Polk PSW150 Sub amp questions

[MuseChaser]

I picked up this sub for $10, knowing it didn't work. Also knowing that it's a notoriously unreliable piece of equipment that, when working, sounds OK.

Upon closer inspection, the driver itself seems sound.. no rubbing. Amp powers on, meaning red indicator LED lights up, but no sound at all, other than a VERY loud sharp crack when turning off the power. Soooo... removed and disassembled the amp. Now.. questions...

Question #1 - Does anyone have a schematic for the amp in this powered subwoofer? I spoke with Polk today and they informed me they did not have anything available for that model.

#2. I'm assuming this jumper wire I found underneath the control board is not original. Can anyone confirm that this is a bad attempt at fixing something rather than factory?



Also noticed that, on the same board, jumper wire W19 is missing, although there are remnants of that wire existing in the eyelets...



... also notice, in that pic, lots of brown goo around the green caps and slight deformation, although they measured within tolerance of the correct capacitance. I don't have anything that'll measure ESR.

And... on the main amp board, the large main 10,000 mfd 50V caps have brown crap underneath them, although they, too, measure within tolerance on my multimeter's capacitance function. Does this mean they're shot? Brown stuff indicated by yellow arrows...



At this point, I realize that the usual advice is, "scrap the amp, put in a new subwoofer plate amp and call it good," but I'm approaching this project as an opportunity to learn more about the repair process.

Anyone have any experience working on this amp? I've found YouLube videos addressing the "heartbeat" constant noise issue, but nothing specifically for no output at all except for a crack on power-off.

The phase toggle switch was completely frozen and inop, but I've found a replacement for that.

Any ideas/help would be, of course, appreciated.

 

[Lavane]

The brown goo is glue. You don't want bits rattling inside a sub. Sometimes they get heavy handed with the stuff. Some of the glues used were known to be corrosive. If it got in the cap leads, they can corrode up into the cap. But it sounds like yours are fine and it's not that kind of glue. Maybe try and find data sheets for all the IC's and check voltages?

 

[Ray Gianelli]

I picked up this sub for $10, knowing it didn't work. Also knowing that it's a notoriously unreliable piece of equipment that, when working, sounds OK.

Upon closer inspection, the driver itself seems sound.. no rubbing. Amp powers on, meaning red indicator LED lights up, but no sound at all, other than a VERY loud sharp crack when turning off the power. Soooo... removed and disassembled the amp. Now.. questions...

Question #1 - Does anyone have a schematic for the amp in this powered subwoofer? I spoke with Polk today and they informed me they did not have anything available for that model.

#2. I'm assuming this jumper wire I found underneath the control board is not original. Can anyone confirm that this is a bad attempt at fixing something rather than factory?

View attachment 2509546

Also noticed that, on the same board, jumper wire W19 is missing, although there are remnants of that wire existing in the eyelets...

View attachment 2509548

... also notice, in that pic, lots of brown goo around the green caps and slight deformation, although they measured within tolerance of the correct capacitance. I don't have anything that'll measure ESR.

And... on the main amp board, the large main 10,000 mfd 50V caps have brown crap underneath them, although they, too, measure within tolerance on my multimeter's capacitance function. Does this mean they're shot? Brown stuff indicated by yellow arrows...

View attachment 2509557

At this point, I realize that the usual advice is, "scrap the amp, put in a new subwoofer plate amp and call it good," but I'm approaching this project as an opportunity to learn more about the repair process.

Anyone have any experience working on this amp? I've found YouLube videos addressing the "heartbeat" constant noise issue, but nothing specifically for no output at all except for a crack on power-off.

The phase toggle switch was completely frozen and inop, but I've found a replacement for that.

Any ideas/help would be, of course, appreciated.

Jumper is probably stock from factory.

Is phase switch frozen in one setting or another, or stuck in between? If the latter it could be the whole problem.

Internal subwoofer amps are subjected to enormous amounts of vibration, so with no schematic available a careful examination of all the solder joints would be a good first step.

Large filter caps are probably OK. Check the DC voltage on them and see what it is. Should be close to the 50 volt rating, around 45 at a guess.

Red LED may indicate standby mode. A lot of powered subs have a bicolor LED that switches from red to green when the sub sees a signal.

Small electrolytics are more suspect than the filters. Check those.

Please post a pic of the entire board.

 

[MuseChaser]

Thanks so much, Ray. I've looked over all the traces under magnification, and haven't found anything yet. Measuring the caps' capacitance in place (but without any power, of course, boards out) hasn't turned up anything unusual, although most are over their ratings. I don't have an ESR meter. Didn't do any voltage tests before disassembly, but will do some later. The phase switch was frozen in one setting, but when I removed the switch there was no continuity between the center contact and either of the outer legs, so it IS possible that was, at least, part of the issue.

Here's some pictures of the control board. I'll post pics of the amp board in another post.

Overall top view ..



Left side close up ...



Right side close up, showing missing jumper and indicating where wire on bottom connects, and where trace would to IF W19 were present...



Bottom of board, showing wire and traces ...



..and bottom showing where the phase switch I removed was...



Amp board pics to follow... and THANK YOU!

 

[MuseChaser]

...and some pics of the amp board....



..and top overview...



..... and a few closeups. With the harness from the transformer still attached, it was hard to get good angles. I can unsolder it if necessary.

 

[MuseChaser]

A success story followup, just in case anyone finds it interesting...

I stared at the circuit a bunch, and the more I though about how the added wire routed a signal meant for the 6080D D+ input to the non-inverting input of one of the 4558 chips, the less sense that made. Same with the removal of W19, which PREVENTED the signal from getting to the 6080 D+ input. All I can figure is that, when the phase switch went defective, someone with really poor eyesight attempted some really bad way of bypassing it... who knows.

Throwing caution to the wind, I replaced W19, removed the added under-board insulated jumper wire, and bridged the center and 0deg tabs as a means of bypassing the faulty phase switch I removed.

Bound the speaker leads to a dummy load and powered it up. Red indicator light, no magic smoke left... so far so good.

Hooked up the scope, fed some test signals... Hmmm... traces look good, frequencies are correct, a little bit of oscillation up around 150K, but otherwise nothing remarkable. Low pass adjustment and volume adjustments work. Sooo..

Tried music input instead of test tones. Traces as expected... Sooooo....

Hooked it up to a cheap speaker, just in case, and tried it at low level...

Works! I need to reapply some damping silicone or hot glue before placing it in the original cabinet with the driver, but I think we're out of the woods.. at least for now.

 

[Ray Gianelli]

A success story followup, just in case anyone finds it interesting...

I stared at the circuit a bunch, and the more I though about how the added wire routed a signal meant for the 6080D D+ input to the non-inverting input of one of the 4558 chips, the less sense that made. Same with the removal of W19, which PREVENTED the signal from getting to the 6080 D+ input. All I can figure is that, when the phase switch went defective, someone with really poor eyesight attempted some really bad way of bypassing it... who knows.

Throwing caution to the wind, I replaced W19, removed the added under-board insulated jumper wire, and bridged the center and 0deg tabs as a means of bypassing the faulty phase switch I removed.

Bound the speaker leads to a dummy load and powered it up. Red indicator light, no magic smoke left... so far so good.

Hooked up the scope, fed some test signals... Hmmm... traces look good, frequencies are correct, a little bit of oscillation up around 150K, but otherwise nothing remarkable. Low pass adjustment and volume adjustments work. Sooo..

Tried music input instead of test tones. Traces as expected... Sooooo....

Hooked it up to a cheap speaker, just in case, and tried it at low level...

Works! I need to reapply some damping silicone or hot glue before placing it in the original cabinet with the driver, but I think we're out of the woods.. at least for now.

Damn good job!

 

[MuseChaser]

Thanks, Ray! Someone was kind enough to send me a schematic, after the fact, and my rambling about "signal getting to..." blah blah were way off, of course, since I don't really know anything. Pin 12 of the 6080D, the D108 cathode, D109 anode, and pin 3 of the 45580 in question all go to ground. I couldn't see how the trace from pin 12 got there on the board, and maybe the last person who "fixed" this amp had the same issue so he/she just rerouted the diode node directly to ground at pin 3 of the 45580. Who knows. In any case, the amp works fine with W19 back in place.

One small setback.. once installed in the original enclosure and hooked up, the driver made a lot of buzzing and rattling noises at anything above very quiet levels. Pulled it, and the corrugated spider had separated from the basket. Regluing that now. Should be all set soon!

I really appreciate the help and encouragement. Thank you.

 

[PNMRED]

Thanks, Ray! Someone was kind enough to send me a schematic, after the fact, and my rambling about "signal getting to..." blah blah were way off, of course, since I don't really know anything. Pin 12 of the 6080D, the D108 cathode, D109 anode, and pin 3 of the 45580 in question all go to ground. I couldn't see how the trace from pin 12 got there on the board, and maybe the last person who "fixed" this amp had the same issue so he/she just rerouted the diode node directly to ground at pin 3 of the 45580. Who knows. In any case, the amp works fine with W19 back in place.

One small setback.. once installed in the original enclosure and hooked up, the driver made a lot of buzzing and rattling noises at anything above very quiet levels. Pulled it, and the corrugated spider had separated from the basket. Regluing that now. Should be all set soon!

I really appreciate the help and encouragement. Thank you.

Would you be able to share the schematic with me? I would like to try and repair mine. Mine has w19 missing and the "homemade wire bypass" as well. I always wondered why the phase switch never made a difference on this sub...

 

[ARC Tech-109]

(snip)

One small setback.. once installed in the original enclosure and hooked up, the driver made a lot of buzzing and rattling noises at anything above very quiet levels. Pulled it, and the corrugated spider had separated from the basket. Regluing that now. Should be all set soon!

I really appreciate the help and encouragement. Thank you.

The torn spider is a common problem with these, I have two the PSW250 and have had to repair this. Great sub if you keep things conservative. I've since bypassed the internal amp and drive them with a QSC 1400 and active xover.

 

[PNMRED]

Thank you for the heads up! I agree, I used mine in a 2 sub setup in my Home theater (5.1 channel) and appreciated it's strengths. I am tempted to put in an class d plate amp and call it good. But now I'm going to use it in my weight room with a pair of Polk MiniMonitors and the OCD in me wants to keep it Polk. In all practicality I know the conflict is all in my head but I am a flawed human...

 

[MuseChaser]

Well..... back again after a year and a half. The sub had been working GREAT in our bedroom 5.1 theater setup, but developed a quite noticeable hum recently

I've attached the schematic for the PSW150 sub; some pics and descriptions of the sub as I acquired it and what I originally did to get it working again are above; it worked fine for a year or two, then developed a hum, present even with the volume all the way down. The hum is very faint, if present at all, without anything connected to the RCA inputs, but pretty strong when a cable is connected or the outer rings are touched with a finger. Touching an outer ring simultaneously with one of the plate mounting screws quiets the hum. Attaching the input cable to the output of the receiver makes the hum even worse.

Here's a link to a thread on the EEVblog forum where I got some other help in addition to what I received here on AK.... it has some more details in it, but in hindsight a lot of the stuff I wrote about the schematic was really wrong.... I can follow directions, but my knowledge of electronic theory is pretty bad.

PSW150 issues on EEVBlog

Here's a link to a discussion on MakerPro about similar issue, and it may describe, partially,, why the subwoofer as I acquired it had a strange jumper wire beneath the control board....

Ground issues in PSW subs


To get the sub working, I removed that added white wire, put the W19 jumper back, and added a jumper across two of the terminals of the missing phase switch, SW101. After doing that, the sub worked fine for a year or two. While trying to troubleshoot this hum a few days ago, I did install a replacement phase switch I had ordered, but that didn't change anything.

The possibility of a bad small cap in a path to AC ground was mentioned by a helpful soul... would that be C107 or C108, off pins 6 and 7 of IC102? Do you think the white wire that was originally there and the missing W19 jumper were factory, or an attempted previous repair for a ground issue, since that white wire connected grounding points (anode of D109, cathode of D108, and pin 3 of IC102)?

I've pored over all of this stuff until my head hurts, but obviously I don't know enough to really understand what I'm looking at. I DO have an oscilloscope and multimeter, and am more than willing to do anything you suggest that would help diagnose the issue.

Thanks again for anything anyone is willing to offer.

 

[MuseChaser]

-Desperation bump-

 

[MuseChaser]

-Incredibly desperate bump?-