Decoder & loco apparent suitability issue.

[Ray H]

A recent O gauge arrival on our club layout is an A4 that has a fairly old motor, although it doesn't draw as much current as its size might suggest. It is similar in style to the old Tri-ang X04 motors and probably double the size thereof.

 

It has been fitted with a Zimo MX645 decoder which is hard wired and the loco is" American" wired - pick-ups on opposite sides on the loco and tender.

 

It runs as sweet as anything on DC with no decoder fitted. It performs likewise with an MX600 - a non sound decoder - and on the MX645R with analogue operation enabled and controlled via a dc controller.

 

However, its low speed performance on dcc is best described as (very) jerky. Adjusting its CV2 value to something like 45 all but eliminates the jerky movement but has two side effects. The initial speed (from a standstill) is quite fast and the jerkiness is still there when the loco is brought to a stand.

 

We've even removed the chassis connection on the loco and electrically connected the appropriate rail direct to the relevant decoder wire to eliminate any electrical continuity problems through the wheels/bushes and gearing.

 

There is no difference in the performance when the loco is placed on a rolling road.

 

The motor runs as would be expected when the drive is mechanically disconnected. Unfortunately the only way to extract/replace the motor requires the motion and wheels to be removed as the motor is screwed into the chassis behind the wheels.

 

We suspect that the problem is associated with back EMF and using the manual as a guide have tinkered with a few related CV values to no avail.

 

We even tried copying the CV values from the MX600 and applying them to the MX645R. Mechanically it made no difference and it introduced significant audio interference so we reset the decoder.

 

Can anyone suggest what we could try to overcome the problem.

 

Many thanks.

[Steven B]

Have you tried disabling the BEMF settings?

 

Steven B

[Nigelcliffe]

"    tinkered with a few related CV values to no avail  "

 

So, tinker with a few more to find the ones which do work ?     Having no idea which have been tried, and it what method of systematic or random change was adopted, its difficult to give advice.  

 

I wrote up my approach on the 2mm website.   http://www.2mm.org.uk/articles/Zimo-decoders.htm   

 

Also, the firmware version in the decoder may be relevant.  If you have the hardware to do it, update the firmware to the current version.  

 

 

- Nigel

[Ray H]

2 hours ago, Steven B said:

Have you tried disabling the BEMF settings?

 

Steven B

 

Which CV (or group of CVs) do that please?

 

1 hour ago, Nigelcliffe said:

"    tinkered with a few related CV values to no avail  "

 

So, tinker with a few more to find the ones which do work ?     Having no idea which have been tried, and it what method of systematic or random change was adopted, its difficult to give advice.  

 

I wrote up my approach on the 2mm website.   http://www.2mm.org.uk/articles/Zimo-decoders.htm   

 

Also, the firmware version in the decoder may be relevant.  If you have the hardware to do it, update the firmware to the current version.  

 

 

- Nigel

 

Nigel, that is an amazing document, thank you for taking the time to write it. I shall print it off and take to the club later this week to use as a guide whilst we endeavour to resolve the issue.

 

The MX645 is a (very) recent purchase whilst the MX600 has been with us for a while so I'd be inclined to try to backdate the firmware in case it is an unreported problem of the more recent firmware. We do have the necessary hardware to change the firmware.

 

I can see we'll be busy at the club later in the week.

 

Thanks to both of you.

[Nigelcliffe]

Don't back-date the firmware.   Its unlikely to make it better, and may make it worse.  

 

 

- Nigel

[mikeg]

It sounds as if the motor needs remagnatising, I think the motor is working hard and as it’s seems not to draw a lot of current but needs a lot of volts I have had this problem and had it remagnatised and it ran well but needed a 1.5amp, 2max to run with the full speed settings from 1 to 128.

 

regards Mike 

[Ray H]

The loco body is crammed full of something very heavy which has been painted over and we haven't plucked up the courage to chip away at it to see what it is.

 

A different decoder of the same model is now in the loco and making a tweak to the value of CV2 seems to have cured the problem at the expense of the initial movement speed. We're planning to try some of the things mentioned in Nigel's notes referenced above later today.

 

I'll also add that the loco was quite happy running at its top speed - which is faster than we normally set top speeds to - and pulling eight fairly heavy Pullman coaches so I don't think the magnet is the problem (albeit that I concede that I may be wrong on that count). I say "was" because we haven't as yet let it loose with a train having only just finished fitting the new decoder when it was home time from the club.

[Ray H]

Whilst not 100%, the tweak that we did to CV2 last Thursday - pushing the value up to 45 (from 1) did stop the juddering - was subject to some more work on Friday. We followed Nigel's guidelines with some of the other CVs and were then able to lower CV2's value back down to 8 to get a reasonably slow start with no juddering on starting or stopping in either direction.

 

We then let the loco run for a fair while pulling 5 coaches with the only problems being the coaches derailing a couple of times, which they haven't done before. You fix one problem only to find another!

 

We may try a few more tweaks in due course to see if we can achieve any further improvement.

 

Thanks for the assistance with this.

[Grovenor]

On 05/07/2021 at 17:13, mikeg said:

It sounds as if the motor needs remagnatising, I think the motor is working hard and as it’s seems not to draw a lot of current but needs a lot of volts I have had this problem and had it remagnatised and it ran well but needed a 1.5amp, 2max to run with the full speed settings from 1 to 128.

 

regards Mike 

A motor with a weak field, ie one in need of remagnetising will be poor at starting and draw a lot of current as the torque produced per amp is reduced by the weak field and it consequently needs more current to generate enough torque to start.