#30 How-to: Using a 21-pin decoder in a locomotive with an 8-pin socket

A short tutorial outlining one approach for making a custom 21-pin to 8-pin harness to enable the use of a 21-pin decoder in an 8-pin locomotive chassis.
You can find a wiring diagram/schematic for the MTC 21-pin breakout board here: tinyurl.com/21pin-to-8pin-ada...
Note: by its very nature, an 8-pin plug will not enable all functions of a 21-pin decoder to be used. This is unlikely to be an issue in most cases as 8-pin chassis tend to have few function connections.
If used with a sound decoder, separate wires will need to be soldered to the adaptor for the speaker connections. Alternatively, in some cases, speaker wires can be soldered directly to the chip.
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Items required for this project are:
- MTC 21-pin breakout board (e.g. Laisdcc or ESU)
- 8-pin plug (pre-wired, if possible)
- Reel of decoder wire (for non-wired plugs)
- Soldering iron, solder & liquid flux
- Kapton tape or insulation tape
- Various tools, including: small side cutters, wire stripper and an exacto knife.
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Thank you very much for watching.
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Visit us at: www.strathpefferjunction.com
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Please note: If you choose to follow any of the steps or suggestions outlined in the video, you do so at your own risk and any damage or injury to yourself, your models, your equipment or others is your own responsibility.

[@JohnPW22]

Just saw a 21 pin decoder that would work for me, and thought I needed this to go to 8pin, so looked for a video! Well done putting this out! Very helpful and just what the Doc ordered!

[@bradwilson8550]

Sad to know that no adaptor is available but good to know it can be done. I put together an adaptor for an 8 pin decoder to a 21 pin loco. Now I shall build this one (because I need it!). Brad

[@TheWGLOVER]

Another well presented tutorial. I have no plans to do this, but still enjoyed watching your approach.

[@StrathpefferJunction]

Thanks, William.

[@raymondwillem6370]

Excellent video again always clear and to the point. What a breath of fresh air you are.

[@StrathpefferJunction]

Many thanks, Raymond. Really appreciate the feedback :)

[@raymondwillem6370]

Strathpeffer Junction i know this is a long shot but are you familiar with the Hornby class 60? I dropped mine fitting an led light in the engine room. No damages to body or chassis but the wires to the running lights all came away from the two plugs and one from the PCB as far as I can see. The wires are all black ones and I'm lost to be honest. Tried to source a wiring diagram but apparently none exist. I thought of you and knowing your a wizard at electronics I wondered if you could help or even repair it. I would gladly reimburse any costs for time and postage. I could send pictures of loco if you decide to have a go. Like I said it's a long shot but I'm desperate! No problem if not. All the best Raymond.

[@StrathpefferJunction]

Hi Raymond. I'm very sorry to hear about this. I don't have a Class 60, but I'm more than happy to try to help. I've you're able to take a couple of photos of the loco with the body off, showing all the wires and the terminals, I may well be able to work out what goes where an guide you. Failing that, if I think it's something I can help with in a hands-on sense, I'd be more than happy to try. I've fixed a few on my own Hornby's when lighting wires have come adrift, so I'm sure even without a schematic we'll be able to get you sorted. 🙂 Drop me a line at [e-mail] with some photos and we can take it from there. 👍🏻

[@darrelldeltic807]

Nice and tidy tutorial, thanks for the link to the wiring diagram, I’ve done one for my class 26 sound chip and every diagram online contradicts each other! I’m printing yours off as it’s so much clearer, cheers Dazza

[@StrathpefferJunction]

Thanks, Dazza. Yeah, that's precisely what I've found. Some don't seem to tally with others and some can be confusing. Hopefully this one is fairly clear...using the wire colours *should* make it fairly fool proof!

[@onnomulder9775]

Where can I find the link?

[@StrathpefferJunction]

Hi Onno. It's in the blurb below the video, but you can also get it via the first post here: www.strathpefferjunction.com/blog

[@darrelldeltic807]

Onno Mulder it’s in the drop down in the description at the top of the page 👍

[@SimonLivertonCentral]

Very nice tutorial David, well done again...Simon

[@StrathpefferJunction]

Many thanks as ever, Simon.

[@johncarvil498]

Great tutorial as usual................John.

[@StrathpefferJunction]

Thanks, John. I've got three other videos in the pipeline just now, it's just finding time to edit them!

[@geoffwalker315]

Excellent. Very helpful. Geoff. W.

[@StrathpefferJunction]

Glad you found it of use, Geoff. ☺️

[@jankochanowski3252]

Hi! Decoders and stay alive are ordered as well as Roco z21 start with MultiMaus :-) Here is my question: The Zimo MX617N decoder I have ordered has 6 pins but Hornby Pecket 0-4-0 has a 4 pin socket. Here is my plan: I will cut out the Hornby socket, solder two wires from the motor and from the wheels pick ups to the coresponding pins on decoder. The two pins mentioned in decoder instruction as a front light and back light I will ignore (they remain not connected). Everything must be insulated and done. Correct my please if I am wrong.

[@StrathpefferJunction]

That sounds like the right approach, but please consult the decoder manual for the specific pins. I don't know that decoder well as I don't have any in my own locos, but assuming you have no lighting, you can probably ignore the white and yellow wires as they only control the light functons.

[@MaXXIMuss2022]

The below is the 8pin to 21 pin adaptor from bachman

[@juanmanuelvelazquezfernand8765]

great video, is something similar in MTH?. I want to replace the decoder from MTH to ESU, is possible to do this?

[@potatohead5910]

Do all the functions still work? I want to have a bunch of lights on my loco (there's a beacon and gyralight and a couple others) but I don't know if that will be possible

[@StrathpefferJunction]

You can map the functions of the 8pin decoder to whatever 21pin function pins you want, but you will be limited to how many func channels the 8pin decider supports. Many 8pin decoders will only have forward lights, rear lights and green aux (so only one func channel to control, say, cab lights). Some will also have a purple aux (normally a training wire) and a much smaller number may have another aux in top of that (mostly zimo decoders). So, it really depends on which decoder you are using as to how many of the funcs on a model will work. That said, in theory, and so long as you don't exceed the current capacity of the deciders aux channel, you could use the aux channel on the decoder to control more than one func on the model.

[@potatohead5910]

@@StrathpefferJunction Can I use the extra spaces on the adapter? Are those still hooked up to the 21 pin decoder?

[@TheDaf95xf]

Nice explanation and tidy work David 😊 I’ve just took delivery of the liquid flux your using 👍🏻 What do you use to clean the flux off? Stevie

[@StrathpefferJunction]

Hi Stevie. I tend to use 'no clean' flux, so you can leave it (although it is a little sticky, just doesn't continue to corrode or turn green like other fluxes). However, if I've put too much on or I'm using standard flux, I tend to use a cotton bud with 90%+ isopropyl alcohol (IPA) if it's just circuitry or watered down IPA if there's any danger of it getting onto paintwork or sensitive plastic. Some folk claim to have had conductivity issues with no clean flux, but I've never had any issues at all, so it seems to work for me!

[@StrathpefferJunction]

Sorry. I misread your comment. If you've got the same stuff as me, you don't need to clean the flux off. But, it can be sticky, like I said in my first reply, so if you do get a bit much on or you just want to wipe off the residue, that's where the IPA comes in. The bottle is VERY easy to knock over though, so do be careful with it. I knocked my last bottle over on the kitchen work top and it took a lot of watered-down IPA to clean it off, and it had to be done quickly to avoid damage! I've since designed a little holder on my 3D printer to hold the bottle secure.

[@MFL8378]

like your wire strippers. where are they from.

[@StrathpefferJunction]

They've gone up in price a wee bit since I bought them, but they are excellent and worth the price: smile.amazon.co.uk/dp/B001YHELPS/ref=cm_sw_r_apa_i_mc.XEb0JWFAA8

[@nicksanto882]

Soldering onto a 21-pin decoder….. hmmmmmmm…. Why not solder onto a Decoder Buddy and then plug the 21-pin decoder into the Decoder Buddy. You can connect all the lighting, speaker, track and stay alive wires to the Decoder Buddy. The LEDs will all be protected with resistors on the Decoder Buddy and lights in the shell are connected to a small daughter board allowing for separation of the shell from the frame of the locomotive. Maybe a better way.