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What you saw is correct (even though the photo appears mangled) for "burst fire" mode. Phase angle control generates large amounts of harmonics and requires heavy and expensive filtering components. Burst fire, because switching occurs at or very near zero voltage, does not generate harmonics and thus does not require a filter.  Burst mode can however, if you are on a weak supply, generate flicker. If you read the documentation, you will find the two modes explained.

If you require phase angle control, you need to change the sketch and of course the trigger i.c. You'll also need a filter and to find what limits apply, you'll have to check the local regulations where you live.

Thanks Robert, It was configured with Robin's PA sketch and so I thought it would be using phase angle control. I guess I'll have to go through the code to see why it was firing at zero crossing in burst mode instead.

Here's another attempt to post pictures... My wonderful Windows 8 machine is terribly unhappy with the latest version of Adobe Flash and lets me hear about it in unpleasant ways.

In Europe, we always use burst fire because of the problems with harmonic generation. I need to remember the part numbers! You have indeed got the random phase opto-trigger, but have you got the correct output pin? Two are in use, one for burst fire and one for phase angle. See the top of the sketch for the pin allocations.

"Here's another attempt to post pictures..." Yikes! large quantities of rectification. That's not good!

"My wonderful Windows 8 machine"  Most of the informed comment I've read is - shall we say - not enthusiastic about W8. I guess it's Linux beckoning when I come to replace this XP machine.

You might also have a timing problem as the sketch was designed for a 50 Hz system. So our firing delay is 0 - 10 ms (I think that's the 10000 (µs) in line 238), whereas yours needs to go from 0 - 8333.

I hope that my expression regarding Windows 8 doesn't persuade anyone to (upgrade???) to it. I'll be polite by saying that it was pure sarcasm! Long live XP

Thanks Robert, I'll give that a go tomorrow. I want to review the entire sketch for little things like that so it may take me a while.

Thank you again Robert... after making several appropriate changes to accommodate 60 Hz operation, the phase angle firing appears to be what I expect to see. It appears that my snubber is also functioning properly since there's no apparent ringing or harmonic distortion. The fuzzy picture is an indication that I had the screen intensity too high lol!

(Hmm - can't look at the pictures - I think the hash character in the file name is causing a problem)

That looks (from the thumbnail) OK. Have you checked what the limits for harmonic currents are in your part of the world? You might need to add some filtering.

I presume you needed to change the line I mentioned and the associated limits. I couldn't see anything else that might have affected it - was I right?

Yes Robert, your line definitely needed to change as did the cutoff points for min and max firing delays in microseconds. As for harmonic currents, my application merges a grid inter-tied solar system with an off-grid solar system through a 5 kw transformer. The Mk2 controls how much of the 5 kw/hr solar generated power is allowed to feed back to the exciting inverter in the off-grid system. It can handle a maximum of about 1,500 watts but since the backfed power causes all kinds of other problems with the batteries etc., I decided to avoid backfeed issues entirely by diverting excess power to a hot water heater instead. Between the inductance posed by the transformer and the snubber network, I think I can control harmonic generation satisfactorily. Time and experimentation will tell...