Symptoms: Sub works, but produces a constant low level hum/buzz.
Troubleshooting: The amp uses CapXon capacitors, the first (and only) troubleshooting step was pulling the caps (other than the large power supply caps and non-polar caps) and testing with an AVR transistor tester (upgraded with a precision voltage reference and 0.1% reference resistors):
Replaced all of the above with old salvaged Panasonic 105° caps (KM series) - C12 and C23 have the classic symptom of measuring good on capacity but with high ESR. These caps are surrounded by the power supply caps, a pair of power transistors, and the bridge rectifier - to add insult to injury, one of the power transistors was bent over as close to the caps as possible. I'm sure this was just an attempt by the amp designer to create a nice little oven to keep the caps in a thermally stable environment.
The large power supply caps and non-polar caps measured okay and left as-is, I didn't bother pulling these out and instead used the AVR transistor tester's in-circuit mode:
Result: Success! The low level hum is gone, and the sub works perfectly. This is an anemic sub (8" driver with a 50w amp), this post is for reference in case there are other Polk subs around this era with the same symptom. The first image shows the bad C12 and C23 caps, the second image shows the amp after replacing caps. Hope this helps!
Troubleshooting: The amp uses CapXon capacitors, the first (and only) troubleshooting step was pulling the caps (other than the large power supply caps and non-polar caps) and testing with an AVR transistor tester (upgraded with a precision voltage reference and 0.1% reference resistors):
Code:
Part Original Measured ESR Vloss C4 100uF 25v 75.62uF 2.1 ohm 3.3% C12 22uF 50v 22.54uF 8.5 ohm 4.7% C14 220uF 25v 231.1uF 0.34 ohm 1.8% C15 220uF 25v 227.2uF 0.45 ohm 2.1% C17 47uF 50v 46.29uF 0.52 ohm 1.0% C23 22uF 50v 22.27uF 9.0 ohm 5.2%
The large power supply caps and non-polar caps measured okay and left as-is, I didn't bother pulling these out and instead used the AVR transistor tester's in-circuit mode:
Code:
Part Original Measured ESR C1 2.2uF 50v 2.49uF 3.7 ohm Non-polar C9 22uF 35v 27.8uF 1.9 ohm Non-polar C20 4700uF 63v 4630uF 0.74 ohm C21 4700uF 63v 4650uF 0.75 ohm
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