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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.

2018-01-28 - Randall Repairs: Turnout T130

Category Randall


Turnout T130 (mainline to industrial city, after Sultan).


Trains stop shortly / continue on layout. Some are unaffected.

Cause: Resistance in the frog.

Summary Fix

Bypass the frog polarity contact/inverted.

Description of Issue

Engines often stop / hesitate on the turnout. Some pass unaffected, others totally stop.

Frog issue: Measurement with multimeter shows a DC offset of 3 V on the frog.

DCC is an “alternative” electric current; a multimeter should show a 0 V DC offset component on the track, but between the frog and the main rail, multimeter shows 3+ V.

I suspect this was confusing decoders as it probably resulted in a poor DCC signal.

If they follow track voltage, the apparent voltage drops from 12-14 V to 9-11 V.

Also some decoders are designed to brake on asymmetric DCC signals (that’s a LokSound feature, although in general it’s disabled).

Description of Fix

The switch machine is an old twin-coil since it is part of the old Industrial City. The turnout is currently deactivated, although it did seem to be working when I was trying to revive the Industrial City back in 2015.

The twin-coil features a toggle which is used to invert the frog polarity. This is connected to a terminal on the right. Top 2 wires are from rail A and B. A 3rd wire under it is connected to the wrong.

A potential culprit could be oxidation on the twin-coil polarity toggle. I did not try to fix that.

In any case since it is currently not used, the quick fix was to unsolder the frog from the terminal and connect it directly to the outside rail power.

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