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Thread: Fishman Loudbox PRO-LBX-300

  1. #1
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    Fishman Loudbox PRO-LBX-300

    Hello all. I have a Fishman Loudbox PRO-LBX-300 amplifier that is not working. It was left on by accident for a few days and upon discovery the fuse was blown. I replaced the 2a slow blow fuse and powered it on. It made a loud buzz and then immediately blew the fuse. I checked the googlewebs for a schematic and it brought me here:

    Fishman PRO-LBX-300

    It doesn't seem that the schematics are right for my application. On the main board mounted to the heatsink it is labeled: Loudbox Café PowerAmp, rev. 3.0, 459-000-024. One of the other boards is labeled: Fishman Transducers, rev 2.0, 459-000-032.

    I can use a multi meter and can solder fairly well.

    Where do I start? Thank you in advance for your time.

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    I should also add that one of the boards has a 2005 date.

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    My guess is you have a shorted output device and the loud buzz is the high ripple on the PSU caused by excessive loading. Disconnect the speaker & tweeter, power up the amp with a new fuse and measure across the outputs for the tweeter and main power amps. If you have more than a few mV DC then this points to a short.

    If you have a variac then you can bring up the amp slowly and monitor the output for DC. If you don't already have one then a bulb limiter is a useful protection device while you're troubleshooting.

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    Thank you for the reply. Upon closer inspection, the yellow wire off the HP plug is disconnected. It appears the lug somehow broke off. I tried to attach a jumper to see if this was the problem and my breaker popped. When I reset the breaker and left the yellow wire unattached the noise was there. I ran it through a variac and at 10 volts it was making noise. I then took out the speakers and disconnected them all and put my meter on the wires for the tweeter. No voltage. I looked closer at the Power Supply board that is connected to the heatsink and there appears to be a small cap broken off with the trace from the board attached. I'll try and successfully post some photos. Thoughts?

    dscn7980.jpg

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    dscn7983.jpg

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    Anytime the yellow wire is attached it either pops my breaker, or blows the 2a slo blow fuse.

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    Are you referring to the yellow wire coming off the mains socket? If so, I can't quite see where the wiring runs but you appear to have black,white,green/yellow leads already connected and it looks to me like that yellow lead does not belong there. None of the pictures of the socket that turn up show an additional terminal. The capacitor is another matter - it's probably shorted and needs replacing.

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    This is the yellow I am talking about. dscn7988.jpg

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    Here's the other end. It had a lug in it, so I assume It had to be connected to something...

    dscn7992.jpg

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    There's no obvious place to my eye that the yellow would be, I've looked over the boards and this doesn't appear to have gone to the transformer, it was laying right near the main power socket, I assumed it went here, but I suppose it could have been just extra material cut off.

    dscn7989.jpg

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    I also discovered another capacitor broken or blown off... Here's a few better photos.

    dscn7994.jpg
    Last edited by gibson17; 10-09-2017 at 03:53 AM. Reason: type-o

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    And another:

    dscn7995.jpg

    Sorry for the multiple messages, I don't know how to attach photos more than one at a time.

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    Senior Member Enzo's Avatar
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    Where does the blue wire next to it go, perhaps to the tweeter? And if so is there a second wire on the tweeter? It looks like the male pin for the female on the yellow wire broke off of something, perhaps off the tweeter?

    To assist my theory, do the red and black wires go to the woofer? I mean the red and black right with the blue and yellow, not the lone red and black to the left.

    This is just a thought, it might be something else of course.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by gibson17 View Post
    There's no obvious place to my eye that the yellow would be, I've looked over the boards and this doesn't appear to have gone to the transformer, it was laying right near the main power socket, I assumed it went here, but I suppose it could have been just extra material cut off.

    Click image for larger version. 

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    Is there a 'fuse holder' in thar IEC socket.
    If so, then that is the fuse out power wire. (broken off)

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    None of the pictures I can find of that socket show a terminal coming off in that position. If this were the case it would be connected directly to the unfused live connection. It also looks too small for a mains connector.

    belton_gma.jpeg

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    Quote Originally Posted by Enzo View Post
    Where does the blue wire next to it go, perhaps to the tweeter? And if so is there a second wire on the tweeter? It looks like the male pin for the female on the yellow wire broke off of something, perhaps off the tweeter?

    To assist my theory, do the red and black wires go to the woofer? I mean the red and black right with the blue and yellow, not the lone red and black to the left.

    This is just a thought, it might be something else of course.
    After looking closer at the small tweeter, the lug is definitely broken off of there. The red and black in question goto the woofer. Is it possible that I just had the bad luck of the lug breaking off of the tweeter and falling down near the mains and it made some intermittent contact?

    What about the caps that a broken off the trace? Should I replace them, or just try to reconnect them?

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    Quote Originally Posted by Jazz P Bass View Post
    Is there a 'fuse holder' in thar IEC socket.
    If so, then that is the fuse out power wire. (broken off)
    There is a fuse holder in the socket. A 2a 250v slow blow.

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    Senior Member Enzo's Avatar
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    Then the yellow wire is the output of the tweeter amp, and if you connected it to the mains fuse holder, you probably blew the tweeter amp.
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    Then the yellow wire is the output of the tweeter amp, and if you connected it to the mains fuse holder, you probably blew the tweeter amp.
    How do I test to see if that's the case? ... and if so, what needs to be replaced to repair it?

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    Could contact of the output with the mains caused the caps to pop off like that?

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    Maybe, but who knows. All you know is that if the connector didn't accidentally touch the mains you made sure that it did and you could have more damage than you began with. Disconnect the speaker and tweeter outputs and see if there's DC on either output. Which part of the circuit are the damaged caps located?

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    Quote Originally Posted by Mick Bailey View Post
    Maybe, but who knows. All you know is that if the connector didn't accidentally touch the mains you made sure that it did and you could have more damage than you began with. Disconnect the speaker and tweeter outputs and see if there's DC on either output. Which part of the circuit are the damaged caps located?
    True. Speakers are disconnected. The connector that has red/black/blue/yellow goes to the small tweeter and the small mid. The connector that is away from the 4 wire connector that is a two wire connector that is Red/Black goes to the woofer.

    On the 4 wire connector, there was DC voltage present on Blue/Yellow even at very low voltage, but much less on Red/black and not so much at low voltage.

    On the 2 wire connector for the woofer there was DC voltage present even at low voltage. The unit was also drawing quite a few amps even at low voltage. 4amps before the 2a slow blow gave way.

    The damaged caps seems to be right near the r58 resistor and right below the vr2 trim pot.

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    You have three power amps that need to be checked. Plus your mains voltage could have appeared on the main PSU rails momentarily (by way of an output to supply short in U5) prior to the fuse blowing. You'll need to establish that all the DC voltages are correct and that you're not missing one of the supply rails and eliminate the possibility of excessive current draw in the PSU. The only way to do this is either to isolate the power supply or remove the excessive load so that the amp can power up. The tweeter amp chip is almost certainly blown and I'd carefully remove it to eliminate it. Sometimes the supply pins read low resistance to the output pin and give a clearer indication of failure. Worthwhile just to take a measurement. The output devices on the other two amps need checking for shorts. You should also carefully inspect the board for any other visible damage. Before embarking on any repair I would establish what's shorted/damaged. Could be you have multiple faults. Sometimes it's necessary to remove the output transistors just to get to the point where an amp can be powered without blowing the fuse and then work from there.

    What's your mains voltage?

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    Quote Originally Posted by Mick Bailey View Post
    You have three power amps that need to be checked. Plus your mains voltage could have appeared on the main PSU rails momentarily (by way of an output to supply short in U5) prior to the fuse blowing. You'll need to establish that all the DC voltages are correct and that you're not missing one of the supply rails and eliminate the possibility of excessive current draw in the PSU. The only way to do this is either to isolate the power supply or remove the excessive load so that the amp can power up. The tweeter amp chip is almost certainly blown and I'd carefully remove it to eliminate it. Sometimes the supply pins read low resistance to the output pin and give a clearer indication of failure. Worthwhile just to take a measurement. The output devices on the other two amps need checking for shorts. You should also carefully inspect the board for any other visible damage. Before embarking on any repair I would establish what's shorted/damaged. Could be you have multiple faults. Sometimes it's necessary to remove the output transistors just to get to the point where an amp can be powered without blowing the fuse and then work from there.

    What's your mains voltage?
    Hello there. I've been quite busy so haven't gotten a chance to reply until now. I removed the Power Amp board from the amplifier to get a clearer look at what's going on. The caps on the backside look to be the only things that have sustained any visible damage. While the power amp board was removed, I powered up the amp so I could take some measurements off of the power connector board, which is what I assume you were asking for. When I took measurements from pins 1 and 2, the meter jumped to 5 acv then dropped to zero. Measurements from 4 to 3 provided no voltage, and pins 1 to 4 produced 120v. Pins 3 and 2 also produced 120v. Without the power amp board connected no fuses blew.

    I have not gotten around to removing U5 yet. Should I reconnect the caps on the back since there are two that need attention or should I wait to get walked through it?

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    I think you're measuring the input voltage and assume you're on 120v mains.

    I'm referring to checking the DC output voltages. Take a look at the PSU and you should have +/- 33v and +/- 14v. Also check for AC - those supplies should be pretty ripple-free with no load. These voltages have to be correct before moving on. Don't reconnect the old caps - they need replacing.

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