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Old 12-19-2018, 05:48 AM   #1
Dannyx
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Question JBL SCS140 subwoofer no sound

Good day folks. I've got a sub here in the shop (rhymes ) which requires a bit of attention....actually it wasn't assigned to me, but I'm having a look into the problem nonetheless, since I'm through with being polite, mr. nice guy....damn it

So: apparently it's got no sound. There is power (LED comes on), but no sound. Some chaps messed around with it, took some measurements, removed/replaced some caps and eventually gave up. Unfortunately that's as far as my assessment goes, because I haven't actually done anything to it MYSELF - just had a sneek-peek so to say.

I grabbed the schematic, wanting to have a look myself and was able to comprehend the circuit in a nutshell, but this is where I'd like a bit of help understanding how some parts of it operate. Page 15 is where it starts.

From what I understand: the signal comes in via LL-L (and LL-R) jacks on the left side of the page, then they're combined in a mono signal (since there's only a single speaker) by R216-217. Then, it's fed into U201 which is an op-amp in a non-inverting configuration. It then keeps going right from op-amp to op amp (still on U201 here) which act as buffers ? Is this right ? VR201 is then added for volume control on U202, then some more op-amps and finally it comes out of that connector at the "limiter circuit".

Here's where I'm having some trouble following along: on the next page, the limiter page, from what I gather, if the signal is" hotter" than a reference set via the resistors on the left on U301, the signal is attenuated ? Is this term for it ?

Another thing I can't quite figure out is how the thing turns on: I know it's got that feature I've met in another sub where it only turns on when it senses signal, but this one has the option to override that via SW201 (page 15) I THINK. In the "always on" position, the switch sends 15v into R256, biasing Q206 "on". Q206 forms a resistor divider with the base of Q207 to a certain value. Then, U203 is turned on/off somehow....this is where I get a bit lost, since I don't know when it's on and when it's off. The output of U203 is labeled "STB" and this goes to the base of Q101 on page 17 where it appears to "short" the bases of Q103 and Q104 when it's on. Is this the "on" or "off" position, so to say ?

NOTE: The picture of the board is only the "amp" section - there's supposed to be another part with those op-amps but I don't know where they put it, plus it's not really important.
Attached Images
File Type: jpg 20181219_133406.jpg (654.1 KB, 20 views)
Attached Files
File Type: pdf SCS140 - SCS146 Service manual.pdf (1.35 MB, 5 views)
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Old 12-19-2018, 10:43 AM   #2
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Default Re: JBL SCS140 subwoofer no sound

I would remove those bad glue especially around those big filter cap and on the resistors and see if any of the jumper wires, component wires got eaten up by the glue or not.
I work backward from output section back to the input section.
When switch is in override position:
Do you have the +/-15V, +/- VSW?
Does the speaker relay come on?
What do you have on STB pin (comes from output of U203 page 15) of the power amp connector P101?
LED D209 (page 15) lights up Green?
If all those are OK then at least the power amp section (PDF page 17) should be working.
BTW, be careful when look at the orientation of connector P301 on page 15 and 16.
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Old 12-19-2018, 11:03 AM   #3
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Default Re: JBL SCS140 subwoofer no sound

Yes, the LED comes on green and from what I recall from the boys, but like I said: it's not my project and can't touch it yet, so the discussion is only to have my knowledge ready in case they give up and hand it over to me. I want to have all my homework done so to say, so as not to be caught off guard.

I know for sure the 15v rails are working, as I heard them talking with the "shop master", but never heard anything about the other bridge rectifier, so it could be they overlooked it or just assumed it's the same thing - it's not.

So the output of U203 should be high ?
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Old 12-20-2018, 01:34 AM   #4
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Default Re: JBL SCS140 subwoofer no sound

Subwoofer is now working: according to the service ticket/sheet, whatever you call it, a 2SB772 transistor and a 15v zener diode were blown. These are most likely Q119 and D109 on page 17. Again, haven't got a hold of these myself to check, but since it's now working, it's probably correct They appear to be part of a regulator circuit.
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Old 12-30-2018, 12:16 AM   #5
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Default Re: JBL SCS140 subwoofer no sound

I wouldn't be surprised if that 772 was for one of the pass transistors for the +/-15V rails. Looking at that heat-darkened PCB, it probably was one of the transistors without a heatsink and likely ran way too hot.

Reminds me of the long JBL S412PII speaker repair I posted not too long ago (except in mine, the problem was due to an open-circuited main filter cap.) Speaking of which, whoever fixed that amp should have removed the tan glue and also checked all of the caps on an ESR meter, including the big ones. Many of the small ones will likely show high ESR, especially in the hot areas. And the big ones, if they are CapXon... expect this thing to come back.
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Old 12-30-2018, 03:49 AM   #6
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Default Re: JBL SCS140 subwoofer no sound

Yes, your sub looks very similar. It is still a JBL after all, so they're bound to share some similarities. This guy's principle is that it should be done with bare minimum work and components (preferably by recovering stuff from other boards so you don't even need to pay for parts...don't get me started on that again ). I sure hope the dude comes back with it TBH, call me mean
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Old 01-04-2019, 02:35 AM   #7
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Default Re: JBL SCS140 subwoofer no sound

BTW: forgot to ask, so hopefully you're still reading this: is that capacitor tester featured in the repair of your sub any good ? Better than nothing I suppose. I saw lots of them on Ali and was thinking of getting one, given how cheap they are.
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Old 01-06-2019, 01:59 AM   #8
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Default Re: JBL SCS140 subwoofer no sound

Quote:
Originally Posted by Dannyx View Post
BTW: forgot to ask, so hopefully you're still reading this: is that capacitor tester featured in the repair of your sub any good ?
As far as I know, it's not the best one (in terms of features, upgrade-ability, etc.). But really, it's helped me with a ton of repairs ever since I got it. Almost anything old with crap caps I get now, I check the caps, especially if they've been overheated. Those little caps won't show it, but they often go bad if they are from a crap cap brand.

Also helps for checking the primary caps of APFC PSUs - a lot of times, those big caps may go high ESR without showing it too... or worse - completely open-circuit. Miss that and your APFC could go *kaboom* again if it's a little more sensitive.

And last but not least, it helps if you want to reuse caps. Sometimes you might pull caps that look good. But are they? Can't know until you check their ESR and capacitance.

So definitely get one - well worth the money, IMO.
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Old 01-06-2019, 04:12 AM   #9
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Default Re: JBL SCS140 subwoofer no sound

I never reuse caps TBH (though this place does...more than it should!), unless they're the large kind used for primary stuff, since those are expensive to get new all the time. When in doubt, I instantly replace them, so honestly a cap tester wouldn't be all that indispensable to me, but I'll get one just in case. Already got a capacitance test on my meter, so another one doesn't hurt.
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Old 01-06-2019, 10:48 PM   #10
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Default Re: JBL SCS140 subwoofer no sound

Quote:
Originally Posted by momaka View Post
Also helps for checking the primary caps of APFC PSUs - a lot of times, those big caps may go high ESR without showing it too... or worse - completely open-circuit. Miss that and your APFC could go *kaboom* again if it's a little more sensitive.
Never a good thing...

Quote:
Originally Posted by momaka View Post
And last but not least, it helps if you want to reuse caps. Sometimes you might pull caps that look good. But are they? Can't know until you check their ESR and capacitance.

So definitely get one - well worth the money, IMO.
Quote:
Originally Posted by Dannyx View Post
I never reuse caps TBH (though this place does...more than it should!), unless they're the large kind used for primary stuff, since those are expensive to get new all the time. When in doubt, I instantly replace them, so honestly a cap tester wouldn't be all that indispensable to me, but I'll get one just in case. Already got a capacitance test on my meter, so another one doesn't hurt.

I will add that checking ESR should never be done immediately after desoldering said caps. The electrolyte is much more active when warm, so you can get a false-good ESR reading.

This could affect troubleshooting, after checking ESR of a just-desoldered cap. "The cap checks within spec, must be something else."
This is also a double-edged sword; someone with questionable motives can make badcaps appear to ESR OK by heating them.

Could be a problem if someone wants to "debunk" whether a certain cap/brand of cap is actually bad.
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Old 01-07-2019, 01:08 AM   #11
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Default Re: JBL SCS140 subwoofer no sound

Quote:
Originally Posted by Dannyx View Post
I never reuse caps TBH... When in doubt, I instantly replace them
Yes, if you have all the caps you need in stock, sure.
I keep a small stock of caps and yet still manage to run out of one value or another. So rather than wait a few days to get new caps, I just use whatever I have - and that often means pulled caps.

Quote:
Originally Posted by kaboom View Post
I will add that checking ESR should never be done immediately after desoldering said caps. The electrolyte is much more active when warm, so you can get a false-good ESR reading.
That's true.

And I'll add onto your addition...
Good ESR doesn't always indicate good cap either. If the capacitance is starting to get very near +20% (i.e. cap is reading high capacitance) or even over that, then you likely have an electrically leaky cap. Nichicon HM/HN/HZ from the affected dates (2001 to mid-2005) and Chemicon KZG are notorious for doing that if they start to fail without any visual signs. In one case, I had a very (electrically) leaky Nichicon HZ with 2005 date code. Didn't know at the time though. So when I applied some voltage to its terminals, it went short-circuit immediately. (And no, I didn't have it in reverse polarity. )

Last edited by momaka; 01-07-2019 at 01:10 AM..
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Old 01-07-2019, 01:13 AM   #12
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Default Re: JBL SCS140 subwoofer no sound

I USED to have caps of virtually all values back at my old shop and I made sure to keep them well stocked, but here it's a different story and we lack stuff a-plenty, so often you have no choice but to use recovered caps which I dislike greatly...
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