Pioneer SX727 keeps shutting down

Just started out tinkering with a pioneer my friend gave me after it started cutting out on him. I love the sound, so I'm committed to trying to get my new baby working, and need some help!

SO, I've traced the problem back to a burntout protection circuit. Checked incoming and outgoing voltage levels which seem fine as per service manual, switched it out with one from another unit of same/make model, and switched back on.
Worked fine for about 10 minutes, then shutdown again.
Does anyone have an idea why it might do this? Could it be related to the power suppply, or is there something known in the curcuit that i should check?

Any help hugely appreciated.

I've had a scout around here; can't find anything directly applicable but some discussion near the topic that I'll apply.
Love this site! I'll keep browsing to try and build my working knowledge up a bit.