Discrete Subwoofer Amplifier Repair

[Hara]

Hey guys,

I'm trying to fix my broken subwoofer amplifier. When I plug it in it makes a reasonably loud hum regardless if it has an input plugged in or not. Its a jpl plate amp for my parent's HT system.

Here are the pictures of the amp

http://student.ucr.edu/~bmill003/sub_amp_circuit.JPG

http://student.ucr.edu/~bmill003/sub_amp_close_up.JPG

http://student.ucr.edu/~bmill003/sub_amp_cross_over.JPG

http://student.ucr.edu/~bmill003/sub_amp_plate.JPG

I think one of the resistors is burnt out, can you guys verify this? Its the brown one in the close up picture. This is my first electronic repair project, any advice is appreaciated.

Thanks

[BlackMTX]

If you have the schematic diagram of that subamp, look at the resistor's value in the schematic diagram i think that is R60, and test all components near R60, and find out why R60 was burnt

[Hara]

The only schematic I could possibly get is from the service manual people are selling online. I could also try to create one based on the circuit but that will take time. I'll still have trouble finding the value for that burnt resistor though... I'll take the board apart from the heat sink and see what I find.

[PM650]

Here's the service manual from jbl, comes with schematics:
http://manuals.harman.com/JBL/HOM/Se...XPS10%20sm.pdf

[Hara]

Wow this forum is awesome... I guess my google skills need work.

[Triumph]

PM 650

I see you are very good in finding JBL schematics. I am looking for months now for the crossoverscheme of the P452 carspeaker can you help me with this one?

Its in the Birdbox I am making you see on the left. Love to finish it.

I bought this lovely P452 coaxial driver for a penny without XO.

[BlackMTX]

Hi, PM650, it's good that you have shared to us the schematic diagram, do you have the apex sr. subwoofer schematic diagram? I find this at www.apexjr.com but the schematic diagram is not clear. kindly share to us if you have still information about schematic diagram of subwoofers, again thank you.

Hi. Hara, based on the schematic diagram the R60 is 22 ohms, check Q4=D1563, i'm sure this is shorted, check also Q5,Q6,Q7. But you must the know the characteristics of the NPN and PNP transistors, whether open, shorted or leaky. The loud hum going to the speaker is a high DC voltage that migth destroy the voice coil of the speaker.

[Hara]

I'm debating whether this amp is worth recovering. Judging from the web, other's have had similar experiences with this amp and the ones that fixed it continued to have problems afterwords. Parts of the board have a slight blackened discoloration. Also, I already bought a replacement plate amp for the sub but I was thinking about using the repaired amp for another project...

However, getting this amp to work again is probably worth the learning experience. Thanks for the advice BlackMTX, I'll check into it deeper when I have the time.

(Reading these circuits [as in how they work] must be second nature to you guys. I can read somewhat complicated digital circuits, but analog is still not in my understanding.)

[PM650]

To be honest, I'm not as resourceful as you may think. The jbl service manuals can (usually) be obtained by finding the link for the owners manual and changing it a bit. (.../HOM/Owner's%20Manual/TLXPS10%20om.pdf becomes .../HOM/Service%20Manual/TLXPS10%20sm.pdf).

Triumph: I doubt jbl creates service manuals for their non-electronic components, and if so, then I have been unsuccessful in locating it for your speaker. Also, it's unlikely that the crossover schematic would be in the sm/om anyways. Your best bet would be to find someone who has a set of those speakers (and crossovers, of course) and ask them to make a schematic or something for you.

BlackMTX: The schematic looks a bit fuzzy, but fine to me. I can relabel the components using larger text, if that would help. Or maybe you just need help with a few components?

[BlackMTX]

Hi PM650, thanks for the tip, I found the JBL PB12 subwoofer service manual. Regarding Apex Sr., I just want to know the values of R33, R32, R26, R27, C16 and C17, from the preamp schematic diagram, by the way, do you have the PCB layout of the preamp?thanks in advance....