DSP/Amp Problem (Reversed Polarity)

[Riley1]

New guy here, first post. I should mention I know just enough about troubleshooting electronics to be dangerous but I can follow directions.

I did something really bad and am hoping maybe you guys can point me in the right direction. I have a JBL MS-8 digital signal processor/amp for my car. When I installed it I accidentally reversed the 12VDC power. I heard a pop and the first thing I checked was the 25A fuse which was blown. I replaced it and correctly wired the power. Now when I hit the reset button the power light comes on for about 15 seconds and then goes out. This unit also has a remote power circuit. This terminal receives 12vdc from the head unit to turn it on. It also passes the power back out to turn on 2 other amplifiers. This circuit works fine as the others amps are powered on.

With help from a friend at work I'm trying to see if I can get this thing functioning again. I do have a service manual. Below is a shot of the part of the circuit we are currently looking at. We pulled the diodes off the board to check them. Using a Fluke DMM set on the diode setting we get .618 and .468 and when we reverse the leads they both read 0. We inspected the board under a large lighted magnifier and can't see any visible damage. The only thing suspect is some dark material under the 2 caps (4700UF/35V) shown which I think may just be glue. The caps themselves are not bulging or discolored. Could they still be damaged? Any help with this will be greatly appreciated.

Thanks,
Rick

 

[Gryd3]

New guy here, first post. I should mention I know just enough about troubleshooting electronics to be dangerous but I can follow directions.

I did something really bad and am hoping maybe you guys can point me in the right direction. I have a JBL MS-8 digital signal processor/amp for my car. When I installed it I accidentally reversed the 12VDC power. I heard a pop and the first thing I checked was the 25A fuse which was blown. I replaced it and correctly wired the power. Now when I hit the reset button the power light comes on for about 15 seconds and then goes out. This unit also has a remote power circuit. This terminal receives 12vdc from the head unit to turn it on. It also passes the power back out to turn on 2 other amplifiers. This circuit works fine as the others amps are powered on.

With help from a friend at work I'm trying to see if I can get this thing functioning again. I do have a service manual. Below is a shot of the part of the circuit we are currently looking at. We pulled the diodes off the board to check them. Using a Fluke DVM set on the diode setting we get .618 and .468 and when we reverse the leads they both read 0. We inspected the board under a large lighted magnifier and can't see any visible damage. The only thing suspect is some dark material under the 2 caps (4700UF/35V) shown which I think may just be glue. The caps themselves are not bulging or discolored. Could they still be damaged? Any help with this will be greatly appreciated.

Thanks,
Rick

View attachment 24823

My suggestion would be to stop probing each component as a time and first check ALL power sources.
You will notice that the part you are looking at goes to VCAR, and that other components require things like 5V... which needs to be supplied somewhere
Leave the black lead on ground, and poke around to make sure that all the interior power supplies and regulators are powering things properly... considering the light comes on momentarily, it sounds like damaged logic, or perhaps an internal voltage regulator for something is shot.

 

[Riley1]

My suggestion would be to stop probing each component as a time and first check ALL power sources.
You will notice that the part you are looking at goes to VCAR, and that other components require things like 5V... which needs to be supplied somewhere
Leave the black lead on ground, and poke around to make sure that all the interior power supplies and regulators are powering things properly... considering the light comes on momentarily, it sounds like damaged logic, or perhaps an internal voltage regulator for something is shot.

Thanks for the reply. Would you be so kind as to take a look at the schematic and give me some direction? Schematics are towards the end. https://www.dropbox.com/s/k2xwilr5h70l26q/JBL MS8 Service Manual.pdf?dl=0

Thanks,
Rick

 

[dorke]

Lets give a try.
First thing is to disconnect the speakers from the unit.
Then check that VCAR is the same as VBAT about 12V (green in the pic below),check it while pressing the reset and releasing it.
In pink are the diodes and caps you looked at.



Then you should check all the voltages on U4 the IC TDA3681 (interesting little bugger )and some more(green in the pic below).


Please,post your findings and we can go on.

 

[Riley1]

Thank you so much for the help. I currently have the unit out of the car. I'll dig up a power supply, do some checking and report back.

Rick

 

[elebish]

With a 25 A fuse, you could be in trouble. I would check D34 and D2 for being shorted. D2 is for spikes and D34 could be an input power reversal protection diode which is designed to short out and blow the fuse when power in is accidently reversed. This will protect the unit. D32 should be a heavy diode with thick axial leads.

 

[Gryd3]

With a 25 A fuse, you could be in trouble. I would check D34 and D2 for being shorted. D2 is for spikes and D34 could be an input power reversal protection diode which is designed to short out and blow the fuse when power in is accidently reversed. This will protect the unit. D32 should be a heavy diode with thick axial leads.

D2 and D32 have already been de-soldered and checked with a meter. Additionally, the device partially powers on. If D2 or D32 had internal short, the fuse would have gone again. If they were open, I can't see it preventing operation (Although you would lose the protection of the diodes in this case)

Waiting for the reply to Dorke's marked schematic would be ideal.

 

[Riley1]

Thanks for all the replies. The diodes have been reinstalled today and I plan on doing the other checks tomorrow and will report back.

Thanks,
Rick

 

[Riley1]

Here's what I came up with. The readings on the right side of U4 are after I put 12V on the remote in terminal. When I did that the power light stays on but appears to be resetting every 20 seconds or so as you can see a quick blink. Thoughts?

 

[dorke]

The first test that shows a drop of VBAT and VCAR from 12V to less than 9V need to be fixed first.
Are you using a Power Supply or the Car Battery when testing?
A power-supply may enter current limit condition and thus drop it's voltage.

 

[Riley1]

It's a power supply (3A), no good? Not sure if it matters or not but it's not a slow drop it's pretty much instantaneous.

Thanks,
Rick

 

[dorke]

I think not,
You have a 25A fuse on that line,worth trying in the car.
Can you see how much current is drawn from the PS?

This scenario is also possible:
One of the voltages that U4 generates is overloaded/shorted so that when U4 is activated it will draw a large current that will cause the power supply to enter current limit and drop it's voltage.
Another possibility is a faulty U4.

But,
(look at the blue in the pic below)
the most disturbing is the 3.27V measured instead of the 1.2V.
This is the DSP's(U21 on page 94/95) core voltage.
If that voltage is indeed 3.27 ,then most likely the DSp IC is a goner.
This also means that the SMPS IC(U24) circuit has a fault somewhere.

Another issue is the 5V instead of 12V
zener-D33 the main suspect here and C376 ,Q8-10
You can check voltages in the "area" and post.

 

[Riley1]

Thanks for all the help. I'll get some more measurements next week. I left this mess on a bench at work. I'm beginning to think I might have an expensive door stop on my hands. Hard to believe this thing was designed to self destruct when some bone head makes a simple mistake.

Rick

 

[Riley1]

There might be hope yet. I don't have the board in front of me but looking at the drawing in the manual I don't remember checking anything where these components are located on the board. I may have wrote down the 3.27V measurement at C278/R203 by mistake. Keeping my fingers crossed.

Rick

 

[cjdelphi]

IC's may have been destroyed, but a 25amp fuse!

I would check the capacitors, they will short when reversed pulling in that kind of amperage .. (the TVS diode is most likely the cause of the pop but i bet those caps took a small beating)

 

[cjdelphi]

Yeah you'd think basic protection of reverse power would be normal these days, but i guess a 30c power diode would bankrupt them lol

Although a diode is a waste of power to protect a circuit (but could protect the main amplifier ic's though) a simple odd shaped connector for positive so you can't mix them is a much better idea

 

[Riley1]

IC's may have been destroyed, but a 25amp fuse!

I would check the capacitors, they will short when reversed pulling in that kind of amperage .. (the TVS diode is most likely the cause of the pop but i bet those caps took a small beating)

Which capacitors are you referring to, C95/C38? D2/D35 have been replaced even though they checked out fine.

Thanks,
Rick

 

[cjdelphi]

3am and obviously my brain switched off on me... the tvs diode is fine, you reversed polarity, nothing else duh...

3am i'll sleep on it!

 

[cjdelphi]

Maybe it is just the IC causing the issues....

 

[Riley1]

There might be hope yet. I don't have the board in front of me but looking at the drawing in the manual I don't remember checking anything where these components are located on the board. I may have wrote down the 3.27V measurement at C278/R203 by mistake. Keeping my fingers crossed.

Rick

I rechecked this and I have 1.2V so I guess that's a good sign. My voltage at the terminals is also 12. I ordered a new voltage regulator and a couple of caps. I'll throw them in and see what I have.

Thanks,
Rick