Hi Jerry, Richard,

I'll take some pictures when I can, but it's probably clearer to see how it goes together in the sketch attached below.

Some of the bits are truly tiny, and soldering to the decoder is certainly not for the feint-hearted. At least on the DCX77 the solder pads are at the edge.

I feel slightly nervous that I might be encouraging people to destroy their expensive decoders! They don't necessarily blow up when you ruin them... they can just make a slight smell and stop working. Yes, this is the voice of bitter experience - you don't get much in the way of pyrotechnics for your 30 quid!

The procedure I followed was:

• Read Nigel's article on the 2mm Association website thoroughly (it includes a circuit diagram).
• Follow his example and file a pointy soldering iron bit to an even smaller point. It has to be at least as narrow as the pads on the decoder!!!
• Tape the decoder securely to the desk to work on it
• Tin the pad from the iron (At first I tried putting a crumb of solder paste on the pad with a cocktail stick, then heating it. While this "worked", it spattered everywhere and needed careful cleaning up. Better not to need to do this.)
• Tin the wires. These were spare bits of wire I'd trimmed off other decoders in the past and saved. Leave them long for now so you can thread them more easily through the loco - only trim to length when the decoder and stay-alive unit are physically installed and it is time to connect them permanently.
• Hold a wire in position with tweezers (as close to the end as possible) and touch with the iron. This is the hardest bit. You have to hold the iron a comparatively long way from the very tiny tip. I have reasonably steady hands, but it looked through my magnifier like I was shaking violently when I was doing this. The "standard" colours for these wires are blue for the common positive (some decoders have this already, but the tiny CT ones don't) and black with white stripes for the negative.
• Check VERY carefully with STRONG magnification that you haven't made a solder bridge from the pad to any other component.
• Assemble the stay alive components, and check several times that the polarities are correct
• TEST the decoder still works before connecting the wires to the stay-alive circuit.
• Install the decoder in the loco and test again.
• Assemble the stay-alive circuit, and chek at least 3 times against a circuit diagram that the polarity of the components is correct.
• Connect the stay alive and test yet again.
• Crack open the Rizla paper and show off to your friends.

Some additional points to note:

• Tantalum capacitors are polarised. SMD capacitors have a stripe to identify the positive pole. (The opposite of non-SMD Electrolytic capacitors which have a stripe to denote the negative lead - just to confuse!)
  • If you connect them the wrong way round, tantalum caps can explode more spectacularly than a decoder... be warned!
  • Make sure the capacitors are connected in parallel. You get the sum of the capacitance this way. If you connect them in series, the total capacitance is reduced, but the voltage increases, and will kill your decoder.
• The Mini-MELF diodes have a band at the cathode end (same as regular diodes) and this is colour coded according to what type of diode it is. Schottky diodes have a grey-ish blue band, and Zener diodes have a blue-ish grey band. Something else to get confused about!
• If you forget to disable DC running on the decoder before you connect the stay-alive, you'll have to un-solder it before you are able to alter any CVs.
• In my experience, CT Elektronic decoders don't like high track voltages anyway, but it is worth checking that your DCC system is not putting more than 16v on the track. (If you're reading this and want to build one of these for a larger scale, I would recommend using 25v capacitors - they're bigger, but you'll have room!)

The components I used are as follows:

Capacitors: 16v 220uF Tantalum chips, case D - also known as 7343 (metric) and 2917 (imperial)

10PCS 7343 Case D 220uF 16V 7.3x4.3x3mm SMD SMT Tantalum Chip Capacitors | eBay

Zener Diode (to prevent over-voltage to the capacitors): 16v (to match the capacitor voltage) 1W

20x ZMD16-DIO Diode Zener 1W 16V SMD MiniMELF tape ZMD16 DIOTEC SEMICONDUCTOR | eBay

Schottky Diode (to protect the capacitors from reverse voltages - I chose Schottky because the voltage drop across the diode when the caps are discharging through it is significantly lower): 40v (not critical) 1A (needs to be at least as high as the rating of the decoder)

20x SGL1-40-DIO Diode Schottky rectifying 40V 1A MiniMELF SGL1-40

Carbon film Resistor (to prevent the DCC Command Station seeing a charging capacitor as a short circuit): 100R, 0.25W

50x SMDMM0204-100R Resistor carbon film SMD 0204 minimelf 100Ω 0.25W ±1% | eBay

Have fun if you decide to make one, but don't blame me if your loco blows up!

Any help?