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Experiences of CT Elektronik SL51-4, SL75 and GE75 Decoders

YouChoos Resouces

Here I will attempt to present some of the experiences that I have had while installing, programming and using CT Elektronik's SL51-4 sound & motor decoder. While I don't claim to know everything that can be done (far from it), I hope that these insights will prove useful to anyone delving into this marvellous decoder and wanting to get a little more than 'the default' from it. I have also used the its smaller siblings, the SL75, GE75 and SL76 as well as big brother the SL82, and will mention the differences as I ramble on.

CT SL51-4 decoders

CT SL51-4 Decoder

Why Choose the SL51-4?

My passion in model trains has always been steam, and I had spent several years playing with lighting and smoke generators, which are fabulous in themselves. However, with the maturing DCC world, and a growing choice of sound decoders on the market, I finally took the plunge and decided to investigate how to go about putting sound into my own steam locomotives.

Quickly it became apparent that steam was much much harder than diesel or electric, largely because of the space available inside the loco's body. Unless you looked at the big engines, such as the tender ones, it was going to be tricky to get most of the available sound decoders inside, let alone a decent enough loud speaker to make it all worthwhile. The other problem of course was the price.

ESU LokSound seemed to be the market leader at the time, and certainly still are at the time of writing, although there are a good number of other manufactures producing a variety of pre-programmed sound chips for specific locos - particularly diesel and electric varieties. I started out by compiling a list of the available chips, which features they had, and in particular, their size.

As well as size, it was also quickly apparent that another criteria was going to be very high on the list - programmability... and I don't mean CVs... I mean sound-sets i.e. the ability to download my own sound recordings onto the decoder.

My final requirement was that there must be a minimum of 4 function outputs and that they needed to be powerful enough to directly drive a single Seuthe smoke generator (around 150mA on an output), to avoid the need for a miniature relay.

Seuthe Smoke Unit #22 Seuthe Smoke #22 Kit

Seuthe Smoke Unit #22

So, with size, function output ability, and full programmability in mind, the remaining list of suitable decoders was in fact pretty small, and my short list was set at:

  • ESU's LokSound 3.5 Micro (standard 3.5 was too big for my liking)
  • Zimo's MX640
  • CT Elektronik's SL75 or SL51

There did appear to be one or two others which might have been feasible, such as QSI, DigiTrax and MRC, but at the time I plumped to look into these in detail! After studying the data sheets over and over, I finally decided to start with CT decoders if for no other reason than they were by far the smallest (certainly in terms of thickness) and appeared to have all of the features that I wanted and more, as well as being the cheapest. It seemed a winner. Even today the choice has not broadened vastly - there are upgraded offerings in the LokSound range (V4), and from Zimo (MX648,646,645 etc.), and a low-cost alternative from DigiTrax.

CT SL51-4 with various speakers

CT SL51-4 shown with a variety of speakers

First Reaction - SL74

Well, the first CT decoder to arrive was an SL74, which was by all accounts VERY tiny. Impressive, I thought.

So, I rigged up my first test harness with an 8ohm speaker (an old Hornby Thomas engine's innards) and switched it all on. I was expecting great things, and that's what I got - out of the box the SL74 appeared to do the trick, coming pre-loaded with a semi-decent steam sound-set... reasonable chuffs, a nice throaty whistle and some well designed auxillary sounds - brakes, water filling, ventilator etc. Even the micro-speaker supplied with it seemed incredibly loud considering its 15x11x8mm size. Nice. Well, nice for about 5 minutes, until the decoder overheated, caught fire and flashed a brilliant white flame onto my work table! (my work bench is wooden, in case you wondered, and I keep that little burn mark as a souvenir now!)

The replacement came very quickly, in the form of an SL51-4 (the SL74's bigger brother). Basically identical, but an only slightly larger footprint (same thinness), but with a greater capacity for motor output. I assumed that the motor has just been too much for the SL74, although that has never been confirmed, and according to the stats, it should have been well within means.

Anyway, I decided at that point to give up with the SL74, and not risk it, but instead just go for the (brand new at the time) SL51-4 instead.

The '-4' is the revision number, having superceeded the SL51-2 (whatever happened to SL51-3 we'll never know!) Don't you just love the imaginative names that these decoder manufacturers come up with!?

The 4th revision was important for me as it has various improvements over the previous revision, including lower operating temperature, more function outputs, more lighting effects and a better sound engine (16-bit with approximately 200 seconds of recording space, over 3-channel playback). I'll be the first to agree that the sound features are not as rich as today's LokSound or Zimo decoders, but they are perfectly sufficient under most conditions.

Second Reaction - SL51-4

So the first SL51-4 arrived and I set to work doing an install in a Hornby Patriot Class. Nothing fancy at this stage - just use the pre-loaded steam sound-set and put in some simple lighting features - front and rear lamps, firebox light and a cab light (4 functions).

This install went incredibly well, and the result was just awesome - it was the first DCC Sound steam engine that I'd properly got to tinker with, so I guess was always going to seem impressive to me. The most impressive thing was the volume coming from that tiny CT speaker, nicknamed the 'sugar cube'. Amazing.

That seemed easy, so I put in an order for a few more SL51-4 units.

Programming Troubles

It all seemed great, and I had great plans to use many more of these decoders for my entire steam fleet (at the time only around 15 locos).

CT Programmer PC Interface

CT Programmer - PC Interface

However, I was very keen to change the pre-loaded sound-set for something else, so I bought byself the CT programming device known as 'SoundProg', to hook up to the PC. After all, one of my main criteria in choosing CT decoders was that I could do just this. You can find my Experiences of the CT Programmer in another resource on the YouChoos website.

The short and curly of it is that the programmer was eventually configured, after a number of blown decoder attempts, a variety of cables and lots of experimentation with CT software versions. I have now figured out how to reliably put together my own sounds-sets and download them into an SL51-4 and have done a large number of installs this way since then.

CT Support and Documentation Woes

CT Elektronik are a small company based near Vienna, Austria. The founder, Mr Tran, is a very technical guy, formerly an employee at Zimo. So, it is no surprise that there are a lot of similarities between Zimo's products, and CT's.

Now any product is great if you can get it out of the box and it just works how you'd expect. What really tests a manufacturer is what happens when things go wrong. Before I plumped for CT, I'd read quite a lot of negative comments from various people about CT's products, from shoddy build quality (wires falling off) through to a total lack of support. Being technical(ish) myself, I had foolishly thought, "no problem - I'm sure it can't be that hard"!

As it turns out, it doesn't matter how technical you are - if the documentation and support you are working with is incorrect, incomplete, non-existent, or simply badly translated, then you're in for tough times. And I was. CT's documentation is almost entirely in German, with only a few of the older revisions of decoders having English translations. My German is less than half decent, but I struggled through and did a translation of a few documents. This helped, but it was clear that the original German documents themselves were incomplete or inaccurate.

Many of my early queries were directed towards DCC Supplies (the very helpful Fiona and Andy) and also to Arnold Huebsch (Arnold's Train Web), rather than directly to CT. DCC Supplies offer fantastic support and they try very hard indeed, but unfortunately hadn't experimented with the latest revisions of CT decoders quite as much as I wanted to push them at the time. Likewise, Arnold hadn't got to the bottom of every facet of the 4th revision either, although his knowledge of both Zimo and CT equipment were probably the best in the world, next to Mr Tran himself of course!

So, what I would say, is that it is probably possible to get the CT equipment to do what you want, and there are resources out there to help... just not that much. Support is, frankly poor from CT directly, which is very disappointing, although he has replied to most of my queries to one extent or another. I truly hope that changes in the future. In the meantime, I have mastered a fair bit of it, and I hope that you'll find my comments on this page, and in other YouChoos resources useful. I really like the CT kit, as you've probably gathered by now, but there are areas of the products which fall far short - namely documentation and support. Hopefully I can plug some of the gaps here with these resources.

The Physical Install

So, let's take a look at the SL51-4 itself first...

It is roughly the same size as a Bachmann 36-553 3-function decoder, perhaps a little smaller. The '-4' revision comes with a black protective tubing to guard against short-circuits. Previous SL51 revisions did not have any protective clothing, so it was much easier to accidentally short and damage them.

Baby brothers, the SL75/GE75 and SL76, do not come with any covering, which is fair enough, as you'll most likely be using it in the tightest of installations.


CT SL51-4 Decoder Close-Up

SL51-4 Decoder Close-Up

Either end of the decoder is exposed outside the tubing, so that you have some access to the wiring points and solder pads - just enough to be able to neatly solder new wires on, or to add more wires for the additional function outputs. Talking of function outputs, the SL51-4 actually has 8 function outputs - yes, 8! That was a surprise to me in fact, as I had understood from the documentation that it had the usual 4. The 'standard' 4 are pre-wired in the usual NMRA colours: white; yellow; green; purple. The extra 4 outputs are provided as solder pads on the reverse side of the decoder, which are pretty easy to attach a thin wire to, assuming your soldering skills are half decent (my soldering skills are pretty average, and I can do it, so it cannot be that hard!).

The first 4 function outputs are identical - same output capability (250mA each) etc. According to NMRA standards, the white and yellow are designed specifically for directional lighting, although there's no reason you couldn't use them for something else. The additional 4 function outputs are similar in function but can handle a total of 250mA between them (rather than each), which, for most lighting, is still perfectly adequate. Their function mapping allows for separate programming for forward and reverse, and there's a reasonable range of lighting effects and related CVs which make it quite flexible.

I had heard of problems with build quality of CT decoders in the past, but it seems that these issues have been improved and this revision looks as sturdy as any other decoder on the market. The attached wires themselves are a little thicker than most manufacturers use, but not really an issue as they are perhaps more flexible. I use very fine decoder wire when attaching to the solder pads - various manufacturers produce suitable wire including CT, ESU, TCS and Zimo.

All current CT sound decoders drive 8 ohm speakers, rather than the more common 100 ohm equivalents like LokSound 3.5 use (or 4 ohm on LokSound V4). In these scales there really isn't much to choose between 4ohm, 8ohm and 100ohm in terms of evident sound quality and there is a wide choice of speakers. There are other factors which influence the volume and the quality far more, such as speaker location and obtaining a true seal.

Examples of some smaller 8 Ohm Speakers available

A selection of smaller 8 Ohm Speakers

I did a quick comparison between the tiny 'sugar cube' speaker from CT against a 23mm round speaker on a LokSound setup and although the 23mm speaker sounded a little 'fuller' (as you'd expect), the sugar cube actually produced a much higher volume and sounded a little crisper. Sound quality is of course a matter of preference anyway, and with there being so little real-world difference between them, I tend to choose smaller speakers simply because they are easier to squeeze into models.

CT's 'Sugar Cube' speaker

CT's tiny but awesome 'Sugar Cube' 8 Ohm Speaker with resonator attached

All the usual rules apply for decoder installation: protect against short-circuits; allow sufficient breathing space for heat dissipation; do not cover in electrical tape etc. I admit though, that for practical reasons I often use a thin strip of electical tape just to give the decoder some stability, but do use it sparingly! I also tend to stick the decoder into position using some sort of thick double-sided pad, again ensuring that the exposed parts are not 'stuck' by it. I try to position the decoder so that the extra solder pads are accessible in case I want to wire any more functions later on - saves me unsticking the decoder.

The speaker is attached in a similar fashion using a double-sided pad somewhere. Preferably away from the decoder a little - remember that all speakers contain magnets, so you don't want that too close to the decoder's flash memory chips. Having said that, this is just a theory, and I've not actually seen any evidence of damaged caused this way. Wiring to the decoder is simple - 2 wires attached to the decoder's 2 brown wires.

Where possible, I try to fit the decoder and the speaker inside the loco's body rather than wiring it through to a tender. This is surprisingly often straight-forward, despite the lack of space, although the first thing I do in a new install is to remove all the gubbins such as the TV interference capacitor, and all DCC sockets etc., just to free up the maximum possible space before I decide where everything goes.

Playing with CVs

The CVs (Configuration Variables) of a DCC decoder are essentially a collection of parameters/variables which influence how the decoder behaves. Many CVs are standard, in that they follow the guidelines set out by the NMRA organisation. However, any fairly advanced decoder will have in addition to this a whole bunch of CVs specific to itself. This is especially true for sound decoders, which provide far more functionality than was ever designed for when NMRA came up with the original standards.

The SL51-4 is no exception, having 177 CVs for various purposes. In fact, it is such a big topic that I'm not going to go into detail here. Instead you can read my commentary on that subject in the Understanding SL51-4 CVs document.

How the SL51-4 Sound-Set is Used

Rather than go into a detailed discussion here on how the Sound-Set is put together, and how it is used in the loco at 'play' time, another document is available to help you with that aspect: Tips on Sound-Sets for CT Revision-4 Decoders

Baby Brothers - the SL75 and SL76

Just by means of a final word on the CT decoders for now, I have also used the SL75, GE75 and SL76 decoders in many tighter installations, including N gauge. It took me a while to grow brave enough to attempt this, after the original disastrous attempt with the SL74. Fire extinguisher at the ready please! Fear totally unfounded of course.

The SL75 is featured almost to the same spec as the SL51-4, providing the same 16 bit, 3-channel sound and an almost identical set of CVs. The only notable differences are in the output capability for the motor and the fact that there are only 4 function outputs, rather than the 8 of the SL51-4. The lower power ability means that you probably wont be able to reliably run this decoder on the biggest OO motors, but it is still quite capable. Rather than overloading and frying itself, it tends to just ignore requests for things that it cannot handle, which is obviously much better than fireworks!

For example, if you have the loco running around at full steam, with sound activated, it might occasionally decide to ignore a request to play the whistle - so all in all, not a big deal. In fact, the LokSound Micro decoders suffer exactly the same problem. It is just a fact that you are pushing them a little harder than they can cope with, but unlike the SL74, it wont damage anything.

The size, of course, is the BIG little factor in the SL75! It truly is astoundingly small...

The SL76 is a replacement for the SL75, and chops almost a third off the dimensions in every direction, making it even easier to install in many N gauge locos. Spec is almost the same again, but with slightly higher motor output. Awesome!


CT SL75 Decoder

CT's SL75 Combi-Decoder

The only other point of note on the SL75 is in its programmability... on my Lenz's programming track, I cannot read back any CVs from the decoder, including the current loco address. This has something to do with the strength of the acknowledgement from the decoder, but despite best attempts I've never had it working. There is a CV which allows you to tweak this ACK signal strength, but it does not appear to help my Lenz system. Interesting I have exactly the same problem with the bog-standard Hornby decoders. The good news is that it does accept direct programming of CVs, so it is easy to tweak it - just not to read-back the data. Read-back works perfectly via a SPROG II or SPROG 3 though, so perhaps it is actually the Lenz system that's at fault.

Sound-Only Decoder - GE75

Worthy of brief mention here is CT's GE75 decoder, which is almost identical to the SL75 in spec, dimensions and functionality, except that it has no motor control output and only has 2 function outputs instead of 4.

Being so similar to the SL75 it enjoys all of the good stuff that the whole Rev4 range from CT gives, and programming works the same way too. It makes sense to use the GE75 in dummy cars to get sound+lights, and works out cheaper - which is great if you have already invested in standard DCC decoders for your motor control.

The GE75 is supplied with just the pickup wires and speaker wires attached, so you have to add your own wires to solder pads to make use of the 2 functions, or the reed-switch inputs (for synchronising steam chuffs with wheel speed). The photos below show the GE75 firstly as-supplied, and then with the necessary wires added for the 2 functions (white for the 1st function, yellow for the 2nd function and blue for common positive)...

Expect the GE76 to enter the market some time in 2012 (August?), which will no doubt replace the GE75, and fit the same footprint as the SL76.

CT GE75 Decoder as supplied Reverse side of CT GE75 Decoder as supplied

CT GE75 Decoder function wires added Reverse side of CT GE75 Decoder function wires added


Please note that these guides are provided as useful resources for you, as-is. YouChoos cannot be held responsible for errors in the information, or for any damage caused to your models or equipment if you choose to follow any of the steps detailed here.

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