PLS HELP! Strange Mcintosh MC431M problem

[stevexyz]

Hi all. My Mcintosh MC431M amp is acting funny lately (it's been working perfectly until couple weeks ago). Let me describe the problem in detail first:

After leaving my car off for several hours (like parking over night), if I turn on my audio system right after I start the car engine, then my MC431M will go into protect mode. It'd scream my speakers really loud for a second or so before it goes into protect. If I detach all the speakers from the amp before turning it on, the amp's both power meters (all four channels) will shoot over the scale, but it won't go into protect mode at all. I have to turn it off right away in order to prevent it from blowing off.

But the amp works fine if I just turn the key to on without starting the car engine. And if I let the amp stay on like this (keep playing music) for a few minutes then start the engine, then the amp will keep working even after the engine starts.

I measured the voltages feeding to the amp before and after the engine started. they were about 12.6V and 14.6V respectively, which seemed normal. I also tried swapping my 431M with my 420M. the 420M didn't give me any problem at all. So I believe my car's electrical system should be ruled out. I also checked all my speakers. There were nothing wrong with them either.

It first sounded like a capacitor problem to me. You know, after I turn on the 431M at low voltage (12.6V running on the battery only) for a while, all the capacitors would have been charged up, then it works at a higher voltage (14.6V after the engine starts). But if the amp is left off for a few hours, all the capacitors would have been discharged. When I supply 14.6V to the amp at this point, the problem occurs.

So I opened the amp and did find out a few leaking 50v 10uF capacitors (not too bad tho). Then I replaced these leaking caps. But to my surprise this didn't solve the problem. Now I'm really confused. All other parts look just fine, no burnt mark or anything. So what else could be the problem here? Can anyone help me please? Thanks!

[stevexyz]

anyone pls?

[llwhtt]

diyAudio does have a car audio forum, try there.

Craig

[stevexyz]

Originally Posted by llwhtt

diyAudio does have a car audio forum, try there.

Craig

Great, thanks! Will give it a try over there