Polk PSW250 subwoofer amplifier constant hum repair

[taligent]

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):

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%

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:

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

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!

[stj]

Re: Polk PSW250 subwoofer amplifier constant hum repair

if it seems a bit limp, try watching the voltages while playing something with some deep bass.

i found on harmon kardon soundstix subs that if you replace the 1.5A psu with something bigger - i used surplus 5A laptop supplies, they generate so much more bass that they can walk across the table!

[momaka]

Re: Polk PSW250 subwoofer amplifier constant hum repair

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.

That's very funny!
Technically speaking, if you NEVER turned the sub off, those hot regs would keep the caps warm, which would indeed decrease their ESR and keep them running.

Then again, with CapXon, you never know if that will work either. They just seem to like to fail, especially in audio equipment from that era. I rebuilt a set of JBL tower speakers with built-in subwoofer amps for someone, because one of the speakers was buzzing and popping badly. Replacing all the small caps didn't do the trick in my case, since it was one of the big CapXon 63V, 4700 uF caps that had gone open-circuit. Though JBL's design was probably a bit at fault too, because the 63V caps in those speakers were sitting at 58-60V DC with the amp unloaded - a bit too close to the caps' maximum rating. I replaced them with 80V caps. 100V would have been ideal, but there was no space on the board / in the box these amp were sitting in.

This is an anemic sub (8" driver with a 50w amp)

Still sounds better (pun intended) than a Panasonic SA-PT770 5CD / 5.1 CH receiver / "1000 Watts" system I fixed for a family friend. Now that thing is really sad - sub has only a 6.5" dia. 30 Watt RMS driver. Bur of course, PMPO is 500 Watts according to the manual.

i found on harmon kardon soundstix subs that if you replace the 1.5A psu with something bigger - i used surplus 5A laptop supplies, they generate so much more bass that they can walk across the table!

Nice!

Only question is, will the amp have sufficient cooling to drive the speaker like that for a prolonged period of time? I suspect they used a small PSU precisely because of that reason.

[stj]

Re: Polk PSW250 subwoofer amplifier constant hum repair

the soundstix are like a giant bubble, the air movement from the speaker draws air across the board!

[momaka]

Re: Polk PSW250 subwoofer amplifier constant hum repair

the soundstix are like a giant bubble, the air movement from the speaker draws air across the board!

Yeah, I've seen pictures of them before - looks like a quirky food processor or humidifier, lol. I didn't know there was a PCB inside there, though. Looked just like a speaker only. Kind of neat that they have air going over the board through the port hole(s). BTW, that design is sooo early 2000's "futuristic".

[stj]

Re: Polk PSW250 subwoofer amplifier constant hum repair

there are 2,
one originally intended for macs has a usb interface and an onboard mixer/soundchip and amplifier.
great on mac, great on Linux, never works right on windows though - some driver problem.

the better model has a regular audio line input.

and the common thing is they need to be recapped because they are filled with crapacitors.