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Re: aaarrrrrgggGGGHHHHHH!!!!

8/24/2000 7:43 PM
Rick Erickson
Re: aaarrrrrgggGGGHHHHHH!!!!
"I mentioned that the symptom had appeared with different speakers "
I caught that after I posted, still it's a good thing to check. As for the variac - I still suggest - use a series lamp. It will save your ass when working on SS amps. I used to think of it as a poor man's variac but now I consider it one of my most important items on the bench. You can adjust the current up in steps while observing circuit operation under load. If a problem develops it can respond much faster than a hand on a variac. It can also be used to find intermittent shorts without blowing fuses. Try it - you'll like it.  
One more thing - check all wirewound & f/p resistors to be sure they aren't open. I had another GK amp blow it's new outputs last night because I forgot to do this. Similar situation, It seemed to be working, I bypassed the series lamp and proceded to tweek their little junctions. I'm still not sure why I didn't catch the problem before throwing full current at them. Sometimes stuff happens, especially late at night when I'm getting tired. Fortunately TIP35's & 36's are cheap.  
As before, good luck & happy troubleshooting! :)  
8/24/2000 4:35 AM

This might sound like a weird question but when the original mosfets were installed did they have insulating mica under them? Or where they just bolted straight to a heatsink? I would think that it would blow straight away, but them again, strange things happen.  
I'd also try changing the output caps - both of them if there's two. I had an stereo amp blowing caps once, due to the other being faulty.  
8/24/2000 11:38 AM

Thanks for the tip. The MOSFETS used teflon insulators w/o heatsink grease.  
8/25/2000 2:56 PM
Stephen Conner

Make sure you used the right protection diodes the right way round. Each MOSFET should have a Zener diode (of about 12V) connected between gate and source terminals. If you replaced the Zeners with ordinary diodes by accident, that would explain your troubles exactly.  
Steve C.
8/25/2000 3:20 PM

Thanks, but.... have already replaced the zeners twice with the specified zeners - in4744s. These are connected anode-to-anode between the gate and source on this design.  

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