plate amp troubleshooting

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himhimself
Posts: 562
Joined: Fri Nov 13, 2015 9:34 pm
Location: Bloomington IN

plate amp troubleshooting

#1 Post by himhimself » Sat Aug 28, 2021 8:14 am

I have a little JBL SUB145 powered subwoofer that recently just stopped working. Looking for some help in troubleshooting the problem, if possible.
Pulled the driver first and tested it - it's fine.
Pulled out the plate amp, hoping it might just be the fuse. Nope, mains fuse is fine.
Nothing looks obviously burned or bloated.
There is a sticker on the transformer (TRANSTEC TT0869906580-E) that says it has a built-it thermal fuse. Thought that might be it, but then checked the outputs (two red wires, one black) and found 24V on both red wires.
There is a power indicator LED downstream on the board that has the inputs that is lighting up. It lights up red only. It should turn green when running, via a signal when in Auto On mode. That's not happening.
Audio electronics are definitely not my forte. Any thoughts on where to look next?
thanks!
2xT30 (20", 3012LF)
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J_Dunavin
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Joined: Sun Jul 09, 2006 3:13 pm
Location: Appleton WI
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Re: plate amp troubleshooting

#2 Post by J_Dunavin » Sat Aug 28, 2021 8:36 am

I had a plate amp that had the gain potentiometer go bad. Something to check....
One way I like to troubleshoot that sort of problem is to turn on the amp and inject a 1K tone and start measuring around to see where it stops. Or if there is an on board filter, inject a tone within the passband, like 50hz. (A schematic really helps at this point)
PLEASE BE CAREFULL AROUND THE MAINS VOLTAGE!
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