Polk Audio PSW350 No Balls

[mmartell]

Re: Polk Audio PSW350 No Balls

An approach is to inject say a 40Hz sinewave maybe 100mV RMS, and use a scope/multimeter to measure ACV at the various stages.
At least to narrow down where the loss of signal/gain is happening.
Try the power amp in and out, or work from the RCA in's forward. You have to make measurements, staring at a schematic doesn't pinpoint much of anything.

The problem for me is I can't yet calculate the expected gain at each stage. I mean I can check for negative gain but failing that I would be lost.

[mmartell]

Re: Polk Audio PSW350 No Balls

I would lift R13 so it's only connected to op and inject audio signal at "lifted" side of R13.
Overall closed loop gain in the power amp should be 21 times.
Do you have a sinewave generator?
If not, audio will do, hook up the scope across the output terminal (remove the speaker) and measure the amplitude the amp can provide.

The bias for the power stage is created by 4 series diodes D2, D4, D5, D6 and trimmer R7. +/- 1.7V across Q3 and Q6 seems normal. It gives a bias current in this stage of around 1mA.

I do not have a signal generator. I have been using my phone with a usb-c to rca cable to input sine waves to the LFE input of the amp, will that work at the lifted side of R13 ?

Do you mean measure the amplitude before clipping ?

And I guess I would need to measure the input signal to see if the amplified signal is in the 21x ballpark ?

[redwire]

Re: Polk Audio PSW350 No Balls

An approach is to inject say a 40Hz sinewave maybe 100mV RMS, and use a scope/multimeter to measure ACV at the various stages.
At least to narrow down where the loss of signal/gain is happening.
Try the power amp in and out, or work from the RCA in's forward. You have to make measurements, staring at a schematic doesn't pinpoint much of anything.

[PowerAmpFreak]

Re: Polk Audio PSW350 No Balls

I would lift R13 so it's only connected to op and inject audio signal at "lifted" side of R13.
Overall closed loop gain in the power amp should be 21 times.
Do you have a sinewave generator?
If not, audio will do, hook up the scope across the output terminal (remove the speaker) and measure the amplitude the amp can provide.

The bias for the power stage is created by 4 series diodes D2, D4, D5, D6 and trimmer R7. +/- 1.7V across Q3 and Q6 seems normal. It gives a bias current in this stage of around 1mA.

[mmartell]

Re: Polk Audio PSW350 No Balls

Still looking for some guidance on this...

[mmartell]

Re: Polk Audio PSW350 No Balls

I've recapped the non polar caps with some from Digikey and it didn't resolve the issue. After poking around some I'm seeing voltages on the base of Q3 and Q6 that are +/- 1.7v give or take a little.

Can someone have a look at the schematic posted in the previous post and let me know if I should be seeing closer to +/- 0.7v as I was expecting ?

[mmartell]

Re: Polk Audio PSW350 No Balls

I still have this haha just pulled it out again from its hiding place under my bench.

Found a schematic will attach.

How to go about using the schematic to pinpoint fault ?

[mmartell]

Re: Polk Audio PSW350 No Balls

Thanks I think that's pretty much what budm said in post#17. Never done this before will give it a try when I get some time.

[momaka]

Re: Polk Audio PSW350 No Balls

To bypass the input section can I use the tip of the sub out RCA connector and attach it directly at R13 as indicated by budm ?

Not sure, as I haven't tried tracing the circuit myself. But I'd trust Budm on that one, so maybe do give it a try. And just to be safe, put a capacitor in series between the sub RCA connector and R13. Something between 1 and 100 uF should be plenty to block DC voltage, but pass signals.

[mmartell]

Re: Polk Audio PSW350 No Balls

If the main filter caps seem alright, then I think the next thing to do is to isolate the audio between the output amp and pre-amp. That is, you'd be inserting an audio signal directly at the amp output IC / output stage to see if the distorted signal is there. If not, then the distorted audio is more likely originating from the pre-amp section. To verify that as well, you can insert an audio signal into the sub the regular way and see if the audio (or at least bass) comes out properly from the pre-amp section or if it's distorted.

To bypass the input section can I use the tip of the sub out RCA connector and attach it directly at R13 as indicated by budm ?

[Retro-Hipster]

Re: Polk Audio PSW350 No Balls

Ya I changed the smaller ones and same result. Maybe this sub is just for shit ?

Seems I can't trust the readings on my esr meter. At least not the capacitance readings, some read grossly under spec but read ok with my DMM. The main filter caps read about 1000uf low on my esr meter but they won't measure at all on my DMM so not sure about their health...

I would guess that the reason you are getting strange readings from your ESR meter is due to the frequency at which you are testing the capacitors. For instance, a higher frequency (higher than 1khz say) might not return anything at all on big electrolytics because that isn't enough time to charge/discharge them for a lot of LCR/ESR meters. (Such as the de-5000). The accuracy can also suffer on really low-frequency test (120hz say) because that might be TOO much time to charge the capacitor, meaning it will stop accepting current and artificially bloat the ohms reading.

Essentially the take away is that ESR tests are frequency dependent. That is why the charts look so incredibly different from one spot to the other. If you want a really accurate understanding of the capacitors ESR, look at the datasheet for that capacitor and see what testing conditions the manufacturer used. Replicate those with your ESR meter and then test them out of the circuit. A decent rule of thumb though would be that really large capacity capacitors would give more accurate results with lower frequency and lower capacity capacitors will get more accurate readings with higher frequencies. I would recommend just experimenting and looking at datasheets on capacitors to see what testing conditions were used. That will give you a much better idea of what you can and can't do reliably in circuit.

[budm]

Re: Polk Audio PSW350 No Balls

https://www.badcaps.net/forum/attach...7&d=1554522518
The pre-amp board ground plane can use as the GND ref. for you scope. Do you see D15? The Cathode leg is soldered to GND plane, you can scrape off the green solder mask and solder a good size solid wire to it to make a post for Gnd connection to your scope.

[mmartell]

Re: Polk Audio PSW350 No Balls

Wow just watched a video by Dave from EEvblog about op amps and they're a lot more complex than they look at first...

I have scope if someone can point out where to put the negative probe so I don't blow it up maybe I can trace the signal with it ?

[mmartell]

Re: Polk Audio PSW350 No Balls

Should the output at pin 7 of the op amp be in the same range as the V+ and V- input voltages ?

[mmartell]

Re: Polk Audio PSW350 No Balls

No distortion at all even at max gain. At max gain the sub outputs less bass required at any volume level.

So I though maybe the LINE IN was too soft for the sub and cranked up the Subwoofer Level to max on my Marantz and at max gain on the sub there was probably enough bass level but it still didn't sound right.

I'm comparing this to a cheap Kenwood sub that makes the Polk sound even cheaper btw.

[budm]

Re: Polk Audio PSW350 No Balls

I would expect V+ pin 8 and circuit GND will be about + 14 ~ +15, V- pin 4 and circuit Gnd should be about - 14 ~ -15V since the Op amp probably runs on Bipolar power supplies.
Since the sound is not distorted so more likely the preamp is OK, just the power amp section that does not have punch.
How loud of the sound can you get before it gets distorted?

[mmartell]

Re: Polk Audio PSW350 No Balls

Voltage on the C358C op-amp is

14.5v with respect to V- (pin4) and
29v with respect to ground.

I note the voltage is a factor of two, is this expected ?

[budm]

Re: Polk Audio PSW350 No Balls

OK, "Also the sub does not output a "distorted" sound - the problem is in the lack of volume and punch in the sound." Thanks for the clarification on that, just lack of punch.

[mmartell]

Re: Polk Audio PSW350 No Balls

bud,

As I would expect a lot of what you've said has gone over my head but I can certainly check the supply voltage of that ic.

Will look into using a diy audio probe as well.

[mmartell]

Re: Polk Audio PSW350 No Balls

momaka,

My dmm has a capacitance setting and it works fine but can't charge up those large filter caps to get a reading. So I'm not just checking for shorts I'm getting actual readings, at least on the smaller caps...

Also the sub does not output a "distorted" sound - the problem is in the lack of volume and punch in the sound.

I too am curious if the problem is in the input stage...