400w HPS ballast stopped working.

I have a hydrofarm 400w S51 ballast. I bought it used (for about 1 year) and I've been using it for several months with a new bulb. It's been working great, but yesterday it just went out. The ballast was still humming away, but there was no light.

I thought the bulb went bad, so I got another one. I put the bulb in and plugged the ballast back into the wall. The light flashed for a split second. I barely even noticed it. And then it would not light up. I unplugged and tried this again, and there wasn't a flash anymore. I looked at the base of the bulb and there was a burn mark on the side of the copper base. Just like the original lamp had. I assume this is just the ignitor doing it's job?

How do I trouble shoot this problem?

Happened to me a couple years ago on a 6 yr old 400 watt ballast. I eventually just got a new switchable. Repairing it yourself is fine if you really know what your doing, but I don't and wouldn't risk my family in a fire for my buds.