I have four circuits passing through a TIU.  Recently, the circuit passing through position 4 stopped working.  As I apply power to this circuit, the Z-4000 shows amps increasing to about 8 as the voltage reaches 12.  And, the track that is serviced by this circuit shows no power.  So, I moved the input/output of position 4 to position 3 on the TIU and the circuit worked fine.

I would appreciate suggestions on what I should try to fix the problem.  Perhaps I should send the TIU in for repair or just buy a new one.


Tom Jasper

Original Post

This is the classic sign of a shorted TVS diode, a fairly easy fix.  You can actually do this repair without taking the board out.

John, that's encouraging.  Can you refer me to a reference that describes how to fix a shorted TVS diode?  And, are parts required?  

I probably caused the short when I commanded a N&W J to move forward, but a PRR Q2 that somehow had the same address moved from a siding and plowed into a passenger train shorting out the track circuit that has the problem



Measure these four components, when you find one with a short, that's a shorted TVS diode.  What I do is clip the leads at the failed part, then individually pull them out by heating the wire, and finally clean out the hole and solder a new one in.  You should be able to do that without removing the board and the sixteen terminals.

MTH TIU TVS Diode Locations


Images (1)
@stan2004 posted:

It's one of those you'll pay more for shipping than the part itself.

That's why I like to try and stage my orders until I have $30-40 worth at least and then fire them off.

Thanks John & Stan.  I'm leaning towards buying a new TIU and then attempting to repair the damaged one so I don't have an interruption of rail service.  Tom

First off, see if you can find which TVS has shorted.  If you identify one shorted, remove it and see if the channel is functional again.  If so, I can put one in an envelope and send it to you.

Thanks John for your great offer.  I have an older pre-Rev L TIU that I believe has the same problem.  So my plan is to service two track circuits with one TIU feed from my Rev L TIU, and have a look at the four TVSs in the older TIU to see if one of them is bad.  This way I can continue to run trains as I bootstrap my way back to a fully functioning TIU.  Thanks for your insight and offer to mail a needed part. Tom

As a temporary fix you can simply clip the leads and remove the failed internal tvs. Then fit a new one to the tiu output posts using the handy holes in the output banana plug posts. Standard tvs PN's for train use have been posted many times on the forum. If you have any on hand you are set.


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