Loss of sound when engine starts to move

[yerffoeg]

Okay I have run a few tests already.

First this is a Flying scotsman with a 3 pole tender drive. I have fitted a class a4 dcc in the engine with a stay alive and small 8 ohm speaker. The engine is linked to the tender with a small 3 pole connector. I guess the manufacturing date is around 1970s?

I have stripped and cleaned everything and run the motor in on dc.

The dcc is new.

Everything runs fine except after three chuffs the sound stops and will not restart until the power has been removed for some 30 seconds.

If I do not cause the train to move it will sit there all day making sounds. Push it around the track by hand and the sound stays. Start it in forward or reverse and within 5 seconds the sound is gone.

To me this looks like a software issue but it can't be. This is a volume dcc decoder from Hornby. Firmware revision is 1.4.

Any thoughts would be appreciated.

Is there a factory reset for the part?

[ColinB]

You don't say which DCC sound decoder it is, which will be of enormous help. To me it sounds like decoder is going into current limit mode. If it is a TTS decoder the current limit is 0.5 amps, a tender driven Ringfield motor draws about 0.6 to 0.8 amps on startup, possibly even more. It sounds like the decoder is shutting down due to over current. Usually when I get it wrong the decoder smokes but it sounds you have been very lucky.

[96RAF]

If the current draw is marginal then try to loco with the sound off. If it moves then you can assume Colin's prognosis is correct. To prove it do a motor stall current check with the decoder removed and the loco rigged for DC power. How to do this check using a suitable range multimeter is shown on Brian Lambert's web site.

[yerffoeg]

Oh dear...

Thanks for that. I will check the current draw.

Is there a current limit I can adjust? Looks like I might have a real problem with these three recorders I have bought. All for ring field motors.

If I convert to cd motors would that help?

I could try running it off the tracks ie with minimum load to see if that works.

Does Hornby offer a dcc decoder that will work with their engines?

Part no of decoder is R8107

[yerffoeg]

You have got it in one. Put rig on it side and jumped rail connections and it appears to run fine. But how to fix it?

[96RAF]

What we are trying to establish is the actual value of the current the motor is drawing particularly when at full load, i.e. stalled. The only way you can check that is by putting an ammeter (multimeter on DC Amps scale) in series with the motor, then apply a DC voltage and hold the loco from moving until you see the max amps drawn. That is the value you have to compare with a decoder spec to see if it can provide that amperage - generally TTS decoders will not run a ringfield motor in poor condition. Changing the motor innards for a CD tray motor can improve the situation, but you need the right CD motor, preferably 12v and if the loco needs it for clearance of tender or bogie centre wheels a slim motor as opposed to the normal 1/2" wide motor.

Note that some multimeters have a limited range of 200mA on DC amps range. You need one that caters for 2A min, usually the range setting is 10A DC.

[ColinB]

I suspect a CD motor will draw less but I don't know. I do know a 5 pole one draws a lot less and is about the 450 to 500 mA. I was going to fit one to my Ringfield based Duchess of Sutherland. In the end I found my old tender driven body would fit the next version which was loco driven, so I bought one of those chassis and did the conversion.

[yerffoeg]

So I have run the motor on a bench power supply. 22v dc current draw is 800ma stalled as to be expected as there is no back emf.

I suppose I now need to find a way to limit the motor current??

The sense resistor is 100m ohms. Would 200m work or will the motor run out of stuffing at start up?

[ColinB]

Look up a CD drive see what current that takes.

[yerffoeg]

Thanks for the support.

End of topic.