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Re: aaarrrrrgggGGGHHHHHH!!!!

8/22/2000 7:33 PM
R.G. Re: aaarrrrrgggGGGHHHHHH!!!!
"Can u fire it up without the pwr Mosfets in circuit, and then get a reading on the gate connections to see what kind of voltage is showing up there?"
Won't help. The power mosfets will happily oscillate at VHF - where most audio scopes won't even see it! - as source followers no matter what is happening with the rest of the amp.  
You might be able to remove the source followers entirely , and connect the feedback to the output of the voltage amp and see what is happening that way.  
I like the idea that the Zobel is busted, or that the power mosfets don't have 100 ohm gate-stopper resistors right at the gates and are killing themselves.
8/22/2000 8:14 PM

My electronics training is self taught during the late 1960s, early 70s and the MOSFET wasn't around then - hell, the JFET was exotic. Started back into electronics part time about 10 years ago and amp repairs about 4 years ago - so, what is a "Zobel network" and how does it function is a MOSFET output? I have enough trouble thinking about junction transistor circuits and only replace a few MOSFETS per year (although it's getting to be a big year this year :) )  
8/22/2000 8:45 PM
Send it to Ashland GW.. then i send you a mackie board,Its bad enough without the mouse poop N pee..I already know whats wrong with the Mackie,.. the Power tranny is fried..most likely a few other things along with it..I bet that was one of those elusive virginia fairlydiddles..that made that magic mess in there..I know what you mean..I get amps that were stored in barns, even had a big bird nest once in a Laney head, ..I worked on a sound city that had mud dobbers sealed the input jacks..and the amp lookked about the color of cardboard.. or mud.. sit for 17 years.. cause the OT went out and they told him no one could fix it..Its now back alive blasting some honky tonks.. He didn't even think it was his amp .. cause i had cleaned it all up. Sounds like these other posters know alot more about this one than i do.. Hvae you checked the speaker since the last amp check blow out.. it didn't take it out the speaker did it?.  
8/23/2000 8:38 AM
Dave H.

A Zobel network is a resistor and capacitor in series from the output of the amp to ground. Itís usually something like 10 ohm and 0.1u or 4.7 ohm and 0.047u. Itís in parallel with the speaker to try and make the load seen by the amp look more resistive at high frequencies. It offsets the rising impedance of the speaker with frequency.  
8/24/2000 11:36 AM

Thanks Dave
8/22/2000 11:03 PM
Rick Erickson

"worked with my test speaker and them promply fried the IRFP140 when hooked to the cabinet speaker."
Check the speaker, especially if the speaker frame is grounded. Sometimes a flex wire can touch the frame or the voice coil can internally short.  
Anytime you work on a solid state amp you should plug the amp into a series lamp (100W-500W lamp in series with the incoming AC) until you are certain that all is well. This will limit current in case of a short. Use less wattage for smaller amps, higher wattage for larger amps. I have a 100W & two 200W bulbs with switches to add bulbs or bypass them altogether as a part of my test bench.  
Check for possible hairline solder shorts across traces. I had a newer GK amp with a similar problem caused by a solder bridge which was nearly impossible to see. It wasn't frying anything though, it just wouldn't conduct on half of the signal under load.  
Good luck with this one.  
8/23/2000 12:00 AM

As the thread lengthens, detail gets lost - somewhere earlier I mentioned that the symptom had appeared with different speakers - my bench test speaker and the newly replaced cabinet speaker. Generally I bring up all SS amps that I've done output work up on a variac as I did both times w/o the speaker load - and adjusted the bias w/o load as per factory specs - buttt, it appears that I should have brought it up on the variac with the load connected - just have never had to do this before - will definitely do so in the future.  
As to solder bridges, went over it with a magnifying glass both times and I'm in the habit of draggins an x-acto blade backwards between the solder pads to remove any bridge and to clean up the rosin enough to inspect the work. Thanks  

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