Got a 3500.1 here, blew up 8 of 16 irfp064's while at low volume so I'm attempting to repair it.
First thing I did before proceeding with the oscope was remove all ps fets,gate resistors,pulldown resistors and npn/pnp drivers, this amp is old so I'm going to replace those regardless.
I noticed that only half of the banks get a clean gate drive, the other half is showing 8vdc, I have traced it back to the driver board which uses the ka7500/lm494 and noticed pin 9 is showing 8vdc no pwm.
Is the right move here to replace the ka7500? Should I consider changing R10,R13 & R14?
First thing I did before proceeding with the oscope was remove all ps fets,gate resistors,pulldown resistors and npn/pnp drivers, this amp is old so I'm going to replace those regardless.
I noticed that only half of the banks get a clean gate drive, the other half is showing 8vdc, I have traced it back to the driver board which uses the ka7500/lm494 and noticed pin 9 is showing 8vdc no pwm.
Is the right move here to replace the ka7500? Should I consider changing R10,R13 & R14?
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If all of the components you mentioned that you are going to replace are out of the board and there is absolutely nothing connected to pins 9 and 10, the IC appears to be defective. The 494 doesn't have the ability to pull down with some external loads but should pull down if noting is connected to it.
R10, 13 and 14? Aren't those on the comparator?
R10, 13 and 14? Aren't those on the comparator?
Yes all of the mentioned components are removed, I also powered the driver board up while removed from the amp and got the same results, I have no idea what R10,13 & 14 run to because I didn't trace it but they look a little burnt up yes?
Anyways thank you, I will replace the ka7500 and test again with the driver board removed.
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This is a S&I design, not zenon!
If you've removed the pnp/npn driver transistors and obtain 8vdc from one of the 2 7500 outputs, of course you have a defective 7500.
Replace only the KA7500/TL494, the three mentioned resistors are ok because are covered with flux residual.
When u have a perfect signal from both tl494 output, you can proceed with the next step (drivers + gate/pulldown + zener) and so, to the new fet.
If you've removed the pnp/npn driver transistors and obtain 8vdc from one of the 2 7500 outputs, of course you have a defective 7500.
Replace only the KA7500/TL494, the three mentioned resistors are ok because are covered with flux residual.
When u have a perfect signal from both tl494 output, you can proceed with the next step (drivers + gate/pulldown + zener) and so, to the new fet.
Thanks for the correction, new to repairing amps but it appears I'm on the right track.