Vactrol failure mechanisms?

I just fixed a friend's Whirlwind Selector box. This guy is an A/B switcher with a common port, an "A" port and a "B" port. You can route the common to "A," "B" or both. It uses two Vactrols each at the "A" and "B" ports. One is series, one is shunt. Operation is obvious. The slowish turn on/turn off times make for a clickless switch, too.

One of the shunt guys died. Instead of the resistor side being at very hi resistance when the LED is dark, it read about 216 ohms. I'm lazy and haven't turned on the LED to see if the resistance changes when lit up.

So what could cause this kind of failure? The thing is supposed to be able to deal with 100V across the resistor, and the logic that drives the LED is still OK.

-a

Moisture ingress? Leads shorting to adjacent components/chassis? Cracked body of the vactrol? Inappropriate use (eg switching high wattage speaker cabs instead of low voltage instruments)?

I have seen this in a new vactrol also. I bought a bag of 10 (from RS or Farnell), and one of them had a resistance of a couple of k only, instead of the normal 20M+

But I have no idea how/why this happens...

Best regards,

Mikkel C. Simonsen

perhaps sending a signal in through the output and overpowering the shunt LDR...like trying to use it between one amp and two switchable spweakers...the LDR-part is usually only good for about 500mW

..."100v across the resistor"..
..across the input ( both series and shunt LDR's) or any single LDR ?
j