JBL scs175 subwoofer amp fried need help to fix it! - diyAudio
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Old 15th September 2010, 11:35 PM   #1
marcogt is offline marcogt  Italy
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Join Date: Nov 2007
Default JBL scs175 subwoofer amp fried need help to fix it!

Hi there fellows,
nice way to start a post in this community after a while i've been reading you.

Today i put my hands on a brand new USED HT kit from jbl.
I got it by a guy not near my door, so i make a shipment.

When i got it from the courrier, i find out, two satellite's cone with the frame slightly broken (it is in palstic!!!).
This is a mechanical issue, and i can explain that, a bad box has been made for that shipment.

What what sounds strange, is the subwoofer, that came with few scuff, simply did not work.

i take the amp apart, and i find that some tracks on the pcb was overheated.
That was related to a unknown C20 capacitor blown up.
I then fixed the tracks with some wire as you can see.

PICTURES IF YOU CLIC 1 time and then in the corner bottom left, they became big enough to see every details
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Then i bought a capacitor with the same size of on near him , the C19 (on the right in the picture) which is written 221 on it.

The resistor that you see blown up, was still not blowed.
I measured it, and it was 0.56 on the 2k scale on my tester.

I measured it while it was on board (i did not take it apart)
I wasn't able to understand the color, they really seems green-blue-black, but i cant really say what is the last color.

Back to the shop, i installed the new capacitor.
Closed up everything, and it was running fine.

Listen to the sub for a while , then it stop playing music, and it began with a continuous buzz, even at low to mute volume. A bad smell came out from the sub.

Itake it apart and realized that the resistor i was telling before, was now blewn up.

The capacitor was still there in good shape.


now, i need some help.

I am not able to find the service manual for this sub.

I find the link on this site of this, wich has pretty the same specs, but it is different (still 100w HT sub dunno if 4 or 8 ohm mine is 4)

http://manuals.harman.com/JBL/HOM/Se...XPS10%20sm.pdf

then i find the manual to be paid here

JBL - SCS 175 Sub Service Manual on Windows Drivers Downloads

but it cost around 10 usd.


Do you think i am gonna find the right manual there?

Or do you amp gurus are able to help me someway to find an alternative resistance and confirm that it may be the right capacitor the one i put it in.

The main problem may be the wattage of that resistance.
it is soo big compared to the 2w....


please help!!!

Last edited by marcogt; 15th September 2010 at 11:43 PM.
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Old 16th September 2010, 11:02 PM   #2
marcogt is offline marcogt  Italy
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i pay the money down, and then i got the schematics.

Now i changed the huge (in dimensions) 56ohm 1 watt that was fried, and replaced with a normal size same specs one.

when i plug the power back and switched on (witout no input neither speaker connected) it immediatly fried out again.

What now?

here is the schamatic:

scs175_preliminary_sm.pdf - File Shared from Box.net - Free Online File Storage

the restitor i am talking about is R44, the capacitor i have replaced (now i know it is correct value the one i put into)
is C20.

i really suspect, that since this component are really near the transistor Q11 whch is a C3421 it may be damaged itself and causing the resistance to burn out.

Now a buzz sound come out from the speker at every volume ,and without no input connected.
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Old 16th September 2010, 11:24 PM   #3
Bone is offline Bone  United Kingdom
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Location: Dorset, UK
You really need to check all the transistors Q10, 11, 12, 13 & 14 to make sure they have not blown. If I was servicing this amp I would automatically replace all of them as they may be damaged.
I suspect the amp may have been overloaded at some stage. The subwoofer also needs checking as it could have gone short and caused the damage. Its unlikely the capacitor C20 was the case only part of the symptoms.
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Old 17th September 2010, 07:57 AM   #4
marcogt is offline marcogt  Italy
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Quote:
Originally Posted by bone
You really need to check all the transistors Q10, 11, 12, 13 & 14 to make sure they have not blown. If I was servicing this amp I would automatically replace all of them as they may be damaged.
I suspect the amp may have been overloaded at some stage. The subwoofer also needs checking as it could have gone short and caused the damage. Its unlikely the capacitor C20 was the case only part of the symptoms.
hi bone thanks for you help.

The real story of this sub is this.

It was shipped as hardly used, like new.
It was supposed to be more than functional.
When i got it i found it with the feets support broken, and with the heatsink bended due to a bad stroke, made during the shipment. In fact also 2 satellites were damaged.

I tested it on the arrival, and no sound was going out the speaker only a small fuzz and when i switched off, there was like a parasite current that come back in the speaker like it was possesed, afeter few second after the switch was in OFF position.

I then open the amp, and i find out that the PCB was seriously bended, due to the stroke of before.

Then i remove the electronic and straighten the metal frame where the pcb is hooked to.

Then i checked the tracks, and see they was fried a bit, and replaced it.

Then changed the the cap, and it was running.

I took everything togherter again and runninf fine for like 4 minutes. then the continuous buzz at every volume and the fried smell.

R44 was gone, and i changed it but it immediatly blown again.

how can i check if this transitro are messed up???
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Old 17th September 2010, 01:48 PM   #5
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remove the power transistors and check it one by one.

http://www.kilowattclassroom.com/Archive/AN0007.pdf
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Old 17th September 2010, 02:03 PM   #6
marcogt is offline marcogt  Italy
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ahah funny coz it's the same doc i founded on the topic in the meanwhile.

i just checked the first pnp, which i supposed to be broken, and it isn't.

i'm gonna check em all and let u know thanks again!
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Old 18th September 2010, 12:42 AM   #7
marcogt is offline marcogt  Italy
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ok here i am with the first report:

i have tested some transitor, and they seems fine exept from the two main ones.

Q13 and Q14 in, (which are stated to be D1047 and B817 in schematics) are in fact a D718 and a B688.

one is PNP and the other NPN, i have datasheets for both that helped me to find the right pin for base collector and emitter.

Here is what i read with multimeter in DIODE position (2k range):

D718 PNP :has collector and emitter shorted in one way plus base-emitter = base to collector in both way, rated at a value .038 (in 2k range, reallly near short value)

D688 NPN : base to emitter .027 in 2k range,base - collector OPEN CIRCUIT;
THE SAME VALUES in the other way too (no good)


what do you think?
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Old 18th September 2010, 04:52 PM   #8
marcogt is offline marcogt  Italy
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Join Date: Nov 2007
Quote:
Originally Posted by marcogt View Post
ok here i am with the first report:

i have tested some transitor, and they seems fine exept from the two main ones.

Q13 and Q14 in, (which are stated to be D1047 and B817 in schematics) are in fact a D718 and a B688.

one is PNP and the other NPN, i have datasheets for both that helped me to find the right pin for base collector and emitter.

Here is what i read with multimeter in DIODE position (2k range):

D718 PNP :has collector and emitter shorted in one way plus base-emitter = base to collector in both way, rated at a value .038 (in 2k range, reallly near short value)

D688 NPN : base to emitter .027 in 2k range,base - collector OPEN CIRCUIT;
THE SAME VALUES in the other way too (no good)


what do you think?
please see this post for the definitive results:

Q10 2sc3421 npn:
B-C: 1.119 kohm
B-E: 1.127
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q11 2sc3421 npn:
B-C: 1.094 kohm
B-E: 1.100
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q12 2sA1358 pnp:
B-C: 1.068 kohm
B-E: 1.075
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q13 2sd718 npn:
B-C: 0.011 kohm
B-E: 0.011 kohm
E-B=C-B=0.011 kohm
C-E=E-C=shortCIRCUIT

Q14 2sb688 pnp:
B-C: short
B-E: open
E-B=open
C-B=short
C-E=E-C=open


now what??
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Old 18th September 2010, 05:14 PM   #9
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Q10 2sc3421 npn:
B-C: 1.119 kohm
B-E: 1.127
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q11 2sc3421 npn:
B-C: 1.094 kohm
B-E: 1.100
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q12 2sA1358 pnp:
B-C: 1.068 kohm
B-E: 1.075
E-B=C-B=OPEN CIRCUIT
C-E=E-C=OPEN CIRCUIT

Q13 2sd718 npn: This one Defective
B-C: 0.011 kohm
B-E: 0.011 kohm
E-B=C-B=0.011 kohm
C-E=E-C=shortCIRCUIT

Q14 2sb688 pnp: This one defective
B-C: short
B-E: open
E-B=open
C-B=short
C-E=E-C=open
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Old 18th September 2010, 05:47 PM   #10
marcogt is offline marcogt  Italy
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Join Date: Nov 2007
Quote:
Originally Posted by coolet
Q13 2sd718 npn: This one Defective
B-C: 0.011 kohm
B-E: 0.011 kohm
E-B=C-B=0.011 kohm
C-E=E-C=shortCIRCUIT

Q14 2sb688 pnp: This one defective
B-C: short
B-E: open
E-B=open
C-B=short
C-E=E-C=open
thanks!

should i change the same the whole group of q10-q14 or only defective ones?
do i need to test hFE onn the transistor that are currently working?
if yes i don't know how to do it

do u want me to test also Q01->Q9?

do i need to test caps?

i found the spare transistors on the -bay i want to order and i want to make a global order.

thnks again.
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