Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

[momaka]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

According to Greek audiophile forums, this company uses Vifa speakers, I don't know if they are considered good or not. ...
The previous owner told me that he left the subwoofer on standby and went to work. A worried neighbor called him on his cellphone and told him there is a terribly loud noise coming from his apartment. Then he returned home to find the sub making the noise and he unplugged it. Then he placed an ad to sell it for repair/parts for 40 euros and this is how I got it.

That's a pretty cool story!
If the speaker driver survived all day making loud noises that the neighbors noticed and called... yeah, I'd say it's a "decent" speaker driver, at the very least.
Like I said, the magnet and construction looks "chunky", which is a rather good thing in this case.

So, maybe after Q56 failed, the protection circuit activated to disconnect woofer from amp and either the protection circuit immediately failed resulting on the terrible noise, or the protection circuit stayed too long on this state (protection ON) and this somehow wore and damaged Q45 and Q46 and then there was the noise that worried his neighbors.

Hard to pinpoint exactly who killed what, but I don't think Q45/Q46 staying turned On for too long (protection On) should have killed either of them. After all, they are driving the Base of Q44 through a 10k resistor, so it's not like they can get overloaded with current from this. And on their input, R92 is 27k, so again a bit too high to let too much current through. So I'd still chuck it to dodgy transistors.

It's quite impressive that the woofer survived.

I should also mention that the heatsink of the amp appears non conductive at first but if you rub the multimeter lead on it, then it gets conductive. The reason I mention this is that when I disassembled the unit for the first time, I noticed that the thermal pad of one of the 2 main output transistors 1943 / 5200 was incorrectly seated, leaving the shiny metal back of the transistor directly touch the heatsink. At this point I am just making wild assumptions though.

Good catch there!
Well, even if that wasn't the problem, it could become one. So good that you eliminated this too.

All in all, this looks like a decent sub, but just poorly/hastily assembled/designed in some parts and possibly with some dodgy components. But the "frame" is there for a good subwoofer, so it's definitely a worthwhile repair.

Coil resistance Ω ( ± 10% ) 650

So, according to voltage divider, we will have approximately 33.163V dropped on the relay coil and 16.837V dropped on the 330 ohm resistor!

This means 1.69Watts of heat dissipated on the relay coil. I heard from a tech here in Greece that he received another PB-150 that had, among other parts, a failed relay. This could explain why the relay failed on him.

Could sure be that. Or maybe more failed transistors in the protection circuit "buzzing" the relay On and Off constantly until it failed?
But yeah, 1.7 Watts of heat is quite a bit.

On that note, I do wonder if a higher resistance for R61 can be used - something like 680 Ohms. That would actually increase the heat dissipation slightly on R61, but should reduce the current going through (and power dissipated) by the relay coil to under 1 Watt... so you'd end up saving around 0.5 Watts. Or perhaps go even higher resistance for even lower voltage on the relay's coil. Of course to make sure it turns On, you might have to increase the capacitance of C66, as suggested by kaboom. Then you may be able to realize even further power reduction.

I use 2 parts epoxy glue. It needs more than 24 hours to totally dry but it is strong enough for my needs. I also have cyanoacrylate / super glue but it is not suitable for this type of jobs.

Yes, CA (super glue) generally isn't great for gluing parts that will see lots of vibration. Flexible glues as better for such task... though epoxy will probably be OK as well - just hope in the future you won't have to remove whatever you glue with it. Probably will be a bit of a paint to remove afterwards. And hot glue may also not be idea if the components inside the box get very hot... though if they do get hot enough to melt the hot glue, that's another issue in itself. Hence why I like using hot glue on electronics - if it runs so hot that it melts the hot glue, it's time to go back to the drawing board on the circuit design.

Thank you both for your advice again! This was great fun and learning experience

You're welcome.

Don't worry about TO-126 for drivers- go right to TO-220.

This comment really cracked me up.

That driver isn't shielded, there's no "can" around the magnet.

Or a 2nd (and weaker) magnet in reverse on the back of the woofer.

I'm guessing this was designed after the CRT era, so not as much regard for shielding... though I've also seen plenty of 90's gear with unshielded speakers.

[kaboom]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

According to Greek audiophile forums, this company uses Vifa speakers, I don't know if they are considered good or not. Could you tell from the pics if the woofer is magnetically shielded?

They seem to be better than average. That driver isn't shielded, there's no "can" around the magnet.

I should also mention that the heatsink of the amp appears non conductive at first but if you rub the multimeter lead on it, then it gets conductive.

Probably just the probe scratching thru the anodizing.

Perfect, Thank you for your recommendations, now I have to go shopping on my local electronics store

You're welcome...

Btw, you are perfectly right about the PCB supporting bigger Q47 Q48 components, but if you look closely you will see that there is marking on the PCB that clearly says 647 and 667.

If I find datasheets for those, I'll upload them. It's possible they've got the same pinout as TO-220- some of those TO-92Ls were the same. But never assume!

Could you please explain what exactly C66 does in the circuit?

That dropper resistor is shunted by that cap. Looks like it supplies a high initial current for when the relay pulls in, then C66 charges and only the resistor powers it. They're probably trying to reduce dissipation. May not even need C66 if C65 is 47u or greater.

If I remember correct, I measured woofer impedance with multimeter and it was 6ohm. Of course the real impedance will vary depending on frequency.

True. d:

Have fun & don't blow anything up!

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Ok, this will be a really long post!

Nice!


No surprise there.
Looking at the magnet of the woofer driver, it has a pretty chunky magnet. That's pretty much the #1 thing needed for a good woofer to go down very low in frequency. The other thing is the box, of course. If tuned properly, it should be able to hit 25-30 Hz easily... which is what would give it that earthquake rumbling sound when it needs to go very low.

According to Greek audiophile forums, this company uses Vifa speakers, I don't know if they are considered good or not. Could you tell from the pics if the woofer is magnetically shielded?

As for your Roister PB-150 circuit, I tried to do some calculations, but it doesn't quite add up. Assuming stable 50V DC comes out before R61 and the relay pulls down R61 on the other side to 24V level, then R61 will have approximately 26V across it, dissipating a little over 2 Watts! But I don't think that's the case, though, or you would have seen a charred resistor. I'm thinking more along the lines that perhaps the relay coil is getting a bit "over-driven" - possibly up to 30V. This should leave only 20V across R61, dropping P_d to 1.2 Watts... which honestly is still a lot. So perhaps it could be over-driven even further? Or perhaps the 10 uF capacitor isn't enough to provide a steady 50V DC, and that further lowers the overall DC voltage? Whichever way I look at it, though, it's honestly a mystery/mess to me. There really ought to be better ways to design that circuit and not use many/more/pricier parts.

The relay is HLS-4453(18F) 240VAC 5A 30VDC DC24V

I think I found the datasheet:
https://cdn.badcaps-static.com/pdfs/...f770a29e5e.pdf
Alternate link:


Coil resistance Ω ( ± 10% ) 650

So, according to voltage divider, we will have approximately 33.163V dropped on the relay coil and 16.837V dropped on the 330 ohm resistor!

This means 1.69Watts of heat dissipated on the relay coil. I heard from a tech here in Greece that he received another PB-150 that had, among other parts, a failed relay. This could explain why the relay failed on him.

The power dissipation on the 330 ohm resistor would be 0,86W.

I should have replaced the resistor with bigger one...

That aside, you may want to glue some of the larger caps down (or ones with very long leads), because the vibration from the subwoofer may eventually damage their seals on the bottom. Probably won't happen in many years if at all, since you used Japanese caps... but generally, it's a good idea to glue down any heavy components in a subwoofer amp, due to the vibration. Regular household/bathroom silicone should be OK to use if kept away from metal pins and parts, due to being slightly corrosive while it cures. For places where you have to glue over metal parts, there are non-corrosive silicones for electronics... or just plain hot glue.

Very interesting and totally makes sense. I will try to keep that in mind for future repairs, especially when the item is expected to be exposed to vibrations.

I use 2 parts epoxy glue. It needs more than 24 hours to totally dry but it is strong enough for my needs. I also have cyanoacrylate / super glue but it is not suitable for this type of jobs.

That's what I was thinking as well. If it was a P_d issue, we likely would have seen it in the form of darkened PCB around the transistor... unless it all happens very quickly during the On/Off cycle of the amplifier while the speaker protection is engaged / relay coil dis-engaged.

However, I'm also not quite sure how it can be a voltage issue.
Let's assume worst case here: either the 2SA1943 or 2SC5200 is fully conducting (but not both at the same time) when the output stage is getting biased. This would be / could be sending either full +50V or -50V to one side of R92. But Since R92 forms a voltage divider with R102, that should limit the voltage to about 1.786V (without the transistors in the circuit.) With Q45 Emitter terminal grounded and Base connected to other side of R92, though, really R92 becomes more of a current-limiting resistor for the Base of Q45... and that current should be less than 1.85 mA... which again, I don't think would be enough to cause damage. And the Collectors of the Q45 and Q46 should also be fairly protected, since the 47 uF 63V cap that sets the delay for when the relay turns On, will be discharged. Therefore at power up, Q45 and Q46 Collectors should be at 0V. Perhaps only if the Base voltage of Q44 is somehow allowed to float above 50V, this could then be an issue, given that the Emitter of Q46 is tied to ground with a 150 KOhm resistor (and also a diode or Zener diode to the Base of Q55? )

So I was thinking of another possibility. The PCB construction (particularly the "random" appearance/selection of the resistors) reminds me of ultra-cheap pre-made PCBs/circuits from China (and elsewhere), perhaps the manufacturer just bought the cheapest parts online... so possibly some counterfeit transistors? Now, the outputs may be genuine (or at least decent quality parts), but perhaps these smaller TO-92 transistor were not. That could explain the random failure of Q45 and Q46 without any discoloration around them.

Time should tell, though. Hopefully the amp will stay fixed. If it does, I would honestly just suspect dodgy/fake/counterfeit transistors from China were the culprit.

Q45/46 are DC detect monitors, one for each polarity. These kill drive to relay driver when triggered. Pretty low duty, so conductive/corrosive glue or borderline/"second" parts likely.

The previous owner told me that he left the subwoofer on standby and went to work. A worried neighbor called him on his cellphone and told him there is a terribly loud noise coming from his apartment. Then he returned home to find the sub making the noise and he unplugged it. Then he placed an ad to sell it for repair/parts for 40 euros and this is how I got it. So, maybe after Q56 failed, the protection circuit activated to disconnect woofer from amp and either the protection circuit immediately failed resulting on the terrible noise, or the protection circuit stayed too long on this state (protection ON) and this somehow wore and damaged Q45 and Q46 and then there was the noise that worried his neighbors.

It's quite impressive that the woofer survived.



I should also mention that the heatsink of the amp appears non conductive at first but if you rub the multimeter lead on it, then it gets conductive. The reason I mention this is that when I disassembled the unit for the first time, I noticed that the thermal pad of one of the 2 main output transistors 1943 / 5200 was incorrectly seated, leaving the shiny metal back of the transistor directly touch the heatsink. At this point I am just making wild assumptions though.

Upgrade the drivers, Q47/48. Final gain drops at high output current; driver not only needs to be robust, but also keep its gain up so the previous stages stay linear.
The C5200/A1943 should be driven with something like a MJE15032/33, 15030/31 , or 15028/29 comp pair.
The 028/029 will work since this amp runs ~51v rails; use what's easy to get.
Make sure you transpose the leads; TO-220 is different vs TO-92.
And be sure to put NPN->NPN & PNP->PNP.
These won't need heatsinking beyond the tab.Your PCB is silkscreened for bigger drivers- look close, they cheaped out.

Don't worry about TO-126 for drivers- go right to TO-220.

TO-92 or TO-92(L) is absurd- amazing it lasted this long!

Perfect, Thank you for your recommendations, now I have to go shopping on my local electronics store

Btw, you are perfectly right about the PCB supporting bigger Q47 Q48 components, but if you look closely you will see that there is marking on the PCB that clearly says 647 and 667.

Q49/50 should be replaced with BD139/140. Make sure NPN->NPN & PNP->PNP and you transpose the leads! Otherwise, you're sending +/-51v into the opamps!

more to add to the basket, thanks

Change the two 47u/25v, across the zeners for the +/-15 regs, to 100u/25v. Make the reference for the reg better vs extra filtering "after the fact."
Change C67 to 47u/50v, 105c, PW, HE, FC, etc. if you haven't already.
Change C66 to 22u/50v, also PW, HE, FC, etc.

Could you please explain what exactly C66 does in the circuit?

If the main filters have been in this long, they'll keep formed at the higher voltage.
Electros are initially formed at a bit more than rated voltage; yours will likely tolerate 54-56v.

This is really good news!

The toroidal transformer can/should keep the rails up better than an E/I.

Should make an easy 150w/4ohms, especially with 230 vs 220 in.
Possibly 180-200w peaks if "pushed," but there's only one set of finals; push it only with an external fan on that heatsink.
Don't worry about opamp "rolling," 4558s are just fine in a sub, and besides, those are JRC!


Good luck and "play it loud!"

If I remember correct, I measured woofer impedance with multimeter and it was 6ohm. Of course the real impedance will vary depending on frequency.

Well, I had it running at unbearable high levels of bass and the backplate+heatsink got very hot!



Thank you both for your advice again! This was great fun and learning experience

[kaboom]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

OK, my turn.

Upgrade the drivers, Q47/48. Final gain drops at high output current; driver not only needs to be robust, but also keep its gain up so the previous stages stay linear.
The C5200/A1943 should be driven with something like a MJE15032/33, 15030/31 , or 15028/29 comp pair.
The 028/029 will work since this amp runs ~51v rails; use what's easy to get.
Make sure you transpose the leads; TO-220 is different vs TO-92.
And be sure to put NPN->NPN & PNP->PNP.
These won't need heatsinking beyond the tab.Your PCB is silkscreened for bigger drivers- look close, they cheaped out.

Don't worry about TO-126 for drivers- go right to TO-220.

TO-92 or TO-92(L) is absurd- amazing it lasted this long!
Q45/46 are DC detect monitors, one for each polarity. These kill drive to relay driver when triggered. Pretty low duty, so conductive/corrosive glue or borderline/"second" parts likely.

Q49/50 should be replaced with BD139/140. Make sure NPN->NPN & PNP->PNP and you transpose the leads! Otherwise, you're sending +/-51v into the opamps!


Change the two 47u/25v, across the zeners for the +/-15 regs, to 100u/25v. Make the reference for the reg better vs extra filtering "after the fact."
Change C67 to 47u/50v, 105c, PW, HE, FC, etc. if you haven't already.
Change C66 to 22u/50v, also PW, HE, FC, etc.
If the main filters have been in this long, they'll keep formed at the higher voltage.
Electros are initially formed at a bit more than rated voltage; yours will likely tolerate 54-56v.
The toroidal transformer can/should keep the rails up better than an E/I.

Should make an easy 150w/4ohms, especially with 230 vs 220 in.
Possibly 180-200w peaks if "pushed," but there's only one set of finals; push it only with an external fan on that heatsink.
Don't worry about opamp "rolling," 4558s are just fine in a sub, and besides, those are JRC!


Good luck and "play it loud!"

[momaka]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Nice!

Glad to hear you got it working properly now.

The subwoofer is perfectly working and can get really loud. It's nothing like the mediocre Yamaha YST-SW030 that I fixed in another thread. This thing can really shake the house

No surprise there.
Looking at the magnet of the woofer driver, it has a pretty chunky magnet. That's pretty much the #1 thing needed for a good woofer to go down very low in frequency. The other thing is the box, of course. If tuned properly, it should be able to hit 25-30 Hz easily... which is what would give it that earthquake rumbling sound when it needs to go very low.

Here is the schematic with some comments about changes I made:

https://www.badcaps.net/forum/attach...1&d=1645390601

Cool, it's great that you made this. Now anyone who may have this subwoofer can actually have a schematic for a reference.

But man... again this reminds me so much of the JBL speakers I pointed to in the previous posts. In particular, it's the circuit around the speaker protection & LED. I'm just trying to "wrap my mind" around it of how it all works. Seems like a lot of questionable decisions were made there.

Replaced R201 , 2.2kohm, with same value, 1watt resistor.

Was that resistor running hot? It seems like it feeds nothing else except for the LED (when it's ON)... though through doing some very quick & rough calculations, it's possible it may be dissipating up to 0.33 Watts (1/3W) when the sub is On... which if the original resistor was rated for 1/2W, then I can see it getting a bit hot. 1W definitely sounds safer.

I could do the same and replace R61 that is 330ohm, 1 watt with 2watt resistor but I just tried to move it far from the nearby caps.

I did that to the JBL speakers - replaced the 330 Ohm resistor in series with the relay with same value, but 5W resistor, IIRC. In my case, the relay was 12V, and it was fed from the 30V rail... so the resistor was dropping ~18V, which equates to ~1 Watt of power dissipation. The 2W used originally was getting quite toasty.

As for your Roister PB-150 circuit, I tried to do some calculations, but it doesn't quite add up. Assuming stable 50V DC comes out before R61 and the relay pulls down R61 on the other side to 24V level, then R61 will have approximately 26V across it, dissipating a little over 2 Watts! But I don't think that's the case, though, or you would have seen a charred resistor. I'm thinking more along the lines that perhaps the relay coil is getting a bit "over-driven" - possibly up to 30V. This should leave only 20V across R61, dropping P_d to 1.2 Watts... which honestly is still a lot. So perhaps it could be over-driven even further? Or perhaps the 10 uF capacitor isn't enough to provide a steady 50V DC, and that further lowers the overall DC voltage? Whichever way I look at it, though, it's honestly a mystery/mess to me. There really ought to be better ways to design that circuit and not use many/more/pricier parts.

If you check the pics, you might notice that I tried to do this with every component that generates some heat: move it away from capacitors or other hot components that would make it get more hot and even fail.

Good!

Changed C67 10uF 50V with 22uF to lower the ripple of the power supply of the anti-thump/protection circuit.

That's actually for the relay's coil supply and LED.
Keep an eye on it after running the amp for a while. Higher capacitance could mean more stable (and thus higher) DC voltage... which may or may not increase the heat given off by R61... though at 22 uF, it's probably still not big enough to matter.

The 2x 25V 47uF caps located on the preamplifier board were upgraded to 2x 100uF Rubycon YXF. This might increase audio quality.

These two caps filter the +/-15V rails for the pre-amps... so if anything, this should further decrease any line hum from going to the pre-amps. It may or may not bring noticeable results, depending on if the old 47 uF caps were enough for the filtering task or not (probably OK, given the +/-15V rails are linearly-regulated.) But the extra capacitance doesn't hurt here, so the upgrade is a good thing.

I left unchanged the 2 transistors S8050 , S8550 that drop down the 50V and -50V voltages for the vcc of the preamplifier board but I bent them to be as far away as possible from PCB.

https://www.badcaps.net/forum/attach...1&d=1645390461

That's fine.
If they're not running hot enough to darken the PCB, then they probably aren't dissipating too much heat. The 1 KOhm 1W resistors before them (R94 and R95) likely help a good deal with voltage drop before the transistors. And probably the pre-amp board doesn't use much power.

I also upgraded 220uF 35V caps with 470uF 35V on that circuit.

I soldered 2 pre-driver transistors from the other side of the PCB and attached them on the heatsink with generous amount of thermal paste

https://www.badcaps.net/forum/attach...1&d=1645390601

https://www.badcaps.net/forum/attach...1&d=1645390601

Anything helps at this point.

I think the best fix would be to replace them with equivalent TO-126 parts but I didn't know how to easily find TO-126 transistors with exactly the same characteristics.

D667C and B647C, possibly part of driver circuit of amp normally should have been replaced with TO-126 equivalents too but were left unchanged for the same reason as above.

You'd have to draw a schematic of the output stage and pre-driver stage, then maybe compare it with circuits from other amplifiers. It's probably using a Sziklai pair configuration on the output, like many other amps are. So you may be able to copy-pasta the design of another amplifier (or at least use the biasing/resistor values to get an idea of what you might need to modify.)

But hey, if the subwoofer isn't burning out when you're cranking it up hard, then I guess let it be.

All the capacitors were replaced with Japanese caps of low ESR series, 105C (hopefully not too low ESR to cause issues).

Nah, it's an audio amplifier, it won't matter. You can even use polymers and it probably won't care. Just good luck finding any 50V-rated polymers... or at that high of a capacity.

That aside, you may want to glue some of the larger caps down (or ones with very long leads), because the vibration from the subwoofer may eventually damage their seals on the bottom. Probably won't happen in many years if at all, since you used Japanese caps... but generally, it's a good idea to glue down any heavy components in a subwoofer amp, due to the vibration. Regular household/bathroom silicone should be OK to use if kept away from metal pins and parts, due to being slightly corrosive while it cures. For places where you have to glue over metal parts, there are non-corrosive silicones for electronics... or just plain hot glue.

I should note that all of the original caps that I removed were tested with ESR micro and Chinese component tester and I found that some of them had started going high ESR and somehow high V leakage.

Not surprising. Cheap cap brands are cheap for a reason. :\

Last, I tried to thermally couple the amp heatsink with the metal cover of the sub applying tons of used thermal paste that I recovered from other projects

https://www.badcaps.net/forum/attach...1&d=1645390601

So, what do you think?

Excellent!
That actually should help with the power dissipation quite a bit.
And yes, I -never- throw away recovered thermal paste, unless it's completely dry.

Only thing I can think of at this point is voltage rating.

That's what I was thinking as well. If it was a P_d issue, we likely would have seen it in the form of darkened PCB around the transistor... unless it all happens very quickly during the On/Off cycle of the amplifier while the speaker protection is engaged / relay coil dis-engaged.

However, I'm also not quite sure how it can be a voltage issue.
Let's assume worst case here: either the 2SA1943 or 2SC5200 is fully conducting (but not both at the same time) when the output stage is getting biased. This would be / could be sending either full +50V or -50V to one side of R92. But Since R92 forms a voltage divider with R102, that should limit the voltage to about 1.786V (without the transistors in the circuit.) With Q45 Emitter terminal grounded and Base connected to other side of R92, though, really R92 becomes more of a current-limiting resistor for the Base of Q45... and that current should be less than 1.85 mA... which again, I don't think would be enough to cause damage. And the Collectors of the Q45 and Q46 should also be fairly protected, since the 47 uF 63V cap that sets the delay for when the relay turns On, will be discharged. Therefore at power up, Q45 and Q46 Collectors should be at 0V. Perhaps only if the Base voltage of Q44 is somehow allowed to float above 50V, this could then be an issue, given that the Emitter of Q46 is tied to ground with a 150 KOhm resistor (and also a diode or Zener diode to the Base of Q55? )

So I was thinking of another possibility. The PCB construction (particularly the "random" appearance/selection of the resistors) reminds me of ultra-cheap pre-made PCBs/circuits from China (and elsewhere), perhaps the manufacturer just bought the cheapest parts online... so possibly some counterfeit transistors? Now, the outputs may be genuine (or at least decent quality parts), but perhaps these smaller TO-92 transistor were not. That could explain the random failure of Q45 and Q46 without any discoloration around them.

Time should tell, though. Hopefully the amp will stay fixed. If it does, I would honestly just suspect dodgy/fake/counterfeit transistors from China were the culprit.
This is why I prefer to use salvaged parts when not going with verified stuff from Digikey or Mouser. Even cheapo PSUs tend to have somewhat more reliable source of parts (except of course, the really really bottom-dollar garbage PSUs.)

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Finally found the time to finish this!

The subwoofer is perfectly working and can get really loud. It's nothing like the mediocre Yamaha YST-SW030 that I fixed in another thread. This thing can really shake the house

Here is the schematic with some comments about changes I made:

https://www.badcaps.net/forum/attach...1&d=1645390601

Replaced R201 , 2.2kohm, with same value, 1watt resistor.

I could do the same and replace R61 that is 330ohm, 1 watt with 2watt resistor but I just tried to move it far from the nearby caps.

If you check the pics, you might notice that I tried to do this with every component that generates some heat: move it away from capacitors or other hot components that would make it get more hot and even fail.

Changed C67 10uF 50V with 22uF to lower the ripple of the power supply of the anti-thump/protection circuit.

The 2x 25V 47uF caps located on the preamplifier board were upgraded to 2x 100uF Rubycon YXF. This might increase audio quality.

https://www.badcaps.net/forum/attach...7&d=1645390461

https://www.badcaps.net/forum/attach...9&d=1645390461

I left unchanged the 2 transistors S8050 , S8550 that drop down the 50V and -50V voltages for the vcc of the preamplifier board but I bent them to be as far away as possible from PCB.

https://www.badcaps.net/forum/attach...1&d=1645390461

I also upgraded 220uF 35V caps with 470uF 35V on that circuit.

I soldered 2 pre-driver transistors from the other side of the PCB and attached them on the heatsink with generous amount of thermal paste

https://www.badcaps.net/forum/attach...1&d=1645390601

https://www.badcaps.net/forum/attach...1&d=1645390601

I think the best fix would be to replace them with equivalent TO-126 parts but I didn't know how to easily find TO-126 transistors with exactly the same characteristics.

D667C and B647C, possibly part of driver circuit of amp normally should have been replaced with TO-126 equivalents too but were left unchanged for the same reason as above.

All the capacitors were replaced with Japanese caps of low ESR series, 105C (hopefully not too low ESR to cause issues).

2x 12000uF 50V were left unchanged due to high cost

I should note that all of the original caps that I removed were tested with ESR micro and Chinese component tester and I found that some of them had started going high ESR and somehow high V leakage.

I also redid every solder connection and I applied large amounts of solder and created ugly solder balls to help with the overheating of the big wattage resistors and transistors.

https://www.badcaps.net/forum/attach...1&d=1645390601

I also replaced the terribly thin AWG28 cables used to transfer the audio signal from the preamp board to the main amp board and banana plugs board to the preamp board. I used AWG24 speaker cable from some cheap Philips speakers that were part from a 90s mini Hi Fi system from Philips . If I ever need to use those cheap Philips 2 way speakers I will use some heavier wire, because I think AWG24 is too thin for speaker cables, but perfectly fine for interconnect cable.

https://www.badcaps.net/forum/attach...1&d=1645390601

The AWG28 cables used to power LED and preamplifier board were left unchanged.

Last, I tried to thermally couple the amp heatsink with the metal cover of the sub applying tons of used thermal paste that I recovered from other projects

https://www.badcaps.net/forum/attach...1&d=1645390601

So, what do you think?

I would love to get your feedback

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Only thing I can think of at this point is voltage rating. Try a mps a43 or something like that...

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

R102 1Kohm

R92 27kohm

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

okay... now that Q44-Q46,Q55 circuit looks more like protection and integrated anti-thump. The two weak/bad transistors appear to be the protection portion but if the schematic is correct they shouldn't be very stressed under normal situations (i.e. if the PO transistors didn't die) ... but what are the values of R92 and R102?

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

BTW it would be nice to put component designators like Q45, C3, R6, etc., etc. so we could textually mark which device we're talking about.

done

Please check the new version of schematic attached bellow.

Question next is where does that 10K resistor on the right of the drawing go... to prove that's the anti-thump.

Please check schematic, I added the rest of this circuit at last.

Now, that we have the circuit of Q45 and Q46 complete, can we expain why Q45 and Q46 failed?

The area has no PCB discoloration implying overheating or parts like big wattage resistor next to the failed transistors. So, why they failed? They also don't appear to be connected to the other part that was found failed, Q56...

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

At 150W you're passing around 30-odd amps or so during the peaks.
The current gain of final transistors passing this much current drops down to around 10, so the base current will be around 2-3 amps.

This is way too much for a TO-92, so yes you probably should upgrade to TO-126 or TO-220 transistors, but other mods might be needed upstream.

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Most of the schematic you drew is the PSU.

Exactly

As far as I can tell the discolored area is the auxiliary power supply for the anti-thump circuitry to prevent the speakers from bumping when powering up... so none of the known failed transistors are here or is it drawn?

Precisely.

Now, we know that 1 of the 2 failed transistors, Q56, is part of the pre-drive circuit of the main large complementary transistors 1943 and 5200.

BTW it would be nice to put component designators like Q45, C3, R6, etc., etc. so we could textually mark which device we're talking about.

You are right, that was necessary and I failed to do it. I will try to add on the next version of schematic.

Question next is where does that 10K resistor on the right of the drawing go... to prove that's the anti-thump.

It goes to the collector pin of both Q45 and Q46 S9014 that one of them was found bad and the other affected too and replaced both. These are the ones with red and yellow color, located near relay:

https://www.badcaps.net/forum/attach...0&d=1642457707

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

**EDIT**
I had a quick trace around the circuit, and Q56 seems like a possible pre-driver for the big 2SC5200 transistor... so probably a little too weak for it.
/edit

Correct, I also checked the traces and they are connected. There is a big resistor R83 56K 1W that could also generate a considerable amount of heat making things worse for Q51 and Q56. What if I unsolder it and solder it from the other side?

Or else, solder Q51 and Q56 from the other side of the PCB and make them touch the external heatsink of the subwoofer? Is this crazy enough?

The external heatsink I am talking about is this:
https://www.badcaps.net/forum/attach...3&d=1642379917

Look at this pic:

https://www.badcaps.net/forum/attach...5&d=1642457707

The transistor in the middle, Q43 2N5551 was originally mounted to the heatsink with thermal paste. They didn't use any enforcing method other than its legs though. If I understood correct, this transistor should be touching the heatsink to provide some type of thermal feedback to the driving circuit for the main transistors to avoid burning them, or something like that.

So, why not try to do the same with Q51 and Q56? Solder them on the other side of the PCB, apply some thermal paste, then screw board trying to get them to make contact with the heatsink?

This is very similar to what I saw in a pair of JBL S412PII active speakers I posted about here:
https://www.badcaps.net/forum/showthread.php?t=73625

Just finished reading the entire thread, thanks for the reference!

Teribble design though. If you check the buying trends in audio forum, youtube etc you will see everyone pushing people to buy active speakers/monitors.

Active vs powered:

Powered is just speaker system with integrated amp while active means different amplifier for twitter and different amp for woofer. And also active crossover before the amps and also heavy tweaking with DSP etc.

That way they say that you get the perfect pairing of amps/speakers and the amps are specifically designed for the speakers, taking into acount the cabinet, etc

I have no knowledge to question any of the above claims.

My view on this though is that this is the perfect plan to turn speakers into comsumer products with expiration date, just as like all the other stuff (TVs, laptops, monitors, Bluetooth speakers, soundbars, smartphones, wireless earpads etc). The integrated amp(s) will last at best a couple of years, long enough to pass warranty period and then randomly die. But a passive speaker from a reputable manufacturer and rubber foam suspension can easily last for 10 years or even more....

and if you wish to keep costs lower, perhaps lower the capacity one notch. 10000 uF caps should still be plenty adequate for the 150W RMS of one lone speaker driver.

I found pictures from the internet of the exact same model and the main caps were of lower capacitance, 10000uF. So, I guess that 10.000uF probably would be enough.

So FWIW, you probably could leave those caps in there, provided your AC line doesn't spike up a lot and too often above 230V. But IMO, it's not a good idea. According to several Panasonic cap data sheets I've read: if you run a cap at or very near its rated voltage, the chances of the cap failing increase a lot more (typically the cap may fail open-circuit.) And in my case above, one of the caps was indeed failed open-circuit. Granted it is CapXon and not a good brand... but I just don't like the idea of caps running that close to their rated voltage. For my repair, I used 80V caps. As for your model, I suggest going up a notch in the voltage to 63V...

Checked my caps stock and unfortunately the only caps I have that are 63V are 2x15000uF and 2x18000uF. And those were really expensive, about 8-10€ each. I am not wasting those caps on this unit.

edit: Also found 2x Elna Audio caps 7500uF 56V from a Kenwood 90s amp that has blown output transistors. But 7500uF is quite low for 150rms.

Well, if the failure is always those 2-3 small transistors that you replaced, that's probably not due to pushing the design too far, but rather pushing cost-savings too far with using really small transistors.

Actually, now that I mention this, it may also be worthwhile to check the pre-drivers for those large transistors. Are those TO-92 stuff too? If so, that's just too cheap. Again, I would expect to see transistors with packages more along the lines of TO-126 or bigger.

All transistors are TO-92 except 2 transistors: D667C and B647C, those are TO-92 Mod.

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

BTW it would be nice to put component designators like Q45, C3, R6, etc., etc. so we could textually mark which device we're talking about.

I don't think any of the capacitors that you query in the schematic - whether increasing capacitance would affect things much. You might improve audio quality if you're hearing hum but this would more be of a PSRR issue of the preamp, otherwise it may increase turn on time.

The two transistors drawn in the upper right of the schematics are what I'm slightly concerned about. These will probably heat up. Question next is where does that 10K resistor on the right of the drawing go... to prove that's the anti-thump.

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Most of the schematic you drew is the PSU.
As far as I can tell the discolored area is the auxiliary power supply for the anti-thump circuitry to prevent the speakers from bumping when powering up... so none of the known failed transistors are here or is it drawn?
Seems this auxiliary PSU will constantly draw power and thus heat up as long as the amp is powered up and after it connects the speaker after the power settles, so I'd expect that 330Ω resistor along with the two transistors in the anti thump circuit would constantly be drawing power and heating up though tough to gauge how much the components would heat up - maybe a little, estimate indicates the semiconductors may be brunting part of the heat while pulling the relay.

[goodpsusearch]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

I drew a partial schematic of the thing.

I focused on the power supply of the pre-amplifier and an auxiliary power supply that I am not sure how it's used. I am sure that the working led light of the unit is powered from this aux psu. It's a 2 color led, red color to show it got power and green light when everything is OK and the relay connnects the amp to the speaker. So, I thought it might also be part of a protection circuit.

On the links I posted above with the Chinese boards that use the same main complementary transistors, it is strongly recommended to add some type of protection to avoid damaging speaker in case amp blows.

Unfortunately, you will not find on my drawing none of the 2 transistors that failed.

It is getting really late here, so I have to go to sleep now, but I promise that I will return to read again your posts and thoroughly reply.

I also attach pics with the areas that I was able to reverse engineer marked in circles to help you understand my terrible work.

As you can see on the pics and the drawing, based on their role on the circuit, I am thinking to change some cap values with higher to help the audio quality and more importantly lower the stress / overheating of some parts.

[momaka]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Now I wonder about this small amplifier:

https:// www.ebay.com / itm / 224789720143

How to tell if this is AB or D by just looking at the picture?

Good question. I don't think you can tell just from the picture alone. Class D is supposed to have an L-C filter on the output, but I don't see any on this board. Perhaps there is something integrated inside the IC? I doubt it, but who knows. Or maybe it's really just class AB and mislabeled as class D. OR OR you're supposed to add on your own L-C filter?
In any case, it's probably best to find a datasheet for the IC (if you can read its part number) and go from there. I never solely trust the listed specs in the listing.

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Hmm... indeed a lot of recent discrete amplifiers are class D, definitely not larger, older gear with class D or A, and definitely not B.

Now I wonder about this small amplifier:

https:// www.ebay.com / itm / 224789720143

How to tell if this is AB or D by just looking at the picture?

[momaka]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Then started checking every component on the amplifier board and found 2 bad and 1 that when testing with multi-meter found values slightly off so replaced all 3 of them anyway
https://www.badcaps.net/forum/attach...1&d=1642457710
https://www.badcaps.net/forum/attach...1&d=1642457710
green color means component OK
yellow = suspicious
red = bad

The components that I replaced are 2n5401 and S9014. Total repair cost less than 2€

I also carefully removed that terrible glue that gets conductive and corrosive.

Then checked subwoofer and it works!

But now I have to decide what else needs to be done to help the unit not die again.

Well, if these two transistors burned... and assuming that's what went wrong the first time around... then they must clearly be undersized for the task. The question is, where are they lacking? It could be the power dissipation rating, it could be the current... or maybe even the voltage, despite nothing in the circuit being able to provide a voltage that high.

So to answer that, check what these two transistors are connected to. I suspect you're going to find these are the "linear droppers" for a complementary +/-12V supply for the op-amps. Since the transformer seems to have only one set of complementary output (the +/-35V AC taps... meaning 50V DC, as you found), then that means somewhere on your main amp board, there will be two transistors that generate the +/-12V (or could be up to +/-15V) rails from the +/-50V DC of the big caps. I only see small TO-92 transistors from the pictures you provided (except for the main output drivers, of course), so that means two of these TO-92 transistors will be tasked with generating those lower voltage rails. And because they would be dropping close to at least 35V DC (well, possibly less if there are any resistors in front of them to "assist" with the power dissipation), then it really wouldn't be too hard for them to overheat and fail.

Thus, I suggest you find which transistors generate your pre-amp complementary power rails (+/-12V or however much) and see if these are the ones that failed. If yes, switch them out for bigger transistors, like B772/D882... and maybe have them connected on an aluminum piece of heatsink, since they are enclosed in a box with no ventilation.

**EDIT**
I had a quick trace around the circuit, and Q56 seems like a possible pre-driver for the big 2SC5200 transistor... so probably a little too week for it.
/edit

I have noticed on this forum that active subwoofers of different brands and price range are prone to amp failure due to a variety of reasons such as bad caps, overheating because the amp is in an enclosed space with limited airflow.

Typical design of modern-day electronics.

I also noticed something absolutely stupid.

As you can see on the pic attached, they used 50V rated main caps but the transformer outputs 71V AC.

I tested voltage across the caps with multimeter and each one had around 50.2 to 50.5V

https://www.badcaps.net/forum/attach...1&d=1642457710

But this unit has been working for years and the caps tested with my esr/capacitance/leakage testers and were perfectly fine.

Buying new caps rated more than 50V and keeping original capacitance of 12000uF or even going one size more to 15000uF , will make the total cost too high and getting this subwoofer for 40€ wont be good deal anymore.

What do you think I should do? Leave them alone? Do you think that the fact that the sub was made in 2009 and the caps lasted all those years means that they will probably be OK?

This is very similar to what I saw in a pair of JBL S412PII active speakers I posted about here:
https://www.badcaps.net/forum/showthread.php?t=73625

^ For those, the main caps were rated at 63V and the transformer was outputting about 42V AC (59-60V DC after rectification.) The speakers were from the early 2000's and I did the repair in early 2018... so they lasted about 15 years - and that's with those main caps being CapXon brand.

So FWIW, you probably could leave those caps in there, provided your AC line doesn't spike up a lot and too often above 230V. But IMO, it's not a good idea. According to several Panasonic cap data sheets I've read: if you run a cap at or very near its rated voltage, the chances of the cap failing increase a lot more (typically the cap may fail open-circuit.) And in my case above, one of the caps was indeed failed open-circuit. Granted it is CapXon and not a good brand... but I just don't like the idea of caps running that close to their rated voltage. For my repair, I used 80V caps. As for your model, I suggest going up a notch in the voltage to 63V... and if you wish to keep costs lower, perhaps lower the capacity one notch. 10000 uF caps should still be plenty adequate for the 150W RMS of one lone speaker driver.

This might mean that the people who designed this sub woofer pushed the circuit to its limits to get more Wrms and this might be one of the causes of high failure rate.

Well, if the failure is always those 2-3 small transistors that you replaced, that's probably not due to pushing the design too far, but rather pushing cost-savings too far with using really small transistors.

Actually, now that I mention this, it may also be worthwhile to check the pre-drivers for those large transistors. Are those TO-92 stuff too? If so, that's just too cheap. Again, I would expect to see transistors with packages more along the lines of TO-126 or bigger.

I am no expert in amp circuits, I cannot even tell if this is class A or B unfortunately, I can only tell this is not Class D, please correct me if I am wrong.

It's class AB.

You won't see Class A used for anything high-power due to it being very inefficient (20-30% at best, IIRC.) Only on pre-amp stages, where power is low.
Class B, on the other hand, has pretty good efficiency (70-75%... again, IIRC), but has terrible cross-over distortion, so you won't find this used anywhere.
Then there is Class AB, which is a combination of class A's low distortion and class B's efficiency. Essentially, class AB looks just like class B on the circuit level, but the output transistors are biased to conduct slightly at no-signal so that cross-over distortion is eliminated.
And then there's Class D - very high efficiency (90% or more typically) and pretty low distortion nowadays (with modern ICs.) But you would indeed recognize a Class D fairly easily, as it will have a toroid/inductor and large PP cap on the output for the speaker for the L-C filter.
Oh, and there's also Class G and H... which are basically just class AB with enhancements to the delivery of the power rails in order to increase the efficiency. The JBL S412PII are class G/H

Anyway, I think that the reason of the failures is mainly the fact that all the components that you see here:

https://www.badcaps.net/forum/attach...4&d=1642457707

are totally sealed:

https://www.badcaps.net/forum/attach...2&d=1642379917

And you're probably right.

Usually the amplifier parts get fresh air through the subwoofer bass reflex port(s) and there is quite a respectable amount of air circulation when the subs are used at moderate to high volume.

Yeah, I like subwoofers that do this. It's much easier on the electronics, especially if they are indeed located near the port(s).

Can you please tell me if this is some type of novelty, totally sealing the amplifier parts ,or you've seen it on other subwoofers too?

Again, have a look at the thread for the JBL S412PII above - it's the same thing with those plastic boxes on the plate amps.

One reason why the electronics on really high power amps are sometimes enclosed in a box is so to keep air from escaping and making "whistling" noises through various holes on the plate amp (like knobs, switches, input jacks, and etc.) Otherwise, they would have to pot the whole thing in glue. Also, a separate box may reduce resonance noises produced by the plate amp. But yeah... the tradeoff is a box full of hot electronics.

And again, since I don't see anything but TO-92 transistors anywhere, I imagine it's the linear power supply droppers that are running too hot and dying there. On the JBL S412PII, these ran mad-hot... and they were in TO-126 packages. The heatsinks I added probably barely mattered... but better than nothing!

Pretty much all audio power/speaker amplifiers, regardless of manufacturer, are class AB.

Not so much anymore nowadays.
Good low-distortion class D amps have been going for a good number of years now (at least 10) and are pretty much the only thing used anymore. Only in "high-end" home theater amps you can still find class AB. Otherwise pretty much everything has moved to class D due to smaller size and higher efficiency (and hence possible to use lower-cost power supplies and other parts.)

[eccerr0r]

Re: Roister Pb-150 Subwoofer repaired, how to prevent from failing again?

Pretty much all audio power/speaker amplifiers, regardless of manufacturer, are class AB. However surrounding the AB drivers lies the secret sauce, so you need to get the *exact* schematic to analyze, especially if just that one is failing...