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I'm helping a neighbor with an Irritrol RD-600. The problem is that the solenoids chatter when turned on.
I disconnected all zones from the RD-600 panel and connected a Solenoid directly to the panel.
When that solenoid is turned on, it will make a screwdriver chatter when put close to it.
I checked the input voltage and found it to be 27 VAC. on a digital meter.
I checked the voltage across the solenoid I had connected to the panel and don't get a constant AC voltage. It jumps around from 5 to 14 VAC. If I check DC, it also jumps around.

It seems to me that the controller is chopping the output to the solenoid. It's neither DC or AC coming out. (If I had an oscilloscope, I would know exactly what it looks like)

All zones behave the same.
Tried two different (new) solenoids right at the panel and get the same result.

I opened up the RD-600 to see if there was any evidence of smoked parts or anything obvious.....nothing.

Anybody have any Ideas?

My neighbor took his RD-600 to a shop and they found replaced a bad cap on the controller board.
All is good now.