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Looking for some help with my TIU.  I typically only run trains during the holiday season.  Today I pulled out my temporary layout and fired up my RK PRR L-1.  Stated up just fine and ran a few inches until a spark at a switch which tripped the TIU circuit breaker.  Replaced the circuit breaker and plugged everything back in with no train on the track.  Immediately a spark and the breaker blew again.  I tried once more, this time with the TIU not connected to the track.  Same problem.  Apparently the "Fixed Voltage IN1" circuit in the TIU has a short.  I can apply power to the "Fixed Voltage IN2" terminals with no problem.  The output voltage from the Fixed (DCS) Out2 terminals checked fine.  

Has anyone had a problem like this before?  is there an easy fix? since the IN2 and OUT2 terminals seem to work fine, can I just use those?  If so, how do I set up the TIU to use the IN/OUT2 circuit instead of IN/OUT1?  Thanks much for any help.

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Thanks much for the responses.  GRJ the picture and diagram especially helpful - thanks for taking the time to post those.  I'm traveling this week, but back on Friday and I will check it out.  Couple of more questions since I am definitely not an electronics expert:

How do I check for a short across the diode?  I do have a multi-tester if that will help.  If I do have short in that diode, where can I get a replacement?  I imagine it is something I will have to order, but from an electronics store or MTH?  

In the meantime, can I set up the TIU to run my trains via the fixed 2 input/output?  If so, how do I do that?

Thanks again

Having the same issue with my TIU that @Scott R had. I am electrically challenged so was hoping for some help in answering these questions.

1. I have a Z-500, do I plug that into aux power input before checking for continuity across the leads of the D300 diode?

2. Do I check continuity using DC or AC on the meter?

3. I purchased the TVS diodes from Digikey, one side of the lead has red tape on it and the other white. Can I cut the leads down to fit into the TIU?

4. Are each of the leads on the TVS Diode the same? does it matter which direction I solder it to the TIU? Not sure if one is hot and one dead???

Thanks for your help

You can check the four TVS diodes in circuit with a standard ohmmeter.  Just measure across them, any bad one will usually read a direct short.  If it's just a TVS diode, I clip the leads next to the body, heat the solder joint while holding the lead in forceps, and pull it out.  You can then solder the new one in from the top without undoing a sea of binding post nuts.

The TVS diodes are bi-polar, that means they don't have polarity.

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