MTH signal 30-11009

One of the signal heads is acting up.  When it is red, everything is fine.  When it is green, the yellow light also lights.  This is part of the cycle for this item, yellow for about 10 seconds before turning green.  I have swapped the head to the other location on the signal and to another signal elsewhere, the problem is isolated to this signal head.

Is this something that can be fixed?  If so, what do I look for.  Otherwise buying another pair is a good excuse to visit the hobbyshop.

Original Post

Well, it sounds like you've figured out where the problem lies. Is there a way to open up the signal head to check the wiring to the LEDs? Those are pretty tight spaces so it could just be a crossed connection between the leads of the LEDs or a bad external connector. I'm not sure if it can be fixed.

Years ago, member fec fan posted this schematic of some MTH signal heads which probably represents what's in the 30-11009. 

mth 30-11016 schematic by fec fan

If this is meaningless to you, maybe you have a buddy handy with a meter and soldering iron that can help you out on a rainy day.  For example, IF the green LED dims slightly when the yellow LED is also on, I can see a failure mode where the IC chip (555 SMD on schematic) has a defective "high" output.  The chip itself is only 10 cents or so but a nuisance to replace since it tiny.

555 smd


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stan2004, thanks.  From the looks of it the price in grief trying to work with a part that small far exceeds the cost of a pair of signal heads.  Also if one failed, another will also fail eventually so why not have a spare?  thanks for the input.  Henry

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Alan Mancus

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