lights on and off with speed step changes

[34theletterbetweenB&D]

I have a quite delightful OO Dapol 21, which started operation with a Zimo MX638 decoder, but since it ran so smoothly I tried it with an old Bachmann 21 pin 36-554 I had spare, a decoder that usually works well with intrinsically smooth running centre motor drives; and the model runs just as sweetly as with the Zimo. So that's good, I have the Zimo to use in a small motored steam mechanism, where its adjustments will be more fully utilised to optimise performance.

 

But late yesterday evening's running session, a puzzle. I accidentally switched on the end lights (F1), and they went on and off on alternate speed steps. So this evening I switched the other end lights on F0, these are constant. Then I found I had a cab light on F2, one end only, and that flickers. Reset the decoder just in case it had become deranged by static while out of use, got address 03, readdressed it: lighting behaved as before (the reset should have ensured it is in 28/128 speed step mode, running it in 28 step).

 

These effects don't matter to me as lights aren't used, but I am curious. This is a basic 3 function decoder with the functions only on or off, and one output level setting for all, as the two control options. I have quite a number in use and don't recall anything like this before: locos with lights have them constant output, on or off. I'll try a decoder exchange with a different model with lights tomorrow to see if this 'travels' with the decoder.

 

Anyone able to suggest what is happening?

[Dagworth]

if the decoder is in 14 speed step mode and the command station sending 28 step instructions to it you will get exactly the symptoms you describe. The light control for F0 is the least significant bit of the speed instruction in 14 step.

 

Andi

[34theletterbetweenB&D]

That's why I did the reset to make sure it was in 28/128. It was running in 28 step, before and after the reset. It was the fact that it was doing the speed step flash one end only, and the cab light had a flicker effect that really puzzled me

 

Anyhoo, did the decoder swap with another 36-554 in a Bach 24. Checked both out on programme track, did decoder reset on both, then gave them the correct address for the locos they are now in. The decoder from the 24 works as expected in the 21, lights switch on and off normally. The decoder previously in the 21, ran normally for a couple of minutes, but with no light functions available. So I stopped the loco for a think, and it then 'self diagnosed' by bolting off at warp speed and commenced emitting really realistic diesel clag. So that's a whole £8.50 worth of decoder from about a dozen years past totalled: I shall write to BoJo telling him here's something else he must fix, urgently...