I have a Space Invaders that I'm putting back together and a number of the playfield controlled lamps were out. All but two of these were solved by replacing the old crappy bayonet bases. Here's what else I've checked out so far for the last two:

-The remaining two bases are fine, when I hotwire them to adjacent lamps with an alligator clip, they light up just as they should.

-The wires are fine, I checked continuity from the lamp terminals to the connectors in the backbox.

-One of the lamps was working yesterday when I first powered on, and stopped at some point.

-The contacts and pins are new and were replaced by a previous owner.

-One pin leads to Q7 and the other to Q27. When I check continuity from the pins to the legs of each SCR, I get a tone, so it's not a cold solder joint.

-Board is an AS-2518-14 which technically is incorrect for Space Invaders, but other than this it works fine, and everything I've read/heard says they're interchangeable.

What is my next step to determine the failure point? I currently have the board out of the machine. Please assume in your answer that I am a complete idiot when it comes to stuff like testing transistors, because I am.

Yeah, here's where the "I'm an idiot" part of my post comes into play....

Okay, found a section on measuring the SCR's on Pinwiki. All the working parts measure right around 0.7, right in the middle of 0.5 and 0.8. Q7 measures 0.9 and I get no reading at all on Q27. I assume both parts are bad. However, when I swap the red and black leads for the second part of the test (where good = 1.4 to 1.6) I get no measurement on any of them. Is the board supposed to be in the game and powered on when testing?

Thanks Quench, this is exactly what I needed. Looks like both SCR's are bad and I'll get some next time I order from GPE.

As it turns out, and I don't know why I didn't do this right away, I have a spare Space Invaders head with backglasses that I display on my "wall of heads". I had previously robbed the auxiliary light board and sound board from it when I had problems with my existing ALB (damaged trace) and SB (needs a capacitor rebuild) and they both worked perfectly. The lamp board wasn't the same part # as the one giving me problems, but as it turned out, the one in the actual game was the wrong part # (though interchangeable) and the one in the other SI head was the correct part #. I pulled that one, cleaned it up a bit, wire brushed a little oxidation from the header pins, and it fired right up with all lamps working. So now I have a few boards to practice my soldering and desoldering skills on and see if I can bring them back to life.

Any idea why I wasn't getting the other measurement (1.4 - 1.6) when I swapped the leads, even on the good SCR's? I followed the instructions from Pinwiki exactly.