Is the trigger transformer close to the bulb?
Any chance of arcing from the trigger wire
Did you try to reverse the two main electrodes of the bulb?
At this point a problem with the trigger transformer seems likely.

Peter van Hoof



----- Original Message ----
> From: Jinx <joecolquitt@clear.net.nz>
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Sent: Friday, July 17, 2009 5:53:20 AM
> Subject: Re: [EE] High voltage opto-coupler
> 
> > When U4 is activated the voltage across the U4 transistor output  
> > should drop from 65V to a least below 60V.
> 
> Yes, I'd expect that as C15 discharges. I might expect though that
> some sort of discharge curve would be visible, not a near-vertical
> nose-dive
> 
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