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I have been using a power supply to run a trolley loop on my layout through a lionel power master.  UL listed input 120 VAC 60 hz 107 watts, output 19 vac 4 amp class 2 transformer. This was a salvage from a printer.  Trying to run the loop I had no response from the power master.  Working backwards there were no shorts, all connections were intact.  Placing a meter on the track resulted in zero.  Placing a meter on the wire from the transformer resulted in 7 VAC.  I put another power supply on the line and everything responded as expected.  The MTH power supply I used from a z-500 shows 19V output.  Clearly the problem lies with the transformer and it is now relegated to door stop mode.  In order to not repeat this failure with another transformer what is the possible cause of this?

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This is transformer similar to a MTH brick.  Output says 4 A.  Placing a meter in the track showed voltage of zero but that may or may not have been due to the power master.  Doesn't matter at this point.  Replacing the transformer got everything running fine.  The "doorstop" by itself only shows 7 volts output with no load.  It had previously provided 19V and ran this trolley loop just fine.  I could provide a picture of the offending piece but this isn't something to be fixed so there is no point.  I was just wondering why it would fail and if there was something to look for going forward.  I have a duplicate here at work that is on 24/7 and has no issues.  

  If it only puts out 7 volts now, there is likely a short in the internal windings wires.

Sort of like a variable transformer , it is tapped in a new spot along the windings.

  Likely it would have died soon anyhow, or it took a bad bump and tweaked during dissasembly.

In my experience, the ruggedness of small industrial transformers varies greatly.

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