# Replacing the MTH SD70M-2 PS3 DCS/DCC Decoder w/ LokSound 5 DCC Direct w/ PowerPack



## webnext (Dec 15, 2021)

Did not find any good information about this anywhere else. Hope this help others faced with the same issues. I spent 3 months trying to contact MTH with no response (phone disconnected and email returns NDRs). Realizing MTH sold the HO business to ScaleTrains, ScaleTrains advised I replace the board because they will not support the electronic aspects of the HO models. I tried asking questions at the Local Train Store (to their MTH tech) - the owner would not answer any questions - assuming b/c he did not want to feel responsible if I blew it up. In the end, I just did research and figured it out.

*Intro*

MTH SD70M-2 with onboard PS3 DCS/DCC Decoder experienced two issues. First, the decode did not retain programmed CVs between power cycles. Second, acceleration demonstrated random profiles at low speed. Overall, the model worked extremely well prior to the rebuild despite the issues noted above. I once heard that if a loco doesn't run well well before installing or replacing a decoder, most likely it will not run any better. In this case, there was obviously something wrong with the decoder and not the motor/contacts/etc.

Layout configuration includes:


Digitrax Zephyr Express DCS 52 command station/booster/throttle
Raspberry PI 4 with JMRI as primary throttle and switchboard

*Approach

Step 1: Removed housing and walkway*
Go slow. It comes of pretty easily if you look at what you are doing.

*Step 2: Map existing wiring*

Existing wiring was bundled neatly using a wiring harness with a plug to the decoder. Each component was connected to the harness plug using a secondary plug. Mapping took some time as there were a lot of components in this model.

Components included:


Headlight and number board PCB with LEDs (purple negative common)
Rear light and ditch light PCB with LEDs (purple negative common)
Front ditchlight PCB with LEDs (purple negative common)
Proto coupler PCB front and rear
Tach reader
Motor (yellow +, white -)
Speakers (yellow, white)
Potentiometer (volume control)
Front & rear track contacts
Three resistors/powerpacks

*Step 3: Reuse and Remove*

Not all components were required in the target build and were set aside. No wires were cut until everything was mapped and determined where it would plug into the target decoder. Existing component plugs were reused.

Removed the following components:

Proto coupler PCB front and rear
Tach reader
Potentiometer (volume control)
Three resistors/powerpacks

Reused the following components:

Headlight and number board PCB with LEDs (purple negative common)
Rear light and ditch light PCB with LEDs (purple negative common)
Front ditchlight PCB with LEDs (purple negative common)
Motor (yellow +, white -)
Speakers (yellow, white)
Front & rear track contacts

*Step 4: LED Lighting (negative common)*
MTH lighting uses negative common instead of positive common standard with DCC. This was confusing at first until we figured out what needed to happen.

2 options:

1. Cut traces on PCBs and create jumpers to create the positive common
*OR*
2. Turn the LEDs 180 degrees - no cutting of PCBs

We choose option 2. Each LED was turned 180 degrees and tested prior to reinstall. The positive and negative must be identified for each LED prior to re-soldering to the PCB. This was done for the headlight/number board PCB, rear light/rear ditch light PCB, front ditch light PCB.

During the process, we lost 1 LED (blew away). And vaporized one LED when it fell on the soldering iron. We ordered additional nano LEDs for the number boards.

*Step 5: Layout the Target Mapping*

The LokSound DCC Direct with PowerPack has 8 AUX pads, 1 headlight, 1 rear light, 2 L track, 2 R track, and 2 common pole pads. The target pads included:

AUX 1: Front Ditch light left
AUX 2: Font Ditch light right
AUX 3: Future use (Walkway lights - not installed yet)
AUX 4: Rear Ditch light left
AUX 5: Read Ditch light right
AUX 6: Numberboards
AUX 7: OPEN
AUX 8: OPEN
Common 1: Headlight/rear light/ditch/number boards
Common 2: Walkway/stairs
Front light pad: Front light
Read light pad: Rear light
Track Left Front, Track Left Rear, Track Right Front, Track Right Rear as named

*Step 6: Target Wiring & Soldering*

Snipped purple common wire from unused components to create 2 common wires. Reused existing female connections cut from original pin harness. Inline spliced each light component common to the common wires and used heat shrink to protect. After each wiring bundle was matched/extended/spliced soldering to the decoder began.

Soldered primary components in this order:

Motor positive (yellow), negative (white)
Speakers yellow, white
Track connections
Tested to verify movement and direction
Validated speakers using LokProgrammer basic diesel profile loaded to decoder
Soldered lighting starting from highest AUX pad to lowest AUX pad.
Tested all lights and functions.
Found front ditch lights to work sporadically. Determined harness plug was cracked and wiring faulty. Removed both plugs and created an inline splice for front ditch lights.

*Step 7: LokProgrammer Used to Load Sound Project*

Loaded the ESU sound file: S0525-LS5H0DCC-Diesel-EMD-16-710G3C-LATE-EXH-T-V2-R2
Mapped AUX outputs and adjusted configuration per preference.

*Conclusion*
The planning and mapping was the most time consuming part. Total time from first removing the housing to testing was 3 weeks. After identifying all components, wires, and switching the LED +/- the decoder soldering took 15 minutes, reassembly and wiring runs took 45 minutes. Gluing bits and pieces I broke during reassembly took 10 minutes.

Everything works well. Motor performance is fantastic. The existing MTH speakers are great. The LokSound 5 is amazing. Having a blast programming the features and capabilities.

Below are photos taken during the process. Still need to install number board lights, adjust the headlight/PCB mount b/c it was installed upside down, and smooth out the wiring.. Will post final images once complete.

Original Wire Mapping








Headlight/Number board PCB







Spliced Wires







Initial LokSound After primary wiring







Initial Motor and Speaker Test







Next test with AUX pads







Final test coming soon...


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## Jscullans (Jul 8, 2019)

I almost bought a set of ho scale warbonnet f3 locomotives but I couldn’t justify 400+ for them when mth is no longer around


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## MichaelE (Mar 7, 2018)

Loksound V is an amazing piece of hardware. Using the Loksound program and/or programmer really allows for its capabilities.

The software alone will allow sophisticated programming of CV's that are a snap to do. Loading sound is not possible without the Lokprogrammer, but everything else is accessible.


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## webnext (Dec 15, 2021)

Jscullans said:


> I almost bought a set of ho scale warbonnet f3 locomotives but I couldn’t justify 400+ for them when mth is no longer around


Yeah - picked the MTH up several years ago. I debated on chucking it or putting the LokSound inside. Glad I tried - wasn't much info out there and few folks that had tried seemed to have issues. Now it runs like new - even better - and I can control every aspect with the LokSound.


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## webnext (Dec 15, 2021)

MichaelE said:


> Loksound V is an amazing piece of hardware. Using the Loksound program and/or programmer really allows for its capabilities.
> 
> The software alone will allow sophisticated programming of CV's that are a snap to do. Loading sound is not possible without the Lokprogrammer, but everything else is accessible.


Agreed - beyond CVs - programming the sound slots really adds a level of customization.


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## DavefromMD (Jul 25, 2013)

Wow. I'm impressed. I have an MTH GP38-2 that the motor control has gone haywire on. I too had no luck contacting MTH (somewhat expected), but when I contacted the authorized server center near me, they said they no longer work on MTH HO. They can't get parts and have no support from MTH.

I wondered what it would take to replace it and like you found information scarce. About the only thing I found out was about the reverse light ground. Seeing your description, it sounds beyond what I want to undertake. I guess it will become a static model. The sounds still work fine and it runs fine at slow speed. Above speed step 6 (of 28) it cuts on and off.


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## webnext (Dec 15, 2021)

DavefromMD said:


> Wow. I'm impressed. I have an MTH GP38-2 that the motor control has gone haywire on. I too had no luck contacting MTH (somewhat expected), but when I contacted the authorized server center near me, they said they no longer work on MTH HO. They can't get parts and have no support from MTH.
> 
> I wondered what it would take to replace it and like you found information scarce. About the only thing I found out was about the reverse light ground. Seeing your description, it sounds beyond what I want to undertake. I guess it will become a static model. The sounds still work fine and it runs fine at slow speed. Above speed step 6 (of 28) it cuts on and off.


Yep - MTH emails get bounced back and their number is disconnected. The local train store was less than helpful and refused to allow a conversation with their MTH tech to ask reference questions. Sure would’ve made it faster if there was a service reference manual but it all worked out. What I understand is MTH sold the HO models to ScaleTrains - not the electronics. Otherwise they are still in business. Based on my experience it seemed like they were completely out of business unless that is just their way of doing business.


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