OK guys... the Wirematic saga continues. I haven't had time to look at the machine for a few days and this might have helped things.... Today, I either had a brainstorm or a brainfart... and remembered something that now I think I should have investigated further when I took the SCRs out for testing.
When I removed the SCRs for bench testing, I've noticed that the surface of both SCRs as well as the surface of the heatsink where the SCRs make contact with, have either signs of pitting or corossion. Where there was a bump in the heatsink, the corresponding point on the surface of the SCR would have a dip. At first, I didn't even think that this is pitting or corossion. My initial thought was that this allows the SCR to sit tightly against the surface of the heatsink and/or allows precise repositioning of the SCR in case it is ever removed, etc. Now, don't laugh as I have NEVER worked on high current equipment and I really don't know what to expect when examining such problems.
Now that I'm thinking about it, it doesn't make any sense to me. You'd expect to have a clean point of contact between the SCR and the heatsink to allow the maximum transfer of current from the SCR to the heatsink (and consequently to the gun). The uneven surface that I'm seeing there is probably a result of moisture getting in there and corroding the surface of both, or a sign on sparking. It may not be as critical in low current equipment but I'd think that it would make a heck of a difference in instances where a very high current is needed at a steady voltage.
Can you guys confirm this for me? I am planning to go back to my friend's either tomorrow or the day after.
Your thoughts would be muchly appreciated.