Hi all,

I purchased a very nice used 39621 Great Northern F7 engine from a seller in Japan last week. It's from 2016, so mfx+, but not the most recent. It worked fine for a week, and then it locked up dead.

The engine worked well on my Mobile Station 2 test track and then on my 6001/6021 main layout. I updated CV 49 to "03" using the 6021 and so got access to all the sounds with three addresses (07, 08, 09). I adjusted the Vmax up, modeling the scale speed of an F7 Passenger used on the Great Northern Empire Builder, and adjusted Acceleration and Braking delays to be more realistic.

Everything was working fine until the engine locked up last night traveling at a medium speed on my 6001/6021 main layout. It would not run on the Mobile Station 2 layout either.

I got a beer and started reading forum posts. One said to put the train on my old analog track and drive it forward and backward. The engine responded! After that, it worked again on both the MS2 and 6001/6021 layouts!

What going on here? I looked at the DC bias and AC voltage between the MS2 track and 6001/6021 layout and they are quite different, which is probably a surprise to no one. I'm pretty sure I understand that the 6021 is using MM and the MS2 is using mfx+, but neither worked when it was locked up and both worked fine before the lockup. All other engines continued to run fine. This 6021 layout has run probably every decoder from Delta (37184) to a brand new mfx+ train (26360) this week with no issues. I'm reluctant to put it back on the 6001/6021 until I figure out why this happened.

Thanks for any help or insight on this.

Steve

Looking up your lok, it's an AA unit with two motors right?

I have a Santa Fe F7 (#37622) original AB that was changed by the previous owner to a ABA, he connected the motor in the second A unit parallel to the decoder in the main A unit..... both 5 pole motors....

Using the 6021 controller lok drives good on high speed, but the moment you go to medium or low speed, lok starts shutting down on the track, stop and go behavior and sounds become erratic.... Removing the 5 pole rotor from the 2e unit "fixed" all issues....I figured out that this original decoder cant supply enough power for the 2 motors and shuts down after a short while....

My electromotive ABA F7, digitized with a Mld decoder and 2, 5 pole motors on the other hand runs perfect in all speed ranges....

As suggested by mvd71, i would indeed take a look at the decoder.....

Regards
Jean-Pierre

The issue is not to do with the 6021 or the voltage you are measuring.

Your loco being factory installed with two motors, will have a decoder with an output stage that is designed to drive two motors.

What may be causing the problem is a build up of carbon inside the motor faceplates. This happens over time, and when sufficient carbon builds up it causes arcing and an increased current draw on the decoder. This may be what is causing the decoder to fault.

John (river6109) has some good advice on motor servicing, or you could try checking out the Märklin of Sweden videos on YouTube.

Either way, if I were you I would service the motors and make sure there is not a build up of carbon first.

Thanks, I found the cleaning video at Marklin of Sweeden and will take a look at the motors to see how dirty they are.

Can you trust your voltmeter? Is it True RMS?
Cheap voltmeters give better results in the DC range with an external bridge rectifier.

Yes, I can trust my voltmeter in all things. It’s a Fluke 87 True RMS. Such a nice tool.

Normally I would expect a higher voltage from the CU 6021 than from the MS2 running on 18 V DC while the load on the track is low.
Maybe that's related to the issue.
The CU 6021 has a DIP switch that allows to reduce the track voltage. I think it is switch 4 which should normally be off.
The CU 6021 has three transistors on the output stage, two in parallel for one voltage, a single one for the other voltage. Maybe there is something wrong at that stage?

yes, and if you have an mfx+ decoder with 3.xx.xx firmware there will probably be a lot of fixes. (note that Marklin has not released updates for all decoders, notably some very early 3.00.xx are missing updates)
example of a changelog: 248244 247 mLD3 - märklin Lok Decoder changelog. Firmware_changelog_mLD3_248244_247_v3-2-2-0.pdf (167kb) downloaded 13 time(s). (note the part number is the key)

but the decoder might also have 2.1x.xx firmware and Marklin stopped updating those with firmware 2.17.19.
ref: https://www.marklin-users.net/forum/posts/m537101-MFX-decoders-and-occupied-addresses#post537101

note that there is also a risk of bricking the decoder during a firmware update, so make sure the locomotive has proper contact and is alone on the programming track, and don't interrupt the process.

Thanks everyone for all the ideas. Here are the possible root causes I have heard:
1. Some problems with the 6021 layout, like not enough power
2. Carbon buildup on the motor causing feedback to the motor drive circuit
3. Firmware problem where inputs are not de-bounded or the watchdogs don't hunt and it ends up in a software dead end
4. Hardware problem with the decoder or motor driver

I think I can rule out #1 as @mvd71 said. The Vrms AC+DC for the 6021 layout is over 20V while the MS2 is around 18V. The 6021 should support 47W of stuff, and there is 21W of bulbs on this old M-track layout, but that should be plenty for the F7. It was able just get to 0.3A on the MS2 and full throttle, so about 3W for the loco.

I took it apart and cleaned the motors per the Marklin Sweden video. Both motors were VERY clean, but I took the carbon off the brushes and rotor. Almost nothing on the inside of the faceplate. If this level of carbon can cause this problem, it's not good. It looks like the loco had never been serviced and motor oiling locations looked dry, so they got some oil. I will be very surprised if this cleaning fixes it, but who knows.

I am going to stop messing with it for a few days until my brother in law gets a chance to see it run. His dad worked for Great Northern and saw this engine run in the 60s and 70s in person many times. Don't want it dead when he comes over to see and hear it.
After he has seen it work, I will get on the 6021 track for some extending running and see if I can get it to lock up again and report back.

Well, that did not go well.
My brother-in-law came over to see the loco and it worked briefly on the MS2 programming track. Then it locked up just moving it back and forth on that track.
I was able to bring it back to life on the analog layout.
Then it locked up again on the first time around the 6021 layout. It is now not responding on the analog track either, so completely dead.

So, it's just going to Scott and Helmuts Hobby to see what he can do.