MTH z-1000 DC offset question

Problem with what you are asking. In theory, you said Z1000 with Zcontroller. That said, there are internal design revisions of the Zcontroller and different PCBs.

So, I'm not sure an apples to apples test.

Current most recent version like this using 2 MOSFETs

VS

This is a Z750 board, but again there were different controls boards to my knowledge

I would also add- a failing bi-directional TVS could apply some amount of DC bias conducting one way but not the other.

While this topic starts off with the Z-500, it does also cover the later Z-1000 Z controller, showing the MOSFETs and the TVS diode.

https://ogrforum.com/...stant-18-volt-output

Also this topic:

https://ogrforum.com/...53#23252098530151753

@The Tinker posted:

It's apparent to me that the z-1000 controllers all have some degree of DC offset in their output (+ or -). Either due to the zero crossing detection or minor differences in the positive and negative chopping.

I gotta be honest, I don't agree with this and wondering how you came to this conclusion? I use a Zcontroller (most recent dual MOSFET version) as my test bench transformer, and repair hundreds of trains, and have not once assumed or even considered it to have some sort of DC bias.

Again, the OLD version used a TRIAC to switch both positive and negative halves of the cycle. The new design uses 2 MOSFETs. I might say there could be difference from that major design change.

Also, while not the specific test you requested using just a DMM, this is some scope views of operation under load. This was tested when comparing the new CW80 redesign that also used MOSFETs thus making it fanless- compared to the older TRIAC based version.

Did what you asked, took a new Zcontroller, Z500 brick (brick should not matter), a Fluke 101 meter, set it to DC voltage, set throttle to 1/2, measured a DC voltage of -0.008 while varying the throttle, and every so often saw it read as high as -0.012V.

Thanks all. I make three boards to replace the DC slug relay in Lionel stock that powers the whistles, horn and Teledyne couplers. I have had customers have issues where my board causes the whistle/horn to sound all the time. Common denominator? MTH z1000. My design uses a RC filter and a Darlington transistor that will turn on at around .7 volts DC. Some z1000s work fine. Leads me to believe that some degree of DC offset is present on some z1000s. Negative won't bother my controllers. Anything under .7 volts DC shouldn't either. I assume it can't be as high as 1.5vDC or any other tenders or diesel locos would sound all the time too. I'm thinking of adding a diode to the base of the Darlington to raise the turn on voltage to around 1.4vDC. Of course, if the DC offset is more than that, I may need a zener diode higher than 1.5v but then my controller won't work with Lionel D cell battery buttons so not an options.

The pics Vernon sent are most helpful. The Z boards with two MOSFETs are probably my nemesis. They chop positive and negative independently and our not completely symmetrical. The ones with a single Triac are probably the ones that work fine with my boards.

My apologies in advance if mentioning my products are a no-no on the forum but I felt you all deserved an explanation of why I ask.

-JP

>I would also add- a failing bi-directional TVS could apply some amount of DC bias conducting one way >but not the other.

Interesting. If that is the cause then adding a diode in series with the base of the darlington on my boards may just be the ticket. It would block the effect of the .7v junction drop missing from the positive side of the waveform.

-JP

If there was a design change in the MTH Z1000, can anyone identify when it might have occurred? Are Z1000's serialized?  Also, exactly what is the impact of a DC offset in terms of locomotive performance?

I think the amount of offset I'm dealing with is minor. Under a volt DC probably. If the cause is the TVS diode then more likely .7 volts. I think if it's a volt or more it may cause erratic operation on some accessory boards. Electronic e-units, sound boards and the like. I know it falsely turns on my board. The design I use is similar to the DC whistle tender board on the 80's and 90's Lionel tenders. Probably affects those too.

It looks like from Vernon's observation that the later z controllers with the twin MOSFETs are labeled as "Zcontroller" on the front. The older single Triac models are labeled "Z-1000", "Z-500"

I found Vernon's video most enlightening. It clearly shows the mechanism they use for whistle and bell. You can see on the scope that the positive half of the chopped wave expands while the negative stays the same when the horn/whistle button is pushed and vise versa for the bell.  That asymmetry is by design, and makes me wonder enough of that asymmetry is what I'm dealing with. I was expecting to see the AC waveform ride up or down on a positive or negative DC value.

The classic Lionel full wave transformers just add a few volts DC to the output and let the AC ride on that to turn the whistle on.

Man, would I love to see a schematic for the new Zcontroller...