25B model from Bowser. A friend of mine got this DC engine (see history here) and I offered to add a DCC decoder to it. This was an interesting install so for once I think it’s worth detailing it.

Let’s start with the end result in action:

Shell removal on this engine is straightforward:

  • Remove the couplers.
  • Lift shell from the back. Careful with the rear LED.
  • Once the back has cleared the top, slide it forward to clear the coupler support from the front plow.
  • Reinstall in reverse order. Adjust the rear LED and make sure the rear coupler support clears the shell, it tends to bind a tiny bit.

This is the finished version of the engine:

The engine’s board has an 8-pin NMRA plug.

For the decoder, I selected a Tsunami 2 TSU-2200 GE Diesel 885006. The decoder has a 9-pin JST on one end and 9 colored wires.

Soundtraxx sells a specific harness for the 8-pin plug but instead I just made my own using a 8-pin DIP IC Socket:

These are easy to find around, just make sure to get the round hole versions. They are generally sold in large batches -- I got a 100 pieces batch for a former project so I had 99 sitting around. Just cut them in 2 and use half of it by soldering 8 out of the 7 wires out of the decoder, as indicated in the NMRA spec (only the brown and green from the TSU are not to be wired). Each wire is soldered on top on the round hole. Then the socket is just inserted in place of the DC dummy plug. Finally a bit of kapton tape holds all the wires neatly in place.

On the other side of the TSU, there are two purple wires for the speaker. A good place to put the speaker was on top of the rear truck. I created a small support with two small flat piece of plastics printed using a 3D printer (cutting a piece of styrene would work equally but I didn’t have any around). That’s the two white pieces here:

My first speaker install used an ESU “sugar cube” speaker:

ESU 50321 Speaker 11x15mm 8 ohm 1 watt w/ Sound Chamber, #50321 

I had wanted to experiment with these small form factors for a while and given the space available, this seemed like a good choice.

The ESU sugar cube speaker was installed with the largest possible baffle and mounted at the back next to the rear light, on top of the rear truck. This way the speaker is located right under the rear grills opening.

Although it does sound great for its size, this is still relative and I’m moderately satisfied. It lacks some bass and has some definitive clipping. The video at the top of the article was recorded when the engine used the ESU sugar cube speaker.

The ESU sugar cube absolutely needs some equalization to sound better, which is an area where the Tsunami 2 shines greatly. I really wish other decoder manufacturers had that option (looking at you, LokSound).

The Tsunami offers a full EQ which was programmed as follows:

  • High-pass filter: CV224 = 20.
    • Speaker doesn't seem to render much below 100-150 Hz.
    • Sound changes above value 40 (200 Hz). Leaving to the default 20 (100 Hz).
  • Mode set to "User Adjustable" equalizer with the following EQ band values:
    • CV226, 62 Hz = 96 / -3 db
    • CV227, 125 Hz = 128 / 0 db
    • CV228, 250 Hz = 160 / +3 db
    • CV229, 500 Hz = 128 / 0 db
    • CV230, 1 kHz = 96 / - 3 db
    • CV231, 2 kHz = 64 / -6 db
    • CV232, 4 kHz = 42 / -9 db
  • Bell is heard in the 500 Hz / 1 kHz / 2 kHz segments.
  • Reverb set to Medium: CV233 =2

To avoid clipping, the master volume has to be turned down between 50% and 75%. Unfortunately my friend wanted the engine to be as loud as possible and he was only satisfied when the master volume was up at 95% or similar. There was noticeable clipping at notch 8 or with the horn at any time but that didn’t bother him as much as it did for me.

I’ve used the same sugar cube speakers on two builds and I’ve been equally annoyed by them. One of them gave up fairly quickly. After searching around, I had good success by replacing the sugar cube speaker by one of these: “Speaker Nokia 6300 6110 E66”. These are easy to find on ebay and really cheap. They fit perfectly in place on top of the ESU speaker enclosure, for 1/10th the price.

Anyway, after using the engine for a while, the speaker died. I decided to go back to a more regular speaker, the Soundtraxx 25x14mm Oval Speaker 810112. I’ve used that in the past and was really satisfied with its sound rendition. The only thing Soundtraxx does not sell is a baffle for it. They simply recommend to build one from scratch using styrene. My thing is 3D printing so I did just that:

This is a straightforward enclosure and I have it in 9 mm and 7 mm deep, 25x16 mm footprint.

It’s a really good fit. The speaker is CAed to the baffle and held on the support using silicone caulk:

The Tsunami EQ is now programmed as follows:

  • High-pass filter: CV224 = 20.
  • Mode set to "Small Speaker (1-2 inches)" equalizer.

Here’s a video of the new speaker under test:


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