M44 problems

[BillyP]

Hello everyone, been a lurker for a while and finally had a reason to post. A ton of info but don't see anything that covers the problem I am having.

I have an M44 running a set of Boston 4x6 coxials in the rear on the front channel of the amp and a set of 10" Pioneer subs on the rear channel bridged. Now, the Pioneer's are dual 4ohm voice coils that are wired in series on each sub going to two terminal connectors so I can run them either stereo, or parallel taking the load down to 4ohm and running them bridged on the rear channel. The amp hasn't been powered up for a while and do not know what has happened but I have to run the gains all the way open to get any sound out of them at all, then turn the stereo up quite a bit (it's an Alpine 9857) to around 18-19 just to be able to hear over the normal road noise. I've read where the input caps can go bad over time (these are the original ones from what they look like) is it possible that is what is going on with my amp?

I had it unhooked yesterday showing my father-in-law (he does electronics for a hobby, around 45 years) and he wasn't really for sure, but this morning after I hooked everything back up it was hitting like it was before I put the amp up (still had the gains up and it went into overload and kicked out) and now is back to where I have to turn the gains all the way back up to get sound out of it again.

thanks for any help on this in advance.

[ttocs]

I would be carefull with the amp. Take the cover off and see how the capacitors look as they are known to go bad after a decade or so and can cause a big explosion leading to big $$ repairing them. Take a couple pics and we will see what we see.

[BillyP]

Well, here are some pics. Stupid digi camera wouldn't focus in when I got close so hopefully what I saw will show up.

[Shinju]

Uninstall that amp right now and fix those 4 powersupply caps here in this photo below! You got some massive leakage going on and it will cause some serious damage if not corrected.

[BillyP]

That's what I thought when I saw that, I immediately turned the radio off and pulled the fuse for the amp when It went into overload. Thanks for the info I'll swap those caps out.

[Shinju]

Make sure to clean those area's good with a q-tip and 90% rubbing alcohol, Also make sure that thick cap bed trace is not lifted from the board if it is then use extreme caution as that is a double sided board and might pull something off from the bottom also use a low heat solider iron too .

Just use the search function here there is a tutorial on how to and what to use and the do's and dont's!

Best of luck! Oh and post rebuild photos in progress!

[stipud]

Definitely replace those caps. As for the sound cutting out, have you tried reversing the channels? So the front is powered off the rear of the amp and the subs off the front? It could be an issue with the RCAs or your headunit as well.

[BillyP]

That's the way it was hooked up and when I reversed the channels, it still sounded the same. I'll replace the caps and go from there, this site has always been awesome and a load of information and I'm glad it's here.

[stipud]

That's the way it was hooked up and when I reversed the channels, it still sounded the same. I'll replace the caps and go from there, this site has always been awesome and a load of information and I'm glad it's here.

So either way you had it hooked up, it was the sub channel that was quieter? That most likely indicates an issue with the RCA, headunit or the subwoofers themselves. But it definitely doesn't seem to be an amp issue, otherwise it would have been on the same channels all the time.

Do the caps anyways though

[BillyP]

Well, I have input caps, output caps, rail caps and the two resistors that I've read people recommend changing as well on order. Should be in later this week. I'll post pics after it's all finished.

[finbar]

The electrolyte from the caps may have seeped into the board and made it conductive. When you have removed the old caps test the board before installing the new caps.
Good luck with the refurb.

[BillyP]

Well, as we were taking the old caps out to put the new ones in, my father-in-law found one of the power mosfets had literally exploded for some reason. He ordered 4 to replace the bad one and the other three as well so just a small update on that.

[BillyP]

Well, all the caps are replaced, ended up replacing all 4 of the power fets. The amp powered up just beautiful, we put an o-scope on it to see how the outputs were and now it's going between overload and power , it was drawing roughly 5 amps at idle and he's at a loss without any type of schematic to go by. Has anyone seen this happen before and may know what is going on?

[ttocs]

if the fets went i thought it meant you needed to check and replace the caps/resistors that power them as they tend to go at the same time. I am sure eric will have more info to help with it.

[BillyP]

Yeah, two of the fets were blown from before the caps were replaced so we went ahead and replaced all 4 of them. Hopefully Eric will have an idea on what is going on with it. I ment to take some pics of it while the wife and I were at his house, but didn't.

[00goobs]

If the output transistors have a direct short to gnd from either rail voltage, it would cause the fets to heat up and die. You could test the output transistors to see if any of them are shorted/grounded out.

...also, if one fet or the fused gate resistors burns out, it would cause the other to take the full load of switching the transformer then burn out...

[Eric D]

Could be...

A short in the transformer.
A short in an output transistor.
A bad resistor in the overload feedback circuit. There are 4 resistors (one for each channel) which feed back to a LM339 and shut the amp down. If one is bad you can have issues. I found this on a M44 before and it took me several hours to diagnose.
Something causing uneven output voltage, thus kicking in the overload through that same LM339.