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Bog Snorkeller
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Discussion Starter · #1 ·
I have recently purchased a Hornby 101 three car green DMU, DCC ready and fitted it with a Bachmann 3 function 8 pin decoder.

This one;-


My problem is when I drive it into a station for example, I reduce speed accordingly and then slowly come to a stand still, as you do. All is well, great slow speed, and speed reduction until milli-seconds prior to the actual stop, when suddenly it lurches or jumps forward violently a centimetre or so and then stops. The only way it will not lurch is if I drive in at an unrealistically high speed and then stop it dead.

It happens wherever I bring the DMU to a halt - signal, station or whatever, forwards or in reverse, but none of my other loco's have this frustratingly annoying habit. It seems like an unexpected power surge but I can't work out why.

Anyone any ideas as to what causes this?

My controller is a 5amp NCE Procab.

Thanks

Mike.
 

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Hi Mike

This problem could well be caused by a suppressor capacitor fitted across your loco's motor. These little capacitors are only needed for DC operation and can interfere with the DCC signal. Try snipping one end of the capacitors leads to disconnect it and see if the problem goes away.

Regards
 

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hi , i had same problem with blue &grey version & baccy chip .i swapped mine for a lenz silver and all is ok , apparently hornby,s own chip will do the job aswell
billy
 

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I tried to standardise on Bachmann 3 function decoders for all my stock, but have always had trouble using Bachmann decoders with Hornby. I now use Hornby 4 function decoders with Hornby motors and all works fine. I have a Gaugemaster DCC controller, and I find I have to turn the controller to quite high numbers before the Hornby motors will start, where as Bachmann and Heljan fitted with Bachmann decoders start at very low speeds. Hope this is of help.
 

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Bog Snorkeller
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QUOTE (Dr Diesel @ 1 Jan 2009, 22:48) <{POST_SNAPBACK}>If howzatts suggestion does not work
Try turning off the bemf on the chip

Problem solved (well maybe) as today it died - dead as the proverbial dodo, put it on the tester and its totally and utterly dead, so it looks like a new decoder from TCS is in the offing.
 

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I had this problem with a Bachmann 4MT after I reset it to factory defaults. I was told that it's due to the motor characteristics set in the CVs not matcging the loco's actual motor. I was advised to alter CV 54 to about 12 and CV 55 to about 60, and that did the trick.
 

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Just another modeller
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***The most common causes of this problem have already been covered.

They are (1) The Hornby decoder itself (Still not very good but getting a little better with each generation) and (2) The suppression capacitors. These should always be removed without fail for best performance. They interfere with the backEMF by filtering the communication between motor and decoder which must be direct for back EMF to work well.

Simply cut out the capacitors or remove with a soldering Iron if SMT / surface mount, and slow running problems always disappear.

Richard
 

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Bog Snorkeller
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Discussion Starter · #9 ·
Thanks Richard but..... the decoder is a Bachmann 3 function one fitted to a DCC ready unit (I won't touch Hornby electronic gadgetry at any price) and there are no caps fitted - well not that I can find anyway, certainly not on top of the motor where you would expect them to be. I also have a DCC on board Bachmann 4MT with the same decoder and two caps fitted both of which I snipped off one leg as suggested elsewhere and it did improve the running.

One question that always baffles me is why they fit capacitors to DCC on board loco's, is it in case the purchaser wants to run it DC (strange purchase if it is) or, as I suspect, is it some electrical regulation somewhere?
 

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Bog Snorkeller
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Discussion Starter · #10 ·
Problems, problems, problems. I finally decided to take it apart and guess what... I found both the problem and the capacitor. As you can see in the photo below I have a broken feed wire at 'A'



This however seems to have given me another problem as it has broken flush with the other square(ish) fitting at 'B' and hence cannot be soldered back on.

Does anyone know what 'B' is? Both feed wires appear to pass through it but there are no markings on it and no indication of what it is.

What would happen if I removed 'B' and soldered the feed wires directly to the terminals shown at 'C'
If this is possible, I would of course clip out the capacitor prior to re-soldering.

Thanks for any help.

Mike
 

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Bog Snorkeller
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Discussion Starter · #12 ·
QUOTE (Brian Considine @ 16 Jan 2009, 18:11) <{POST_SNAPBACK}>Looks like a choke to me - bypass it & run the just motor on DC gently before applying full power.

Hope this helps.

Thanks Brian, but what does a choke do? am I likely to damage the motor now or in the future if I do bypass it?

Mike
 

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QUOTE what does a choke do?
It is a counter measure for Electro Magnetic Interference which is also known as Radio Frequency Interference. This is the stuff which causes crackles on your radio or "snow" on your TV picture (at least it did in days of yore). It is used in combination with capacitors to create a circuit which eliminates the RFI which is generated by the locomotive motor.

The performance of these circuits is highly dependent on the frequencies involved and these in turn are dependent on how the motor is driven. For DCC Ready Locomotives, the circuit is designed for motors driven by a variety of DC waveforms from the unsmoothed semi sinusoidal output of a basic variable resistance transformer to controllers with a more sophisticated output.

When a DCC decoder is used to drive the motor, the conditions for which the original circuit were designed no longer hold. The retention of the original RFI suppression circuit has been a matter of hot debate in DCC circles. Some find that leaving the circuit in place causes no harm, but at the first sign of odd behaviour, this circuit is the first to be removed which is why many people remove them as a matter of course when fitting a decoder.

Hope this helps.

David
 
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