In an earlier post, I reported I had a transformer that the current was not stable on the right channel. My research strongly suggests a loose internal connection (outside connections are tight). Also swapping to another transformer corrected the problem and then hooking up the one in question the problem returned.

Question is there anything I should be aware of in opening up the transformer to check for loose internal connection? Also, the handles on this transformer seem much looser than the other Z4000s; I don't see any adjustments to be able to tighten that. Do some of the early makes not have adjustment?


Original Post

The first issue of Z4K's had no adjusters and no friction tensioners installed, though there is a channel in the case molding to enable installing them. The parts are readily available thru MTH. Just call Midge she will set you up.


We are never too old to learn something stupid....

I finally got around to checking out this transformer. This is an early unit from March 1998 made in Korea so no tensioners.  My issue is the erratic amp display while there is a load. I opened it up and found no loose terminals. I also checked/reseated all plug in connectors; no real issues found. Still have erratic amp readings when being used and can be on both channels but seems more prevalent on right side. This happens when using with TIU and just using in conventional.  The voltage display doesn't vary like the amp display.

I powered some incandescent lights and with the voltage at 13V, the lights did not flicker (volt display was stable; amp display varied from 0.4 to 3.0).

My conclusion is this may be a display board problem, but the actual transformer is working correctly since the voltage is stable (even when measured separately with multimeter).

Does this conclusion seem probable and can I still use this transformer just ignoring the amp display?  Any other thoughts on what is causing this?


some of the early ones transformers serial number 039 had drifting issues and mth might sell you the parts but more then likely they wont sell them to you only as ASC tech! call midge and ask! as far as the tensioners are concerned it was just a small adjustable spring in the bottom below the throttle control and there was a rubber washer about the same  size as the spring which would put pressure on the bottom of the throttle control to give more friction! it was a screw driver slot and you just rotate by screwdriver  the amount of tension you want!

o gauge  trains ,music ,computer repair windows 7 and 10!

ASC Tech MTH school completed! 2019 !

Thanks Alan.  Not sure what drifting issue means. The voltage doesn't change on mine from whatever you set the handle. If the current display  "jumping around" erratically is drifting then mine does that. Doesn't seem to impact anything that is connected to the output.

Are you comfortable/capable of component-level troubleshooting?  I think the level of detail provided in the Z-4000 patent can help:

mth z4000 patent

The text in the patent describes how the current is sensed across a resistor (Fig 12B).  The voltage (proportional to current) is amplified by an op-amp circuit nicely detailed (Fig 16), rectified to a DC level (proportional to current), then converted to a digital value by an A/D converter (Fig. 2B), then displayed.  

Since the same A/D converter is apparently used for the Voltage measurements which work, I'd look at the circuitry leading up to the A/D converter.  And since you apparently get erratic readings on BOTH current channels, I'd wonder about any components, voltages, or signals used by both channels.



Photos (1)

Thanks Stan.  There really aren't that many components used only in the current sensing and used in both tracks. There are two quad opamps , U16 and 17 that are only in current sensing and all of the single components seem to be unique to each channel. One or both of those have potential to be bad. Pretty inexpensive part

I haven't yet used my multimeter to measure current output while watching the display. I will try that to see if meter displays a stable value while the transformer is varying.

Kenjr posted:

… I powered some incandescent lights and with the voltage at 13V, the lights did not flicker (volt display was stable; amp display varied from 0.4 to 3.0).

0.4 to 3.0 is quite a fluctuation.  I'd think you can meter the inputs to the box marked "A/D".  The text suggests this will be a voltage proportional to current and probably 0-5V DC range.  I'd think you want to see this test point fluctuating wildly so you can trace backwards thru low-cost generic components to the current-sense resistor.

OTOH if the inputs to the "A/D" are nice smooth stable voltage proportional to current then I'd be nervous.  That would suggest the problem is downstream toward the microprocessor - such as in the analog multiplexing circuit which (given the technology of the time) was probably used to share a single A/D converter amongst the (at least) 4 inputs (2 V, 2 I).  Then you'd need an oscilloscope to see what's going on.  

I'm not an EE.  But have you validated the display readings with an EXTERNAL ammeter?  That would seem to be one way of determining if there really is a current fluctuation, or just a readout issue.  My $.02

Creep, coast, and pull.  We're not talking about cold fusion here.

I finally got some time to check more.  I did what Ted mentioned and found the voltage and current very stable on the meter while the transformer showed a stable voltage but the amp meters were continuing to display erratic readings. The display would match the meter for a few seconds and then jump to higher values for a few seconds then back to the same as the meter.  This at least narrows the issue down to the display board/circuit.

Stan, I may do some more troubleshooting as you suggest, but the good news is I can use the transformer and just ignore the amp reading. Until I got the Z-4000s, I never had a way to monitor current anyway.


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