TS Pyramid PB-400a with STK4040XI

I got this Pyramid PB-400a which uses STK4040XI chips. Power Supply works when the rectifiers are pulled and outputs a healthy +40V. When the PS is connected, it oscillates on and off and a pulls heavy amount of current.

So question is, how do I troubleshoot whether the STKs are good or not?

If they are bad, I'm gonna send it back to the seller for refund, or at least try and get most my money back and see if I can find replacements.

TIA!
Tristan