Converting Hornby Dublo 2-6-4 to DCC

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Matthew
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Converting Hornby Dublo 2-6-4 to DCC

Postby Matthew » Sun Mar 31, 2013 8:31 pm

Hello,

I've converted my Hornby Dublo 2-6-4 to run on DCC. It worked absolutely fine in DC, but under DCC control it barely crawls along. When I tested the loco on DC it drew just under half an Amp when running. I'm using a TCS M1 decoder which is rated for 1 Amp, so it should be OK. I've tried tinkering with the various motor control CVs, but to no avail. Does anyone have any experience on this please?

Many thanks.


Matthew

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

Postby Bigmet » Mon Apr 01, 2013 7:16 am

Remove the decoder, and measure the stall current on DC. The 'crawl along' symptom is probably caused by insufficient current supply. It may 'only' draw 0.5 amp when running, but probably needs more current when starting to get the motor turning at any speed. Consider a new motor magnet too, unless that has already been attended to.

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

Postby Matthew » Mon Apr 01, 2013 2:38 pm

Thanks - I'll check it out. Presumably I need to measure the current draw at from 0V upwards? Yes, new "Neo" magnet just been fitted.

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

Postby K9-70 » Mon Apr 01, 2013 9:24 pm

The brass screw holding the armature in place may be over tighten.
Back the screw off a eighth/ quarter of a turn until the armature turns freely.
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Bigmet
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Bigmet » Tue Apr 02, 2013 9:41 am

Matthew wrote:...Presumably I need to measure the current draw at from 0V upwards? ...

No, just the one full voltage measurement is required. With the motor stalled, apply 12V and take the current draw measurement. That's what the decoder 'sees' as the current that has to be supplied when trying to start the loco from rest, motor in the stalled condition.

A DCC decoder supplies motor power in pulses of full voltage - nominally 12V - switching this on and off very quickly. With many older motor designs the decoder can easily run into the limits of its current output, and then you get exactly what you described in your opening post. Instead of gaining speed as it does on DC, it just 'bogs down' and crawls along because of the current limitation from the decoder; this is quite a distinctive aspect of DCC. DCC for OO and smaller scales was designed around the much better motor efficiency of can motors and similar designs in order to get the decoders small enough to fit easily.

Every DC motor draws a pretty fierce starting current when stationary, relative to its running draw, that's basic to ohms law. But on a modern can motor the moment it starts moving the back EMF effect which limits the current draw is very effective thanks to a strong magnetic field and a very close fitting armature. That limits the high current the decoder has to supply to a brief spike on the first pulse which gets the motor turning.

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

Postby Matthew » Sat Apr 06, 2013 10:26 pm

Hello again, apologies for the late response - I've been away for a few days.

As far as I can tell (using a DVM connected in series with the DC supply), the loco draws around 600 mA on start up, so presumably a 1A decoder should be OK. However, as the loco didn't work in DCC, is there a CV setting which could be influencing things perhaps?

I appreciate that diagnosing these things remotely is almost impossible, but I'm just hoping that someone has (a) experienced a similar problem and (b) found a cure!

Thanks again,


Matthew

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

Postby Matthew » Sat Apr 06, 2013 10:29 pm

K9-70,

The loco works fine in DC, so I don't think that's an issue.


Matthew

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

Postby Bufferstop » Sat Apr 06, 2013 11:18 pm

Mathew, the current you see indicated as the loco starts is an adjusted average, the maximum current at start up does not last for long enough for you to see, or the meter to register. You must keep the motor stalled for just long enough to see the maximum current drawn then cut the power. If the stalled current is within the capacity of the decoder, take a look at the setup of your motor. Early versions of that model had a screw accessible through a hole in the bunker, which changed the magnetic characteristics of the motor. I think it was included to overcome a problem of controlability versus haulage. It was later omitted and the motors were set at a compromise. It may be worth experimenting with the position of the magnet.
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Bigmet
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Bigmet » Sun Apr 07, 2013 9:32 am

Matthew wrote:...the loco draws around 600 mA on start up, so presumably a 1A decoder should be OK...

Did you follow this procedure? With the set up you have used, leave the DVM probe not in contact with the rail so there is no circuit. Set the controller to full output. Hold the loco on the track, or even better the armature, so there is no way the motor can turn. Then with your free hand complete the circuit by bringing the probe into contact with the rail, and take the current reading, only needs a second or so for the meter to stabilise.

That current, at a full 12V, is what the decoder has to output when the motor is to be started. I strongly suspect if measured that way you will see a reading more like 1.5A based on past experience with H-D motors.

The cure is a decoder with larger sustained current capacity, I would be looking at something with 2A output from Zimo in your shoes. (Well, actually I wouldn't: if my layout needed a BR Std 4MTT then the Bachmann model with a Lenz standard would be what I would choose; that's a 'horses for courses' decision that might not suit you, but the simple fact is that DCC is much easier with modern efficient can motors.)

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

Postby Matthew » Sun Apr 07, 2013 8:15 pm

Hello again,

Apologies for not understanding how to measure "stall current" and thanks for making this clear. I have now measured this and - according to the DVM I'm using at least - it's 900 mA. Still less than the 1.1 Amp the decoder's rated at, but probably close enough to cause a problem (especially if the DVM reading's not all that accurate). Unless there is some CV trickery to be applied, maybe I should get a higher rated Zimo decoder as suggested.

I fully appreciate your comment regarding the worth or otherwise of trying to convert old locos to DCC. As you will see from my layout (www.youtube.com/fiskersham), all my locos are recent. It's just that I have two 100% pristine Wrenn's and I'd really like to put them to good use if I possibly can. The Hornby 2-6-4's being used as a bit of a test bed for this!

Thanks again,


Matthew

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

Postby Bufferstop » Sun Apr 07, 2013 8:46 pm

I would say that it's well worth persevering with the HD as it and the two Wrenns should still have years of good service to give. Go for higher rated decoders. Go for decoders recommended for use with 0 gauge locos. As has been said DCC as designed around more modern motors, watchmaker jobs rather than 20th Century heavy engineering.
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Bigmet
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Re: Converting Hornby Dublo 2-6-4 to DCC

Postby Bigmet » Mon Apr 08, 2013 9:52 am

Definitely needs a bigger output decoder, that's close enough to maxing out a 1A continuous output decoder. I would want 50% margin, a 1.5A continuous type as a minimum. These typically are premium decoders and come with a good range of adjustment; output frequency may well be what needs adjustment for better response from these older motor designs.

May as well test the Wrenn locos for 12V stall current as well while you are at it, and also give them a running test on DC, in case there is bad news lurking there. My limited experience of Wrenn is that their motors were not a patch on H-D. H-D was a system intended for operation by children, as such it was designed to be reliable. The motors would draw a hefty current, which the H-D controllers were up to supplying; and the build quality - weight of metal in the motor - was such that the motors would only get warm, and keep going.

Wrenn was aimed at adult collectors; most had no intention of even taking it out of the box, let alone have a layout to run it on. As such the motor was a skimpy thing that would draw a similar current to H-D when starting, and then quickly overheat and slow down. They were easy to replace with something better like the Romford Bulldog or MWOO5, and that's what I would do in the long ago when using Wrenn chassis as a method of powering a whitemetal body. No decoder can overcome the limitations of a poor motor design...

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

Postby Matthew » Mon Apr 08, 2013 12:55 pm

Bigmet / Bufferstop,

Many thanks indeed for taking the trouble to help. I'll do all recommended and let you know the outcome! Only downside I suspect will be that I won't be able to fit sound (my ultimate aim) as most sound decoders are 1.2A maximum as far as I can see.

Thanks again,


Matthew

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

Postby Matthew » Tue Apr 09, 2013 9:46 pm

Gentlemen,

Some progress to report! I was getting nowhere with the Dublo 2-6-4 loco, so I decided to give the Wrenn a go. Its stall current was the same as the Dublo (ie 900 mA), but it seems to run OK with the TCS M1 decoder. However, it's very borderline and it needs a nudge half the time to get it going! I suspect that I'm using the wrong decoder. The M1 has a continuous current of 1.0 Amps whereas the T1 has a continuous output of 1.3 Amps, so I think I'll give that one a try.

Watch this space.

Thanks again,


Matthew

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

Postby Bigmet » Wed Apr 10, 2013 8:15 pm

Do report how you get on, as it is helpful for anyone else who tries this sort of thing.


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