Sttng - some gi lights on string not working

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The second playfield gi string on my sttng has a bunch of lamps not working (although 3 of the lamps are working). The bulbs are checked and all good. Tried swapping j121 and j120 to see if using a different gi circut made a difference but it didn't. Measured voltage at a few of the bulb holders where the bulbs are out and i'm getting 6v, which I don't understand - how can there be voltage but the lamps don't light? At this point I'm stuck.

Any help much appreciated.

 

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Turns out it was the bulbs! Don't know how that happened, check with a known working bulb first I guess.

 

[JT.]

I had a car where both headlights stopped working at the same time. Took it to an auto-electricals place who ran full diagnostics on it, couldn't find a problem. Decided I'd try replacing a bulb, on the off-chance they'd both happened to go at the same time...doh