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Old 22nd May 2007, 11:16 AM   #21
Bonsai is offline Bonsai  Europe
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I assume in your testing you are connecting the load hrough the jack plugs - are you sure they are not damaged/shorting?

also, be aware that mosfet vendors have a habbit of shrinking die (beleive me, I know aboujt this stuff) and often that leads to changes in device specs - subtle, but if a design is on the edge, a small change may be enolugh to tip it over the edge.

Have you checked that the Zobel network is ok? Resistor not open circuit or cap shorted?
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Old 22nd May 2007, 07:45 PM   #22
Tarzan is offline Tarzan  Belgium
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I wouldn't try power-on the amp without preamp connected.
They have the habit of oscillating.
With a signal source on it it should run fine.
I try these amps (repaired a few of these 350's) with a regulated psu.
Youknow; a bench top thingy of +/- 32V and 3 Amps max.
Just in case to not kill more then needed.
Could it be that the heatsink is not wel bolted to the pcb?
Is the pcb clean?
Check the driver transistors. Are they well soldered in place?
Check with a magnifying glass if there are no thin cracks in the solder.
I had a hell of time with one with that problem.
It ran fine and suddenly started to distord and back to normal again.
Good luck.
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Old 23rd May 2007, 12:38 AM   #23
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FOUND THE PROBLEM! C13 had a cold solder joint! Now the amp runs stable.

C13 is the rail cap after the diode that feeds the front end on the positive rail. I had to dig out a microscope to see that the joint was cold. this is a big cap and vibration must have caused the lead of the cap to pull loose from the solder. Normally i can spot these right away. this one was much more difficult to see.

I touched uo several solder joints even though they looked good. if they even slightly looked weak i touched them up. then added .1uf caps across C13 and C33 just for good measure.

Ran it into a 4 ohm resistive load under full heavy clipped sine into a square wave and no problems at all!

If i run it up on a variac it still oscillates pretty good until the power gets up high enough for it to stabilize. I will have to work on that a bit but. no more craziness at full power.

Now i am debating if i should just replace the output relay with the same type, or upgrade it to something a little beefier. it was a pretty weak relay.

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