I’ve got a home build subwoofer using a Dayton SPA250 plate amp that I recently pulled out of storage. When I first powered it up I got loud thumping followed by a loud hum with the driver fully extended. (No signal hooked up). I opened it up and found some burnt resistors that I replaced. Now when I plug in I get a hum, very load when the switch is off, but quiets a bit when on. Again no input. If I send an input, it does play. The hum happens in my house and at my office so I think wiring is out. My next thought was the main filter capacitors. She I measure resistance across the caps, on one it starts low and gradually rises but on the other it sits at a constant 2500 ohm ( it’s in parallel with a 3k ohm resistor.) Should that be the next logical part to replace? Any help. Thanks!
If you get sound (Hum) in the off position, it indicates a problem with the ON-AUTO-OFF circuit. The STANDBY Board is basically a switching (SMPS) power supply that feeds (or doesn't) the power relay coil. This relay allows 115VAC application to the main power transformer. C8 on the standby board has failed in multiple instances. This cap is the output filter for the SMPS; it should be a 25 WVDC @ 470uF low ESR, high ripple part. The factory part is a generic 16 WVDC, 470uF cap. This is part of the problem . . .
A driver fully extended indicates DC Voltage on the amp's output. A few part changes happened to the power resistors used around the Zener diodes and filter capacitors. These values changed over time; 2k0, 2k2, 2k4, etc. The latest version has all four resistors at 3k0 and 2W. You should change all four resistors and take a hard look at C8. Actually check all the caps with a magnifying glass while looking for buckled end caps. Problems have been reported with P/S decoupling and feedback caps. Good Luck - EVMan
A driver fully extended indicates DC Voltage on the amp's output. A few part changes happened to the power resistors used around the Zener diodes and filter capacitors. These values changed over time; 2k0, 2k2, 2k4, etc. The latest version has all four resistors at 3k0 and 2W. You should change all four resistors and take a hard look at C8. Actually check all the caps with a magnifying glass while looking for buckled end caps. Problems have been reported with P/S decoupling and feedback caps. Good Luck - EVMan