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The Randall Museum in San Francisco hosts a large HO-scale model model railroad. Created by the Golden Gate Model Railroad Club starting in 1961, the layout was donated to the Museum in 2015. Since then I have started automatizing trains running on the layout. I am also the model railroad maintainer. This blog describes various updates on the Randall project and I maintain a separate blog for all my electronics not directly related to Randall.

2018-04-07 - Randall Repairs: Turnout T05

Category Randall


Turnout T05 (Stockton Front Yard).

Turnout toggle button.


Trains stop on turnout / short the block after.

Examination indicate frog has wrong polarity when T05 is Normal.

Summary Fix

Spring fallen in the Fulgurex contact that reverses the frog polarity.

Description of Issue

There are two totally unrelated issues that make turnout T05 hard to use. Make that three issues.

Toggle button issues:

  • Turnout toggle button is loose. It rotates.
  • Turnout toggle button is a 3-position and not a 2-position.

Frog issue: Second issue is an incorrect polarity on the T05 frog.

T05 is a double cross-over. When straight, the frog in mainline track 1 tested as having the same polarity as the lower rail.

What should be expected:

  • There’s supposed to be a gap between the track 1 & track 2 segment cross-overs.
  • The frog is controlled by a Fulgurex to draw power from either rail A or B.

2 possible causes to look for:

  • Incorrect continuity between the track 1 & track 2 segment
    ⇒ check for gap.  
    ⇒ Verified gap done properly by plastic jointers.
  • Fulgurex inverter does not close as expected.
    ⇒ Verified from wiring.

Under the layout:

On Fulgurex mainline track 2:

  • 2 orange wires pickup power from mainline B02, rails A & B.
  • 1 orange output powers the frog.
  • Terminal block:
    • Orange A ⇒ Green Fulgurex Input A
    • Frog ⇐ Orange middle ⇐ Red Fulgurex Output
    • Orange B ⇒ Black Fulgurex Input B

On Fulgurex mainline track 2:

  • 2 orange wires pickup power from mainline B10, rails A & B.
  • 1 orange output powers the frog.
  • Terminal block:
    • Orange A ⇒ Green Fulgurex Input A + Black lead power bus.
    • Frog ⇐ Orange middle ⇐ Red Fulgurex Output
    • Orange B ⇒ Blue Fulgurex Input B + Red lead power bus.

Description of Fix

Turnout toggle: Removed black button, tightened the nut holding the toggle, put black button back. It’s still a 3-position toggle, suggestion is later to solder together positions 2 and 3.

Frog issue: Culprit was a fallen spring in the Fulgurex contact switch.

Here’s what T05 look from below:

On the right side, the 3 wires for the frog polarity can be seen at the top: green (input from rail A), red (output to frog), and blue (input from rail B).

In the circle below is the spring that makes the contact.

What is not obvious on that picture is that it’s missing the spring itself.

On the left side, it’s easy to see the spring. There should be a similar one on the right side.

Explanation of the fault: since there’s no spring, the contact is always closed on the same side. The frog polarity is fine when T05 is reversed, it’s wrong when T05 is normal.

To fix this, the spring “just” needs to be replaced and placed back on both contacts. Except it’s a tiny sheet-metal spring (about 4-5 millimeters) and there are two tiny holes on each side for the contacts, which are about 1 millimeter in size max. All of that to be done upside down outside of reach.

I considered taking the whole Fulgurex off the layout and realized it would be even more work. Plan A was to try to put the spring back, plan B was to take the Fulgurex out.

In this case, I found the spring on the floor next to the turnout.

If that’s not possible, another spring can be taken from another Fulgurex. We have a bunch of old ones removed from the layout.

I took an old one and I practiced removing and putting back the spring at the workbench first using tweezers:

Doing it upside down under the layout was a lot harder. After several tries I managed to put it back:

Also worth noting the Fulgurex was holding only by one screw, which means it was free to rotate. I added a new screw in the reverse corner. I could not determine the exact original orientation, but this is close enough and it moves the point properly.


After fix, tested:

  • With multimeter, frog polarity was as expected with the switch normal and reversed.
  • Running a train with the switch normal and reversed. Train travelled fine through the switch fine forward and reverse.

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