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2mm Coal Tank test build


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Hi,

 

I've added lead to the side tanks and some in the bunker, but none in the smokebox yet as I'm going to try some tungsten putty when it arrives from China. The loco now weighs 33g and it pulled a load of five LMS coaches weighing 103g without difficulty. That equates to 20 -25 of my mineral wagons, so I'd say the pulling power of the loco is already enough for prototypical train lengths. The running has improved after the weight was added too.

 

Nig H

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  • 2 weeks later...

After my last progress report I took a break of several weeks from all things Coal Tank. Having been treated to the sight of Nigel's superlative model "in the flesh" last weekend, however, I have been inspired to get moving again, and below are some pictures showing where I'm up to.

 

The chassis has been completed with the addition of the brake gear... and it even still runs!

One refinement to the mechanism I am considering is filling part of that huge empty bunker space with tantalum capacitors as a "stay alive" circuit... if I can solder the wires onto the decoder.

 

Nigel's warning about needing to bend the brake hanger wires down so that the coupling rods don't hit the brake pull rods was useful. Bizarrely, I only needed to lower them on the right hand side.

The brakes hangers for the middle wheels had to be attached to the frames ahead of the bearings for the gear axle and cranked to hold the brakes in the correct position. I hope this is clear in one of the photos.

 

The real fun and games came when adding the pull rods.

There are holes in the frames to attach the cross-rod for operating the pull rods. Unfortunately, these are behind the flanges of the rear wheels! This may be a feature of the etch-shrinking process, but left me scratching my head.

I thought about bending the shaft round the wheel flanges, but in the end settled on soldering it to the back of the downward protrusions from the main frames that had the holes in them.

Missing altogether from the kit are the cranks that connect said cross rod to the ends of the pull rods. The scrap etch box came to my rescue here, and I found some suitable little cranks left over from some long forgotten wagon chassis. Because the cross shaft is set further back than designed, the cranks look to lean back somewhat more than they should, but I think this it the most pragmatic compromise. Ideally the pull rods should be longer, but I wasn't up for making new ones or attempting to extend those in the kit... I consider myself lucky to have got away without mangling them as it is - they are so thin and fragile.

 

Now I am left with the problem that when I place the body on the chassis, the front brakes are squashed tight up against the guard irons. There should be a gap between them - on the real thing, the front sand pipes go down this gap. I'm not sure at this point whether to try and file the guard irons thinner, or cut them off and try to attach some new ones further forward on the frame extensions. I think the latter course of action would probably look best.

 

Speaking of the body, only a little progress has been made here. I cut the boiler tube to fit over the motor. A 5 thou shim was added at the smokebox end, with the rivetted wrapper fitted on top of that (top half only soldered at this stage). The wrapper was fitted protruding 8 thou. so that the front plate could fit inside it later. This wrapper, incidentally, is from a different shot-down LRM etch I've had in the gloat box for ages - I think two of them might come with a Bowen-Cooke tender! Anyway, after poking out all the rivets on the smokebox front plate, a horizontal bend has to be made under the bottom of where the smokebox door will be. The cylinders of these locos are inclined, so the front of the cylinder/valve chest needs to be perpendicular to them. In many photos of coal tanks there is a noticeable "crease" across the front, so it is worth replicating. Unfortunately, in these photos of mine, it doesn't seem to show up at all!

I attached the handrail to the smokebox front before fitting the front plate to the boiler tube. It was necessary to file some notches in the boiler tube to clear remains of the tails of the handrail knobs (association etched ones). I didn't want to file these completely flush in case they came adrift. For once I had thought ahead and filed the notches before adding the smokebox overlay...

Once the front plate was in place, the bottom part of the smokebox wrapper could be formed around the reverse curves and soldered in place. I found this bit particularly tricky to get right.

 

The smokebox door is my own turning. I started off with the N Brass one, spent ages thinning it, then decided it was still too big (it was covering the rivets) so I determined to replace it. I hope I have captured the subtle dished shape of the prototype, but in these very cruel pictures I can see that I haven't polished it enough! I have to say I'm a bit disappointed with all of the N Brass castings for the Coal Tank. Other castings I've bought seem much finer (a chimney I've recently acquired for my Radial tank, once polished, was first rate). The chimney and dome in the photos are just loosely balanced in place to give an impression. I think I am going to have a go at turning replacements for both of them. The chimney especially looks a bit portly, and the casting has some flaws in it (a chunk missing from the rim, corners missing from the base) that might be just as difficult to correct convincingly as turning a new one. The dome looks a bit lumpy too - but then I'm sure a real one after 60-odd years service would have its fair share of dints.

 

I'm basically at the stage of adding details now, which is where most of my loco projects seem to stall - the last 10% of construction seems to take 90% of the time. That said, not much of the first 90% of this one has been easy, but I'm really pleased with how it is coming along.

 

 

 

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Nick,

 

photo of LRM 4mm chassis attached. The cross shaft is behind the rear coupled wheel (P4 profile) so the lack of clearance in 2mm may be a combination of artwork reduction and wheel profile. However, the instructions do refer to a potential problem if using original profile 4mm Romford wheels, recommending the RP25 profile version for OO, so the clearance is obviously very small.

 

From memory, the short "drop link" was included in the 4mm etch, so should be on the 2mm version. I don't recall having to find a couple elsewhere.

 

post-1191-0-25770600-1498897257.jpg

 

IIRC, LRM do a wrapper etch for the Coal Engine (although not listed on the price list) which was a hangover from the George Norton Connoisseurs Choice range. Brassmasters did their early LNWR loco etches as a series of small component etches to mix and match the various loco kits so it could be one of theirs. However the BM locos were all of larger LNWR prototypes with bigger boilers.

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I spent a pleasant few hours this afternoon making a new chimney - turned using hand gravers and files in my watchmaker's lathe.

The attached photo shows the result in company with the rather corpulent casting it will replace.

post-14390-0-95315600-1498942444_thumb.jpg

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  • 3 weeks later...

You were right about the dome, Colin!

My coal tank now has a new turned one which I'm much happier with.

 

Those who subsctibe to the 2mm VAG may be aware I've been experimenting with DCC Stay Alive over the last couple of weeks.

I have managed to squeeze four 220uF tantalum chip capacitors into the empty bunker of the Coal Tank.

The capacitors are arranged in a block, sitting on a rectangle of thin PCB, which rests on the frames above the radial truck.

Once I'd added the necessary diodes and resistor, the block was wrapped in Kapton tape.

I cut away the bunker floor, for the capacitors poke up into the coal space. They will eventually be completely covered by the toolboxes and coal.

 

The running was good even before I added the capacitors, but there were occasional stalls - inevitable with a short, light loco.

Even this comparatively tiny amount of stay-alive has taken the performance up another notch (once Nigel Cliffe helped me by revealing the secret that the decoder needs to have DC running disabled).

This is how I want all my locos to perform.

 

Here's a little video showing it doing its party trick:

https://youtu.be/NcxEq_deKBI

You may notice I've still got some bits of insulating tape preventing the front guard irons and brakes from shorting.

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Excellent stuff Nick. Any chance of some pictures of the stay alive set up. Nigel has regailed me with the advantages of fitting capacitors on a number of occasions and I've resisted, mainly due to my lack of understanding and confidence re. soldering to the tiny pads on the chips.

The very impressive performance of your coal tank makes me want to fit stay alive to the Bath shunters.

 

Jerry

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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!

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To add to Nick's instructions above...

 

I practise small soldering on little bits of PCB before any micro-tasks.  Cut a piece which is decoder sized, and scrap/scratch/file or otherwise abuse the PCB to create similar sized solder pads to those on the actual decoder.  Tape it down (or fix in the same way as you plan for the decoder), and do some practise soldering.  When happy, replace scrap of PCB with real decoder. 

 

Having some form of block as a rest for the heel of your hand whilst holding a soldering iron helps no end with stability.  A few blocks of wood, or a very thick book does it. Any block has to be stable (screw or clamp it down, with any clamp not in your way).  Make the effort to get the workplace right and very difficult soldering jobs get a little easier.   Again, practise on scraps until any arm/hand supports are working correctly for you. 

 

Of the small CT decoders, I think the DCX76 series are the easiest to access the solder pads.  The DCX75 is the hardest, so use a different decoder !  

 

Keep your filed-down soldering iron tip for these jobs only.   Filed-down tips don't last long in my experience, so keep them back for just the special job. 

 

I tin multi-strand wires, then, before fitting to the decoder, I cut back the tinned end to "very short".   Do any tinning just-before the job, don't do it the day before and expect the solder to flow wonderfully (it doesn't, I suspect there is a little bit of oxidation going on if the solder is left alone).

 

You can probably program a decoder with the combination of stay-alive and DC-enabled on the main line, and thus switch off the DC-running without resort to soldering iron.  DC-running is completely incompatible with stay-alives on CT decoders.    That said, having somewhere in the circuit that you can disconnect the stay-alive does make it quicker/easier to deal with any subsequent issues. 

 

 

 

Nigel

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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.

 

No, the series capacitors won't kill the decoder.    It just alters the total capacitance (lowers it, which isn't good) and increases the possible operating voltage.  

The Zener diode in the circuit limits the total voltage in the system.

 

You can wire low-voltage capacitors in series to increase the operating voltages available, so one can wire several 2.7v rated "gold caps" together to get up to 16v running, and 30-second run-times, but gold-caps won't fit into small 2mm locos. 

 

 

Note that the use of the Tantalums and Zener's right on their voltage limits is against "best practise" guidelines from every component maker, they usually recommend working at only 70% (or thereabouts) of the rated values.  So reliability can't be guaranteed.  

 

 

 

- Nigel

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You were right about the dome, Colin!

My coal tank now has a new turned one which I'm much happier with.

 

Those who subsctibe to the 2mm VAG may be aware I've been experimenting with DCC Stay Alive over the last couple of weeks.

I have managed to squeeze four 220uF tantalum chip capacitors into the empty bunker of the Coal Tank.

The capacitors are arranged in a block, sitting on a rectangle of thin PCB, which rests on the frames above the radial truck.

Once I'd added the necessary diodes and resistor, the block was wrapped in Kapton tape.

I cut away the bunker floor, for the capacitors poke up into the coal space. They will eventually be completely covered by the toolboxes and coal.

 

The running was good even before I added the capacitors, but there were occasional stalls - inevitable with a short, light loco.

Even this comparatively tiny amount of stay-alive has taken the performance up another notch (once Nigel Cliffe helped me by revealing the secret that the decoder needs to have DC running disabled).

This is how I want all my locos to perform.

 

Here's a little video showing it doing its party trick:

https://youtu.be/NcxEq_deKBI

You may notice I've still got some bits of insulating tape preventing the front guard irons and brakes from shorting.

Hi Nick,

 

The Craftsmanship/Clever button does not do justice to what you have achieved with the Coal Tank.  That is one heck of a model (and one heck of a speck of dirt!).

 

All the best,

 

Colin

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I have some photos of the stay-alive capacitor install I have done on my Dapol Terriers (2mm Scale Association etched replacement chassis). I too followed Nigel's blog, and purchased the components from the german website he mentioned. I used the ceramic capacitors, but after reports that their capacitance gets lower with voltage, and that tantalum capacitors are better, I tried making a similar setup with them instead. Unfortunately, they ended up just slightly larger overall, and I just could not clip the body firmly to the footplate, so the ceramics are still in use.

 

Starting with the components:

  • reel of Kapton tape (very thin, transparent, heat resistant and insulating sticky tape)
  • reel of fine enameled copper wire (as used for winding transformers or motors)
  • capacitors (look like liquorice allsorts)
  • diodes (one schottky, one zener, look like tin cans)
  • resistor (labelled 1000 for 1kohm)

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The capacitors were soldered together in pairs (end to end) then combined to make the block of 8. A small piece of Kapton tape was put on one end to insulate the central part from a length of thin copper wire (enamel removed by scraping) that was soldered along one side of the block, run across the tape and then soldered up the other side. The free end will eventually connect to the negative pad on the decoder.

 

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The block was than wrapped in Kapton tape, except along the top. The Zener diode was soldered across one capacitor, and a short length of the wire attaches the centre of the block to the Schottky diode, with the resistor in parallel with it. A wire run from the other end of the diode/resistor combination will run to the positive pad on the decoder.

 

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The diode/resistor combination was then moved to one side of the block, and more Kapton tape wrapped everything up tidy.

 

post-11458-0-87890600-1500761068_thumb.jpg

 

The two free leads from the block were then carefully soldered to the pads on the decoder. The decoder was already protected by Kapton tape, so two small 'windows' were cut into it with a new scalpel blade first. Polarity was checked and double-checked and triple checked before applying power.

 

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The enameled copper wire leads could then be coiled up tight so the block could stand right behind the decoder in the cab.

 

post-11458-0-13390900-1500761406_thumb.jpg

 

A tight fit, but it works.

 

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Ian,   its not clear how many Tantalums you tried to match the Ceramics.  From my experiments, I'd expect two Tantalums to fit within the space of the eight Ceramics (440uF vs 800uF), and the performance to be slightly in favour of the two Tantalums (due to the alarmingly bad performance curve for the Ceramics).   If you can get three (660uF), then the Tantalums will be better.   

 

- Nigel

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