Alastairpell
I have had some interesting responses to a couple of engines with zen decoders with standard stay alives. Every now and again it goes racing off uncommanded usually when the loco is travelling slowly. Then it slows right down before picking up to the commanded speed again. It is a roco Swiss 460 that ran very well with the old lenz 8 pin decoder. Have you seen this before, is it just corrupt signals due to dirty wheels or something else?

Regards 
Alastair
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AHJAY
Generally a runaway is related to confusion in the DCC signal. This is usually related either to something in the layout wiring overall OR something to do with intermittent power delivery. Hard to guess at without being hands-on though.

* How long does it speed up for?
* You could try disconnecting the stay alive and see if the loco just stalls OR still runs away - that will help narrow things down a bit.
* I'd give the loco wheels and pickups a good clean and adjust if needed... let me know how you get on.

I am awlays interested to see such problems so i it is not sorted by cleaning, I am happy to look at it for you without cost but it'd be best to have the loco with the decoder still installed to identify the issue properly, rather than just the decoder which will probably not show the symptoms by itself.

regards, Ahjay
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Alastairpell
Hi Ahjay, 

The runaway lasts between 0.5 and 2 seconds, seems to vary somewhat. I would not have thought the standard stay alive could drive it that long. 

I have snipped off the stay alive and now it does not run away but runs like a bag of knackers. Seems like dirty contacts or I disturbed something when I swapped the decoder. Will clean it up and check it out and see how it goes. I ran another loco round the track which ran super so it is not a track issue. 

Regards 
Alastair 
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AHJAY
Sounds very much like a pickup or cleanliness of wheels/pickups etc. that loco is usually a good performer and we have used that chassis as one of our test fleet.

regards, Ahjay
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Alastairpell
I have managed to clean up the wheels and contacts, disabled dc running and now runs like a dream, the only problem is the lights don't work.
Originally I had a 6 pin zen blue decoder with an 8 pin harness. I assumed the lack of a blue wire on the plug was the issue. So have purchased some 8 pin hard wired zens. However something is amiss, with the hard wired 8 pin the lights are on dim all the time, whether the headlight function is on or not! Also the decoder gets very hot and after 10 to 20 seconds trips out the track short circuit protection device, after several auto reset attempts the decoder blew up!

Being unable to find anything wrong with the loco I fitted another decoder with the same effect, this time I knocked the loco off the track before it destroyed the decoder again. If I fit the 8 pin plug the wrong way round the loco works perfectly, without any working lights obviously. So whatever is going on seems to be related to the lighting circuit.

I have tried a couple more locos, all high quality Roco chassis that worked perfectly with the old Lenz decoders fitted, same outcome. Either you have a bad batch of zens or they are not man enough for 20 year old Roco locos with 8 pin sockets!?

In the short term I can just turn the plugs round and do without lights but obviously I have invested in new decoders to get over shortcomings with my old Lenz decoders not create more problems!

Any ideas? Have the 8 pin standard pin outs changed since the early days when my Roco were manufactured? The lighting circuit does look complicated with 3 diodes at each end but, if I plug my old Lenz decoder back in, it works, lights and all... 
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AHJAY
There is no batch issue and can never be because testing is extreme well prior to packaging - so there are definitely other things in play.

I suspect I know your general problem. It is totally related to overall current draw. You are using a tiny decoder designed for light loads for an old HO loco with overall high current draw. There are far more suitable decoders in the Zen range. They are larger and have higher power components.

Do your locos also have incandescent bulbs? I bet they do. These can draw a lot of current (150mA plus vs 2 or 3mA for an LED) and as a result
* They will over-stress any decoder lighting circuit eventually. A lighing circuit is generally 100mA safe load limit.
* They will probably not work at all if you use a 6 pin decoder - or be very dim due to the way 6 pin lighting has to be wired.
* Any more than one incandescent lamp will overload any lighting function. Expect eventual or fast failure depending on brand of decoder.
MOST importantly
* The total load on the decoder will be significantly increased. ANY brand decoder rating is the cumulative load on ALL outputs, not just the motor drive.

An old motor (already higher current) plus several incandescent bulbs will exceed reasonable limits. Therefore, usually, converting old locos to DCC would also involve replacing incandescent bulbs with LEDs - certainly every loco I have ever done for myself or others has had that done as a standard change, including old Roco, FL, Marklin, Trix etc.... 

AND - a 6 pin decoder has NO blue wire by design. it uses the chassis as the lighting ground and can only deliver half-wave DC to the lights. It is unsuitable for incandescent bulbs. Not our decision - that is the 6 pin NMRA/NEM specification.

With very old locos, you should perhaps also look at:
* The magnet strength of the old iron core motor - this can weaken over time, but can be helped simply by attaching a Neodymium super magnet to the original motor (it will stay in place via its own magnetism so a dead easy job).
* The lubricants: They will harden over time as the volatiles evapourate, no matter how good their original quality was.. A proper clean and new oil makes a huge difference. (sounds silly but I often strip old chassis totally and put them through a warm but not hot dishwasher cycle as its hard to get to the hidden bits - it is remarkable how much better they then run once re-oiled and assembled)

If you cannot do that then please do use decoders with higher current limits, not tiny decoders designed for smaller current draw. I would never use a 6 pin or other form of miniature decoder in anything thats not a new small loco - it is an N scale decoder.

regards, Ahjay
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Alastairpell
Thanks for your detailed reply Ahjay, 

It will be the incandescents then, there are three of them in each direction, tiny but I guess will smash the 100mA limit. Unfortunately the DCCC website is non too helpful just saying the zens are suitable for all rtr HO models, no mention of LEDs or old technology limitations, at least not on the sales pages. Your response is much more helpful, although all the zens bar the Buddha appear have the same light output limit? 

I think I will have to get my head around converting to LEDs as I have bought a few zens to replace the geriatric lenz decoders and I'm fairly confident the zen will be up to powering the 'modern' can motors in my old Roco stock.

I'm still not sure why the bulbs are lit dimly all the time whether the light option is on or off though, seems to be common to all my older Roco stock but doesn't happen with the one old lenz decoder I have which works properly with the NCE, I think that has a 500mA light output to be fair. 

Regards 
Alastair 
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AHJAY
I

No, buddha has much higher function power plus about 5 timers the motor circuit power - it is for O and larger...

Actually the light function limits are all published and in the manuals. the fact that decoder power is a total power usage is common to all decoders. Its actually the total tolerance of the bridge rectifier that sets it.

Your older stock will have one side of the incandescent light circuits live to the chassis, so bulbs will only ever get half-wave power... that is why they will be dim... Another reason to totally rewire older locos and add LEDs.

regards, Richard
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