Background: I have two Z4000s with each handle connected to one of the four channels of a Rev L TIU. The variable channels are not set to fixed. Using the latest software version. The two variable channels are hooked up to one transformer and two fixed to the other transformer.

While running MTH Proto 2 and 3 trains on the variable channels, suddenly both trains stopped. The Z4000 was flashing a red light. After troubleshooting and isolating the problem, I found that VAR 1 channel of TIU causes the apparent short. At low voltage of 6 or 7 volts the transformer does not trigger the overload but around 10V the current jumps and red light comes on; this is with nothing connected to the output of VAR 1.  The transformer works when not connected to TIU. All other TIU channels work properly as well as the transformer.

Questions: Is this potentially related to some TVS issue I had read about with Rev L TIU?  What would cause item to fail suddenly when there were no shorts or derails and was drawing less than 4 amps when it failed?  The TIU is probably around 3-years old, so should this go back to MTH for repair or to a service tech?

Thanks in advance for feedback/suggestions.



Original Post

It's almost surely the TVS on VAR1, it's an easy fix.  It's also possible that it's the FET's for the variable channel, but my bet is on the TVS.  It's possible to replace the TVS parts from the top, but if you need to replace one or more FET's, that requires removing the board.

Since it's doubtless going to have to be repaired anyway, you can just clip out the TVS for VAR1 near the aux power jack and see if it returns to normal operation.  If so, you still want to get that replaced, but you'll know what needs to be done.

OK, will check that out. If it is the TVS. it looks like something I could replace. Do you recommend the 1.5KE36CA? If my local source doesn't have that one would you go to a 33 or to the higher 39?

It still bothers me that there was nothing that happened on the track that I could perceive that would cause the failure. Will chalk it up to within the normal reliability failure rate.  As mentioned by others, if it fails open I wouldn't know it anyway.

I'd probably go with the 39 if they don't have the 1.5KE36CA.

If you're going to do the repair from the top, clip the leads right at the body of the diode and leave some lead.  Then you can grab that lead with either needle nose pliers or I prefer some forceps to clamp on the end.  Heat up the lead at the PCB and pull it out.

Correct, you'll never know if they fail open, fortunately, in the majority of cases, TVS devices fail degraded or shorted.

Kenjr posted:

Can do.

Out of curiosity, if I didn't want to solder to the board, could you just connect across the output terminals of TIU and essentially have the same protection?

It wouldn't hurt to place one on the INPUT side also, if you go that direction. 

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