macintosh analogue board reconditioning

Howdy to the list (and especially to Tony)... I have a Macintosh 512K I would like to use to demonstrate certain UI principles (okay, and also to play Dark Castle). The display hasn't worked properly since I got it though. In an attempt to stabilize the image, I pulled off the insulating fibreboard covering the sweep board, and looked for bad solder joints to fix. The problem is I don't know what a bad solder joint looks like aside I reflowed a few joints randomly, with no clear sense of what I was doing or where particularly to focus my attentions. Predictably it did not change the behaviour of the image on the display noticeably. I just in the last few minutes switched the machine on to photograph the display for your expert appraisals, and found that its behaviour has changed since it has sat. Now when I switch it on the internal speaker makes a repeated popping. I'm guessing the power supply is resetting itself continuously for whatever unknown reason. I could use some guidance repairing this fault. What's my first step? Tony, I'm a module swapper because that's what I know, and usually the problems I have are in software anyway. I'm handy with a soldering iron, though, and I'm showing a willingness to learn something new. Help! ok r.