Overland Models AMD-103, original drive - worth Resurrecting?

[peteski]

Thanks for the pointers. I read those and learned a few things, but nothing that clarifies what I'm seeing.

It's a brand new DZ126 decoder, and I haven't changed anything other than the address.
Is the default 28 steps or 14 ( I know it's not 128)?
It sure seems like my controller thinks it should be using 28 steps, and if I understand Digitrax documentation their decoders should default to 28 as well.   I'm a little confused.  Is it possible that this is just how the motor behaves under DCC?

The only problem with step mismatches is between 14 and 28/128.  There is no decoder setting specifically to select 128 steps. If you want to use 128 steps you set your DCC throttle (command station) for 128 steps.  As long as the decoder is then set for 28/128 steps (it is a CV29 setting), it will accept either 28 or 128 speed steps commands seamlessly.   That is touched upon in the first URL I posted above.

I still recommend resetting the decoder.  Even if new, it might not contain default settings (for some reason).  That will set the CV 29 to 28/128 steps and disable the speed table.

I doubt it is the motor - it is a standard run of the mill motor used in brass locos (not even a coreless motor). If it has the same speed between speed step 12 and 28 on the throttle then it sounds like DCC problem (decoder?).

[nickelplate759]

I tried resetting the decoder - the behavior doesn't changes.    Worthwhile experiment nonetheless.

I'm thinking that this is just as fast as the motor goes.  I've got a number of other models that seem to reach their top speed well before speed step 28 too.  Maybe an artifact of the motor running on PWM rather than pure DC (I'm guessing - I'm terrible at electrical engineering)?

[peteski]

I tried resetting the decoder - the behavior doesn't changes.    Worthwhile experiment nonetheless.

I'm thinking that this is just as fast as the motor goes.  I've got a number of other models that seem to reach their top speed well before speed step 28 too.  Maybe an artifact of the motor running on PWM rather than pure DC (I'm guessing - I'm terrible at electrical engineering)?

I guess it is possible, although you should see (or even hear) some change in the motor's speed between step 11 and 28.

The average voltage produced by PWM circuit and seen by the motor at step 11 will be quit different than at step 28 (assuming the decoder is at the factory setting which is set for a linear speed curve).

Assuming that the motor drive at maximum can supply 11V to the motor (at maximum speed the PMW voltage is very close to straight DC). But at speed step 11 the average voltage derived from the PWM motor driver will be around 5V.  There is a big difference between 5V and 11V which should reflect speed change of the motor.  A DC motor like we use in our models will change its rpms depending on the voltage powering it.  You can even overdrive a motor by supplying it with high enough of a voltage which will spin it as high enough rpms to self destruct.

I really have no explanation as to why the motor would not change rpms for such a wide range of speed steps (assuming the decoder is functioning correctly).  Maybe if the mechanism's friction was increasing proportionally to the motor's rpms, so the moad on the motor increased with rpms just at the exact rate to keep the motor's rpms constant?  But why only starting about half way through the model's operating voltage range?  You mentioned that in DC this loco seems to behave correctly. This is a mystery to me.

As for different speed forward and backward, that could be explained by a problem with the mechanism, or even by the motor's brushes timing being off (maybe a damaged brush).

[davefoxx]

If you are able to read back CVs, see where CVs 5 and 6 are set, for example, if the decoder is using the three-step speed programming.  If, for instance CV6 is somehow at 255, your engine is going to hit top speed at the middle of your throttle.

DFF