MTH Z-750 CONTROLLER.....AGH!!!!!

Hi Peter.  There are so many on this Forum who know more about electrical circuits than me.  But have you tried testing it with a resistive load, such as an incandescent bulb or a motor?  Also, is your meter digital or analog?  Sometimes inexpensive digital meters give a spurious reading.  GunrunnerJohn has a whole thread about that.  Good luck and please post what you find out.

I was curious too if this is one of the situations where you want to have a load so I went to my z 1000 and tested it. I hate to be the possible bearer of bad news but with nothing on the output of the z controller but my digital volt meter, the voltage varied. Ugh! Let's hope we are missing something!

Run a train with the Z-750 and your question will be answered.

That's a bummer Peter. Looks like they came off the same layout too. I know that Cabin Fever often sells items "as is". Any chance they had tested these before listing?
I found this thread which may offer some guidance. The triac under the board was my thought as mentioned in the thread.

I have a Z750 that I use for my Christmas tree loop. I recall testing for track voltage this past year and the voltage to the track does vary with no load. With the controller at the top position I had 18V at the rails with no load.

Bob

The triac is the top suspect, but I believe there's a diode in there that could be the issue as well.  Cheap parts if you want to replace them.

Those are the older Z750 bricks, they went from 24V to 21 volts for later models.  24 volts is way too high for Lionel stuff, MTH stuff is a little more forgiving on max voltage.

The Q2015L5 Triac is now considered 'obsolete', you may or may not find it at a normal parts house, but will be available on Ebay.  My experience with  Triac supplies is limited compared to a  linear supply, from looking at a couple schematics online, it appears a triac supply controls the AC going to the rectifier, as opposed to a linear supply that controls the DC coming from the filter bank, so basic voltmeter tests of the triac may not reveal the problem, a schematic would help, but they seem to all be proprietary.

Plenty of suitable substitutes, consider the STM BTA16-600BWRG.

Digikey Delivery to Australia

Thank you one and all (Ted S, Hokie71, ADCX Rob, Bob, GRJ, & CALNCC)  for your suggestions and help.

I will test the controller again by hooking it up to a direct load, and see what happens. (Fingers crossed???)

Thank you John for the reference to the Digikey Triac....I found I can get this same component from Element 14 here in Australia, which will save the AUD $24 on the international shipping costs.

Next chore will be identifying the suspect Triac, getting it out, and then installing the new replacement......all using my sausage sized fingers!!!!

Watch this space.

Peter.....Buco Australia.

I definitely will be watching this closely. I have a Z-750 from the mid 2000s that does the same thing. I used it for accessories but I would love to fix it.

UPDATE:

ADCX Rob.....you were right...to a point!

The Z-750 will run my Atlas "bump-and-go trolley perfectly fine, but that's about it!

And yes, it does need a "load" to get the voltage flowing in some form of increments.

I had my multi-meter on the track at the same time I was running the "bump-and-go" trolley, and nothing happens (no voltage) till the throttle handle is one third the way up the dial, and then it starts delivering 10 volts, and the "bump-and-go trolley starts to move, going faster with each increment of the throttle.

Tried one of my early PS1 diesel loco's (GP-9), and this is where I had the same trouble I have with my Z-4000.....these transformers don't graduate back to zero, like the Z-1000 does. As a result the PS1 board starts up and enters "reset" just fine, but then won't come out of reset when the throttle is lowered from the 10V start-up point. The board gets "stuck" in reset mode, and I get the bell chimes, and then it progresses into the factory reset mode...clanks and clunks etc., but still won't move!!!

Shutting the PS1 loco down was also a feat in itself, as the Z-750 was still "trickling" 3.4 Volts, even in the "OFF" throttle position. This was weird, as I could not get it that low when running the "bump-and-go trolley.

These Z-750 transformers would have to be the "poverty pack" of all transformers!!! This weird voltage output is common to both of the Z-750 units, and is basically useless to me. I don't think replacing the Triac will be the answer.....but I will be guided by Gunrunner John or Vernon on this.

If all fails, I have a couple of spare Z-1000 circuit boards in my stock, and I will swap these good boards for the crappy Z-750 boards.

Why would MTH produce transformers that don't run the various different types of their own loco's?????

Peter......Buco Australia.

Sorry for not diving into this earlier.

Yes, the Z-750 is different for a number of reasons- one being the source transformer was a much higher voltage (21 or 24V AC, VS 18V AC for the Z-500 and Z-1000 bricks), but also, it's the earlier chopping design using a single TRIAC output device.

Just like TRIACS on the R2LC and many other devices and loads in this hobby- the TRIAC is a somewhat picky device and needs a certain load to properly chop the waveform.

So no, replacing it probably is not a fix or change in output for your usage scenario.

I'll also add- you also have the problem of 50Hz power there, and many of these designs worked around 60Hz assumed power.

The newer Z-controller is a very different design using 2 FET transistors, instead of a single TRIAC and is not subject to that load requirement- but also, I don't know how well it handles the higher 21 or 24V input voltage from a Z-750 brick. It's really meant for the 18V AC of the Z-1000 brick, or Z-500 bricks I like to use on the test bench.

@Buco posted:

Why would MTH produce transformers that don't run the various different types of their own loco's?????

I think it's just a matter of competing with Lionel  (who let's be honest- the CW-80 isn't compatible either) at the time and hence the 75Watt class.

Again, a couple of things stand out:

#1 the external brick aspect was I believe a way to get around toy and UL regulatory guidance at the time. The brick is UL- the controller doesn't have to be. So, secondary to that- then sourcing transformers at a price, the higher voltage of the Z-750 might be just what a common transformer the supplier was already making for other industry.

#2 The timing of the Z-750 I believe was also later and PS2 engines, and Locosound were out and it's compatible with them. I get what you are complaining about- but the fact of PS1- especially early PS1 being picky on transformer- that is more or less coming from the early conventional era- QSI offering the boards and firmware to MTH, older transformers were what many folks had at that point in time. To then say well MTH always should have made backwards compatible transformers- I understand the argument, but the fact is economics override that requirement.

#3 Later on with the Z-1000 and improved Zcontroller, they did change the circuit, did improve the output, did make it more compatible but no guarantee of 100% backwards compatible- but again, few if any electronic controlled transformers are compatible. And the competition isn't even remotely trying to be compatible.

Thanks for the speed reply Vernon.....don't you guys ever sleep????

I am not too worried about the 24V power bricks.....I can just put them away in storage but.....for your info, I currently use a Buco (Swiss) 220V AC transformer for power on my work bench.

It has a variable output up to 22V AC (running trains) as well as two fixed voltage outputs...one at 15V AC for street/building lights etc., and one to power the Buco turnouts that require 23V AC to throw the points.

I have been using the 23V outlet to power my Z Controller test  station I built from spare parts, when I started going down the "rabbit hole" with these early PS1 circuit boards, nearly two years ago.

You may recognize this board, as it was the one that blew the TVS from being incorrectly "phased" with my Buco transformer, and you showed me how to replace the TVS.....please pardon the "bird sh.t" soldering, as this was early in my "repair career". I'm much better at it now!!!!

Anyway, the Z Controller board has been handling 23V AC just fine for nearly two years now, and is my "go to" test station when I get finicky PS1 boards, or when I check the BCR's I make for proper function/charge.

Thank you also for the insight behind the creation of the Z-750 transformer, and MTH's thinking at the time.

These Z-750 controllers will be up-graded to Z-1000, and put into my spare transformers bin for future use. They get all of the finicky PS1 loco's to run!!! Thank you again Vernon.

Peter.....Buco Australia.

@Buco posted:

These Z-750 transformers would have to be the "poverty pack" of all transformers!!!

These were meant to be simple starter-set controllers and compete with the contemporaneous BW-80 transformer.