View Full Version : Soundcraftsmen MA5002A overheating. PLEAS HELP!!!!!!!!!!

11-17-2011, 06:04 PM
I'm a big Soundcraftsmen fan, and my setup has a Pro Power Ten amp, a Pro Control Four preamp, the best preamp I never heard, and an SC tuner. I bought a Soundcraftsmen MA 5002A in pristine condition for $25 bucks, and thought I was lucky even though I knew there were problems. The crowbar protection circuitry was kicking in every 2-3 seconds. I have a friend who's a pretty good tech, and we both hoped it was just the output transistors. When we popped the top one of the outputs transistors on the left side had one of the mounting screws sticking out, almost like it had been left that way. We tested the transistors, and sure enough that one was bad. I suggested we just replace them all. Might as well. And not with old original stock because I've been told if you can find them they're counterfeit. We knew that somebody else had been inside because some of the outputs on the left side had been replaced, and not uniformly, which is not a good sign.
He called me when he put new On Semi output transistors in and said what we have is an overheating problem on the left side. He said he turned it on and thought it was okay because the crowbar didn't kick in, so he let it sit. After about 20 minutes the crowbar kicked in. He turned it off, and the left side heat sink was so hot it burned his hand. The right side was cool. He let it cool down and put a watt meter on it and turned it on again. Started out normal pulling about 20 watts, but slowly started to heat up on the left. After about 10 minutes it was pulling a 100 watts and after about 20 200 watts and the left side heat sink was hot again The right side seemd to be fine. So he turned it off, let it cool off, adjusted the bias down and turned it on again. With the bias dialed back it still started heating up, just slower. I've been told we have to test the Darlingtons, and also maybe swap the pre-driver boards and see what that does to isolate the problem.
If anybody has any ideas or suggestions let em fly. This unit is in spectacular condition. I mean literally like out-of-the-box new condition. It's too beautiful to abandon. I love my Pro Power Ten, but my friend who's working on mine has an original 5200 with the vu meters, and I think it may be the most natural most unamplified sounding amp I have ever heard. Id love to get this 5200A up and running so if you can help please do.
Thanks Boys and Girls, Martin.