My son has asked for my help in learning to build and repair electronic devices, so after 35 years away from a soldering iron I need help getting started. Our first project is to try and repair a JBL TLXPS10 powered subwoofer amp that started to roar while the stereo to which it was attached was playing relatively quiet background music. I've looked through the web and this site and saw a few threads discussing issues with this amp. The most detailed was a few years ago on diyaudio:
http://www.diyaudio.com/forums/showthread.php?threadid=68551
Many of the symptoms are similar to ours. Darkened spots on the power amp board darkest next to the leads of a resistor. In our case its R74 and not R60. I've attached a link to a photo of the board (in an effort to save server space here) and the circuit board diagrams and schematics for both the preamp and power amp. If they load too slowly, let me know and I'll post the files here.
http://pws.prserv.net/divergence/BurntSpots OnAmpBoard.JPG
http://pws.prserv.net/divergence/JBLTLXPS10BoardsSchematics.pdf
You can see that there are some capacitors and diodes in the darkened area as well. We tried to test them as well as we could (recognizing the limitations of testing in a board). My presumption is that if we get a normal response it suggests the component is ok and if we get an abnormal result it is ambiguous due to possible interactions with other parts of the circuit. Results so far are that the capacitors seem ok. With a DMM on a low ohm scale they started out at a low reading and the reading slowly increased to a plateau. The resistors in the area also checked out with values within about 5-10% of the nominal listed on the schematic (including R74).
The diodes in that region were ambiguous, showing continuity in both directions on the diode test scale. The diode test scale seems ok, as we got a normal diode response on a diode in another area of the board (continuity in one direction and open circuit in the other direction).
The power transisters (Q6 and Q7) tested ok with the DMM. Both behaved like diodes from the center connnectors to the side connectors, and have relatively low resistance in both directions between the pair of side connectors. Q4 and Q5 gave ambiguous results.
We see no sign of discoloration on either the preamp board or the relatively fine wires connecting the amp and preamp (which connect at P1 near the darkened area of the power amp board).
Any suggestions as to the cause of the failure and which device or devices I should remove and test? My guess would be ZD6 failed and shorted to ground. Is that reasonable? Or could it be C34 and our DMM test is misleading me? I'd rather not get into a situation where we're pulling component after component from the board without a clear idea that we're on the right path. My resoldering skills are not good enough that we could resolder several devices with trimmed leads successfully.
Thanks in advance for your help.
http://www.diyaudio.com/forums/showthread.php?threadid=68551
Many of the symptoms are similar to ours. Darkened spots on the power amp board darkest next to the leads of a resistor. In our case its R74 and not R60. I've attached a link to a photo of the board (in an effort to save server space here) and the circuit board diagrams and schematics for both the preamp and power amp. If they load too slowly, let me know and I'll post the files here.
http://pws.prserv.net/divergence/BurntSpots OnAmpBoard.JPG
http://pws.prserv.net/divergence/JBLTLXPS10BoardsSchematics.pdf
You can see that there are some capacitors and diodes in the darkened area as well. We tried to test them as well as we could (recognizing the limitations of testing in a board). My presumption is that if we get a normal response it suggests the component is ok and if we get an abnormal result it is ambiguous due to possible interactions with other parts of the circuit. Results so far are that the capacitors seem ok. With a DMM on a low ohm scale they started out at a low reading and the reading slowly increased to a plateau. The resistors in the area also checked out with values within about 5-10% of the nominal listed on the schematic (including R74).
The diodes in that region were ambiguous, showing continuity in both directions on the diode test scale. The diode test scale seems ok, as we got a normal diode response on a diode in another area of the board (continuity in one direction and open circuit in the other direction).
The power transisters (Q6 and Q7) tested ok with the DMM. Both behaved like diodes from the center connnectors to the side connectors, and have relatively low resistance in both directions between the pair of side connectors. Q4 and Q5 gave ambiguous results.
We see no sign of discoloration on either the preamp board or the relatively fine wires connecting the amp and preamp (which connect at P1 near the darkened area of the power amp board).
Any suggestions as to the cause of the failure and which device or devices I should remove and test? My guess would be ZD6 failed and shorted to ground. Is that reasonable? Or could it be C34 and our DMM test is misleading me? I'd rather not get into a situation where we're pulling component after component from the board without a clear idea that we're on the right path. My resoldering skills are not good enough that we could resolder several devices with trimmed leads successfully.
Thanks in advance for your help.
bkspero;
I hope nothing died in your amp, except one leaking capacitor that spit an electrolyte that shorted the board conductors!
Take it off and wash carefully with a dish soap and a toothbrush, then carefully rinse and dry. Replace the capacitor that is leaking out electrolyte, it should be well visible which one. I had very similar problem with a car radio, you may see my photos:
http://www.wavebourn.com/forum/viewtopic.php?t=476
I hope nothing died in your amp, except one leaking capacitor that spit an electrolyte that shorted the board conductors!
Take it off and wash carefully with a dish soap and a toothbrush, then carefully rinse and dry. Replace the capacitor that is leaking out electrolyte, it should be well visible which one. I had very similar problem with a car radio, you may see my photos:
http://www.wavebourn.com/forum/viewtopic.php?t=476
I personally would check that R73/74 are still in value, replace the capacitors C34 and C35, and the zeners ZD5/ZD6. I'd use higher wattage zeners.
You may want to check the voltages at pins 4 and 8 of U6, to make sure there is the correct 15 volts symmetrical there. If this has gone high, the opamps will be toast.
You may want to check the voltages at pins 4 and 8 of U6, to make sure there is the correct 15 volts symmetrical there. If this has gone high, the opamps will be toast.
Thanks
Thanks for the quick response. We'll try replacing the diodes with larger ones and set them off of the board. And also replace C28, C29, C34, and C35.
What do you think about also replacing the large capacitor that just shows along the lower left edge of the photo? The yellow/white hardened glue-like material that's underneath that capacitor is black along its edges near R74.
My concern about changing the large capacitor is what should we do with the yellow/white stuff. Do you think that we should try to scrape the old stuff off of the board, or might that cause more harm than good. And should we use more white material under the new capacitor? If so, can you recommend a place where we can get some?
As I read this and put myself in your place I think about the saying that no good deed goes unpunished. Sorry about all the extra questions. Please know that I appreciate the help.
Barry
Thanks for the quick response. We'll try replacing the diodes with larger ones and set them off of the board. And also replace C28, C29, C34, and C35.
What do you think about also replacing the large capacitor that just shows along the lower left edge of the photo? The yellow/white hardened glue-like material that's underneath that capacitor is black along its edges near R74.
My concern about changing the large capacitor is what should we do with the yellow/white stuff. Do you think that we should try to scrape the old stuff off of the board, or might that cause more harm than good. And should we use more white material under the new capacitor? If so, can you recommend a place where we can get some?
As I read this and put myself in your place I think about the saying that no good deed goes unpunished. Sorry about all the extra questions. Please know that I appreciate the help.
Barry
Is the hardened white material electorlyte
Thanks for the suggestion.
See the yellow-white material under the large capacitor at the left edge of the photo. Its hard and I thought that it was some sort of glue. Could that be leaked electrolyte that has dried or baked to hardness?
On the other hand, that capacitor is C36. Would its leaking electrolyte cause R74 to overheat?
Thanks again,
Barry
Thanks for the suggestion.
See the yellow-white material under the large capacitor at the left edge of the photo. Its hard and I thought that it was some sort of glue. Could that be leaked electrolyte that has dried or baked to hardness?
On the other hand, that capacitor is C36. Would its leaking electrolyte cause R74 to overheat?
Thanks again,
Barry
The white stuff is just ordinary PVA-type glue, it's used to anchor the component(s). Depending on how well it's stuck to the board, you can choose to leave it or remove it. Do get some PVA glue to rebond any new large capacitors you put in, as there's much vibration expected inside a subwoofer enclosure...
Electrolyte is conductive and corrosive, so it can cause a component to fail... It will also eat into the PCB material, so make sure you clean the stuff off as best as you can....
Cheers!
Alternative to the PVA glue - hotmelt works fine, although bonding may not be as good in the long term...
Electrolyte is conductive and corrosive, so it can cause a component to fail... It will also eat into the PCB material, so make sure you clean the stuff off as best as you can....
Cheers!
Alternative to the PVA glue - hotmelt works fine, although bonding may not be as good in the long term...
Hi bkspero,
I don't see any leaking electrolyte. I do see glue as Clem pointed out. When you remove the components in the overheated area, have a look at the bottom of the capacitors. If one were leaking, it will be hard to miss. Cleanup is now easy since the area is clear of parts.
No biggie.
-Chris
I don't see any leaking electrolyte. I do see glue as Clem pointed out. When you remove the components in the overheated area, have a look at the bottom of the capacitors. If one were leaking, it will be hard to miss. Cleanup is now easy since the area is clear of parts.
No biggie.
-Chris
Thanks all.....going in
Thanks all. After I posted the notes regarding the white material I went downstairs to look again at the board and it's clear that the material was placed there intentionally and not leaked electrolyte. And its pretty clear that the heat never reached the large capacitors attached with the glue, so we're leaving them be. Sorry if I wasted anyones time on that tangent.
Have the 4 replacement diodes and 4 capacitors (2, 100 uF and 2, 220 uF). We'll take out the old ones tonite and clean off the board as well as we can. Will wait for a new soldering station to arrive to install the new ones. Will let everyone know how things work out. Thanks again for the advice.
Thanks all. After I posted the notes regarding the white material I went downstairs to look again at the board and it's clear that the material was placed there intentionally and not leaked electrolyte. And its pretty clear that the heat never reached the large capacitors attached with the glue, so we're leaving them be. Sorry if I wasted anyones time on that tangent.
Have the 4 replacement diodes and 4 capacitors (2, 100 uF and 2, 220 uF). We'll take out the old ones tonite and clean off the board as well as we can. Will wait for a new soldering station to arrive to install the new ones. Will let everyone know how things work out. Thanks again for the advice.
C22 another symptom and advice needed
There is another symptom that might be relevant to this diagnosis. The roaring exists whether there is a signal to the amp or not. I briefly (<1 sec) plugged it the day after the failure; several hours after disconnecting it from the system. It "roared" then, too, even though it should have been off as a result of the auto-on circuit having been deactivated. Correct? Or would a problem in the area of ZD6 drive the speaker even in the absence of the auto-on circuit activating? I'm not knowledgeable enough to work that out.
Is there a way to test the bad-C22 hypothesis from the solder side of the preamp board? That's because the component side of that board is glued to a piece of ca. 1/2 in thick foam that is attached to the metal backplate of the amp (the plate that seals the opening in the subwoofer enclosure after assembly). So I can't really get to the preamp components without doing some serious disassembly.
Or should I just replace the parts 4 capacitors and 4 diodes on the amp board and try things out?
Thanks again for everyone's help.
There is another symptom that might be relevant to this diagnosis. The roaring exists whether there is a signal to the amp or not. I briefly (<1 sec) plugged it the day after the failure; several hours after disconnecting it from the system. It "roared" then, too, even though it should have been off as a result of the auto-on circuit having been deactivated. Correct? Or would a problem in the area of ZD6 drive the speaker even in the absence of the auto-on circuit activating? I'm not knowledgeable enough to work that out.
Is there a way to test the bad-C22 hypothesis from the solder side of the preamp board? That's because the component side of that board is glued to a piece of ca. 1/2 in thick foam that is attached to the metal backplate of the amp (the plate that seals the opening in the subwoofer enclosure after assembly). So I can't really get to the preamp components without doing some serious disassembly.
Or should I just replace the parts 4 capacitors and 4 diodes on the amp board and try things out?
Thanks again for everyone's help.
mrshow4u said:
Yes. While it is in "roar" mode: If the bi-colored LED is not changing from red to green rapidly, then C-22 is not at fault. No need to probe anything.
If you do have a scope and the LED was changing colors, you could look at pin 5 of U5 (4558) and see that the voltage is not smoothed. That point looked accessable with the PCB overlay PDF you provided.
Replace the other stuff too. It's cheap insurance.
Saga continues
Fixed roaring, now no sound. Details below.
Replaced 4 capacitors (C28, 29, 34, 35) and 4 diodes (ZE3-6). There was nothing water soluble on the board. There are areas of greenish, transparent dried coating that I thought might have been dried electrolyte, but it would not clean off and there are fingerprints in it so it must have been there before the last incident. Also no sign of failure on the old capacitors.
Tested old capacitors and diodes, and they seem ok by my DMM. Confirmed no shorts or open circuits around new components. Turned on amp and roaring gone, but moderate hum. Resoldered all of the joints (including some old components on connected pads). Hum gone. Steady red LED (no signal yet).
Attached to home theater receiver and turned on sound. Relay clicked. LED turned green (as it should with signal) and low whine sound. Pulled plug on subwoofer. Turned off receiver. Plugged sub back in. Turned on receiver. No sound, although LED turns green.
Tested power transisters Q6 and Q7 with DMM and got normal 2x diode response with low resistance across collector and emitter. Tested voltages at C36 and C37 with signal off and on and got the expected 41 volts with correct polarity. Clicking sound happens when signal is applied when amp is on, and also a few seconds after amp is unplugged. So those things are normal.
But I do not measure voltage to ground from any of the 3 legs of Q6 or Q7 when signal is applied. If I read the schematic correctly, I should be seeing 41 volts to ground from the emitter legs of both when a signal is being input to the amp. And that voltage comes directly from the relay K1. So it sounds to me like the relay contacts may be fried.
Is my analysis reasonable? Are there possible alternative causes? How can I test them?
Thanks again for everyones help.
Fixed roaring, now no sound. Details below.
Replaced 4 capacitors (C28, 29, 34, 35) and 4 diodes (ZE3-6). There was nothing water soluble on the board. There are areas of greenish, transparent dried coating that I thought might have been dried electrolyte, but it would not clean off and there are fingerprints in it so it must have been there before the last incident. Also no sign of failure on the old capacitors.
Tested old capacitors and diodes, and they seem ok by my DMM. Confirmed no shorts or open circuits around new components. Turned on amp and roaring gone, but moderate hum. Resoldered all of the joints (including some old components on connected pads). Hum gone. Steady red LED (no signal yet).
Attached to home theater receiver and turned on sound. Relay clicked. LED turned green (as it should with signal) and low whine sound. Pulled plug on subwoofer. Turned off receiver. Plugged sub back in. Turned on receiver. No sound, although LED turns green.
Tested power transisters Q6 and Q7 with DMM and got normal 2x diode response with low resistance across collector and emitter. Tested voltages at C36 and C37 with signal off and on and got the expected 41 volts with correct polarity. Clicking sound happens when signal is applied when amp is on, and also a few seconds after amp is unplugged. So those things are normal.
But I do not measure voltage to ground from any of the 3 legs of Q6 or Q7 when signal is applied. If I read the schematic correctly, I should be seeing 41 volts to ground from the emitter legs of both when a signal is being input to the amp. And that voltage comes directly from the relay K1. So it sounds to me like the relay contacts may be fried.
Is my analysis reasonable? Are there possible alternative causes? How can I test them?
Thanks again for everyones help.
Hi bkspero,
I think the relay contacts being fried is spot on. As long as you hear the relay click and the power LED turns green, that 41 volts (if available) should be connecting to those power transistors. You said you did hear a small whine?? Hmmmmm, that would indicate some voltage is available to the output transistors. There's a couple of zeners and their associated filters on the power amp board. ....ZD4, C28, ZD3, and C29. ....as well as a couple of dropping resistors R56, R57. Is access so bad that these points can't be probed?
I think you're getting close. ....at least weeding out all of the possible failures.
Check that power relay first. After that, you might want to rig something that will let you have access to the power amp board (extenson wires, etc) I hope it's the relay. That's the easiest, cleanest, thing
I think the relay contacts being fried is spot on. As long as you hear the relay click and the power LED turns green, that 41 volts (if available) should be connecting to those power transistors. You said you did hear a small whine?? Hmmmmm, that would indicate some voltage is available to the output transistors. There's a couple of zeners and their associated filters on the power amp board. ....ZD4, C28, ZD3, and C29. ....as well as a couple of dropping resistors R56, R57. Is access so bad that these points can't be probed?
I think you're getting close. ....at least weeding out all of the possible failures.
Check that power relay first. After that, you might want to rig something that will let you have access to the power amp board (extenson wires, etc) I hope it's the relay. That's the easiest, cleanest, thing
More test spots
The whine only happened for a few seconds until I pulled the subwoofer plug. No sound since then.
I can access most of the elements you list. No voltage to ground at either side of R58 and R59. 41 volts with correct polarity to the higher voltage side of R73 and R74 with or without signal to the inputs. No voltage at ZD3 or ZD4 with our without signal. Has to be the relay.
I'll pull out the relay and check its continuity.
The whine only happened for a few seconds until I pulled the subwoofer plug. No sound since then.
I can access most of the elements you list. No voltage to ground at either side of R58 and R59. 41 volts with correct polarity to the higher voltage side of R73 and R74 with or without signal to the inputs. No voltage at ZD3 or ZD4 with our without signal. Has to be the relay.
I'll pull out the relay and check its continuity.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.