Working on another Sundown Audio SCV-7500D. Outputs were shorted. Power supply is fine. pulled all the outputs. Working on the driver board. Pulled all the driver IC chips. Powered it up without driver chips installed. I have good drive signal to pad one of the driver chips. Installed new driver chips. Still have good drive into the driver chip. No low side drive out of any of the four driver chips. Haven't checked high side drive. I suspect it isn't there also. I have good 12 volts referenced to negative rail going to the board and on pin 7 of the driver chips. Good plus and minus rails. Good plus and minus 5 volts to drive board. Not in protect. Anyone shed some light on what might be going on?
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Gonna have to order from LCSC. This will take awhile. Do you know of a good sub off the top of your head?
If you feel like it, you can adapt a transistor into SMD "2sc3326" which is great for both KTD1302 (even if the original is not SMD) and KTD1304 (aka "max" transistor). I often use it when I have the same problem as you, and have never failed.
I think the main factor that's needed with these transistors is an extremely low VCEsat. I no longer do repairs but someone who does could easily compare them to one of the OEM transistors.