Converting Hornby Dublo 2-6-4 to DCC

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Matthew
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Fri Apr 12, 2013 5:32 pm

Hello again,

An interesting phenomenon to report.

First of all, whilst it's obviously quite old, the Wrenn is absolutely mint and hasn't been run at all. So, on the advice of several parties, I ran it in for a while in both directions. Having done that, I re-set all of the CVs on the TCS M1 decoder to default and thgen put the loco on the layout. It runs very well except that it stalls on every single point and then pulls away again - even when moving at speed! Before you ask, I've cleaned the points thoroughly and there's not a mark on the wheels. If I run the loco under DCC on a static line it works wonderfully.

Any thoughts? Is there a CV value which is dropping the voltage to the motor if there's a hiccough in the signal from the controller? I'm using a Dynamis by the way.

Thanks,


Matthew

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poliss
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby poliss » Fri Apr 12, 2013 5:52 pm

It's not the CVs. It could be that it's shorting on the frog. Are you using live or dead frogs? Are you using insulated rail joiners? Have you checked the back to back distance between the wheels? Should be 14.4mm +/- 0.05mm. Are the wheel tyres wider than modern ones? How big are the flanges? Possible that the wheels are being lifted as they are going through the points.

krusty
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby krusty » Fri Apr 12, 2013 6:43 pm

It's probably a case of the wide treaded tyres bridging the insulation across the vee of the points. Unless you put insulated fish plates on the vee rails older locos will suffer this stalling with DCC. Apologies if this isn't the case and you are familiar with this problem.
Krusty.

Matthew
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Fri Apr 12, 2013 8:46 pm

Gentlemen,

Thanks for your (prompt!) input. I'm using basic Peco Setrack points which I think are electrofrog (is that right?!). Nevertheless, given that the loco (Wrenn Duchess of Hamilton) stalls on every single point and they're all clean as a whistle, it certainly seems that there must be substance in what you're saying. I'll have a look tomorrow. Can I just clarify, though. Basically the theory is that the large loco wheels are shorting across the insulation at the head of the Vee in the points is that right? Sorry to ask, but I've never really understood the terminology of points!

Thanks again,


Matthew

krusty
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby krusty » Sat Apr 13, 2013 4:25 am

It is explained in a bit more depth here, click on "insulated frog points" from the menu on the left and scroll down a bit.

http://www.brian-lambert.co.uk/DCC.htm#Insulated frog points

As you are using set track points I'm not so sure this is your problem, as they are a little more generous with the insulation on set track.
Krusty.

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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby krusty » Sat Apr 13, 2013 4:41 am

Another thing just occurred to me, are you getting electrical pick up from two or more wheels on each side of the loco? If not power will be lost the second you run over the insulated area of the point.
Krusty.

Matthew
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Sat Apr 13, 2013 2:22 pm

Hello again,

I've checked out the loco again and you are absolutely right in your assessment. Basically, the stall comes at exactly the point at which either of the two pick-up wheels on the loco are directly on the Vee in the point (the loco picks up power / signal from the track via the front and rear main drive wheels). Indeed, at slower speeds it will stall twice on the same point - one time for each of the pick-up wheels. So, what's the solution? From what I can see, as I'm being told that it's not fixable through programming the CVs differently, the only solution is a mechanical one.

I measured the width of the main wheels on the Wrenn and it's 3.3 mm. For a newer loco (Bachmann Class 47) it's 2.8 mm. Clearly the 0.5 mm (20%) extra on the Wrenn is what's causing the problem. A bit drastic on a mint Wrenn I know, but maybe the wheels need to be reduced in width? Any better (ie less drastic!) ideas please?

Once again, thank you for your time and trouble in helping me with this - I reall apprecaiete it.


Matthew

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poliss
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby poliss » Sat Apr 13, 2013 2:38 pm

No need to do anything to the wheels. You just need to put insulated rail joiners on the rails from the frog 'V' as shown on Allan Gartner's page.
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Flashbang » Sat Apr 13, 2013 4:08 pm

Hi
Two IRJs installed on the end of the points Vee rails will overcome the problem.
The problem and the simple correction can be clearly see here... link to item Scroll down to the sub heading A problem found with some Insulated frog points... Krusty linked to the item, but that link doesn't take you to the item directly.

But you seem to have another problem!....
Assuming your old loco wheel treads are able to bridge the two metal rails at their closest place right at the plastic frog, your DCC system should immediately see the short circuit occurring and immediately shut down the system and provide some sort of an indication of the short circuit being detected.
The failure of this to occur and as you say the loco just stalls and moves off again is an indication that you're DCC power feeding is inadequate!
Remove all locos and power up the DCC system. Then apply short circuits across the running rails (Coin on edge is idea) around the layout at many places and every time the coin shorts the rails together the DCC system MUST trip out. Failure to trip should be investigated and the cause corrected ASAP.

If you rely on the rails and the metal rail joiners (Fishplates) to pass DCC power and data rail to rail then its likely you have one or more joiners that have become high resistance. If you use a DCC bus pair of wires then its likely the actual bus wires are too small or there is a high resistance joint somewhere along their length.
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Matthew
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Sun Apr 14, 2013 5:14 pm

Hello FlashBang,

I'm not sure I understand this anymore! Firstly, the point that you make that a short should cause a trip on the controller is a good one. However, I can report that the Dynamis trips out immediately if there's a short circuit. Secondly, I put insulating tape over the (allegedly) offending sections of the Vee on a couple of points and the loco still stalls! On the one hand that sort of explains why the controller isn't tripping out - it probably isn't a short-circuit - but on the other hand what on earth is happening?!

Does anyone have any experience of this please?

Thanks,

Matthew

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poliss
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby poliss » Sun Apr 14, 2013 5:28 pm

You need insulating rail joiners at the end of the rails. By putting insulating tape on the frog you are lifting the loco wheels off the rails leaving the loco with no power.

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Flashbang
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Flashbang » Sun Apr 14, 2013 7:25 pm

Hi
Stalling and shorting are two different things and as you were saying it was stalling I made the assumption that the DCC system was not shutting down and not showing a short circuit that was occurring.

If it is shorting the system then its highly likely its the wheels either being too wide for the two rails at the frog or its also possible that the wheels are not to the correct "OO" Back to Back distance of 14.4mm +/-0.05mm. Two IRJs will overcome this.

So if you now have two IRJs fitted onto the points vee rail ends does the loco now pass through the point correctly or Stall or Short the system?

If its stalling then I would be looking at the point itself. It must be laid flat and level both along its length and across too. With power off use a the edge of ideally a metal 12inch rule to check everything is level by laying the rules edge along the top of points rails. Any low spots can be packed with thin card shims to correct as necessary.
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Bufferstop
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Bufferstop » Sun Apr 14, 2013 9:22 pm

Hi - if I have followed your steps correctly I think you have shown that it's stalling rather than shorting, if the ruler across the railheads doesn't show any high or low spots, look for tight back to back dimension on the drivers or deep flanges lifting the wheels just enough to lose contact. If this is the problem check that the pick ups are bearing on all of the wheels. Stalling when one wheel loses contact often points to the other wheel(s) on that side not being contacted by the pickups.
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Matthew
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Sun Apr 21, 2013 6:38 pm

Gentlemen,

Apologies for the radio silence - I've been away awhile. Once again, many thanks for your tireless efforts on someone else's behalf.

I realise that doing these things remotely is difficult and you're relying on someone else to explain things accurately and in sufficient detail in order to be able to help and I fear that's not happening here as I don't fully understand it all myself! However, the latest situation is as follows:

Firstly, just to remind you, the loco doesn't short the controller out on the points. Instead it stops suddenly and then pulls away again. It does this on every point - not just one or two. Secondly, if I get a real short circuit, the Dynamis shuts down the track immediately, so I don't believe that the effect is down to a shorting across the Vee. From what I can see the wheel separation is correct and, given that this is an "out of the box" condition loco, that''s to be expected.

The flanges are certainly deeper that current models so I'm trying to see if that is having an impact, but it's hard to see what's going on. I'll keep playing around and let you know.

Thanks again,


Matthew

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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Bigmet » Sun Apr 21, 2013 7:53 pm

What is happening is very simple. The plastic point crossing is usually slightly high relative to the rails. The first driven pick up wheel onto the plastic point crossing causes the chassis to tilt slightly, lifting the only other pick up wheel that side out of contact with the rail. The decoder thus sees no power and most are set to a default 'time out' where they power off in this circumstance. Then the loco rocks slightly and the one pick up wheel that can come down on the rail does so, and the loco moves. The process typically repeats when the second pick up wheel hits the insulated crossing.

Live crossing points are vastly superior in this respect. More work to install, and not available in set track. As with so many things in life, if you want better, it does entail extra work. Once tried though be warned, there is no going back.


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