gibson17 10/7/2017 5:37 PM
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:

http://music-electronics-forum.com/t40099/

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.
 
gibson17 10/7/2017 5:38 PM
I should also add that one of the boards has a 2005 date.
 
Mick Bailey 10/8/2017 12:55 AM
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.
 
gibson17 10/8/2017 11:56 AM
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?

[ATTACH=CONFIG]45229[/ATTACH]
 
gibson17 10/8/2017 11:58 AM
[ATTACH=CONFIG]45230[/ATTACH]
 
gibson17 10/8/2017 11:59 AM
Anytime the yellow wire is attached it either pops my breaker, or blows the 2a slo blow fuse.
 
Mick Bailey 10/8/2017 2:35 PM
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.
 
gibson17 10/8/2017 8:40 PM
This is the yellow I am talking about. [ATTACH=CONFIG]45235[/ATTACH]
 
gibson17 10/8/2017 8:43 PM
Here's the other end. It had a lug in it, so I assume It had to be connected to something...

[ATTACH=CONFIG]45236[/ATTACH]
 
gibson17 10/8/2017 8:48 PM
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.

[ATTACH=CONFIG]45237[/ATTACH]
 
gibson17 10/8/2017 8:51 PM
I also discovered another capacitor broken or blown off... Here's a few better photos.

[ATTACH=CONFIG]45238[/ATTACH]
 
gibson17 10/8/2017 8:52 PM
And another:

[ATTACH=CONFIG]45239[/ATTACH]

Sorry for the multiple messages, I don't know how to attach photos more than one at a time.
 
Enzo 10/9/2017 12:16 AM
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.
 
Jazz P Bass 10/9/2017 12:35 AM
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.

[ATTACH=CONFIG]45237[/ATTACH]
Is there a 'fuse holder' in thar IEC socket.
If so, then that is the fuse out power wire. (broken off)
 
Mick Bailey 10/9/2017 1:33 AM
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.

[ATTACH=CONFIG]45241[/ATTACH]
 
gibson17 10/9/2017 8:55 AM
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?
 
gibson17 10/9/2017 8:56 AM
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.
 
Enzo 10/9/2017 10:32 AM
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.
 
gibson17 10/9/2017 10:55 AM
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?
 
gibson17 10/9/2017 11:11 AM
Could contact of the output with the mains caused the caps to pop off like that?
 
Mick Bailey 10/9/2017 1:47 PM
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?
 
gibson17 10/9/2017 2:55 PM
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.
 
Mick Bailey 10/10/2017 8:16 AM
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?
 
gibson17 10/12/2017 11:18 AM
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?
 
Mick Bailey 10/13/2017 8:10 AM
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.
 
gibson17 11/10/2017 10:07 AM
Quote Originally Posted by Mick Bailey View Post
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.
Hello there, been pretty busy so haven't gotten a chance to get to doing more work to this until the last few days. I removed U5, but was still blowing fuses. There amp was still drawing over 2a at about 40v. I did the smell test and Q18 was stinky and very hot. So I removed it. The amp will now idle at full power at .18 amp draw. I tried testing various points now that the amp would stay running. I took a look at the schematic and it looked like to test for those values that I should test U10, U11, BR1 areas to get the values you are looking for. I tested for both AC and DC. I did not get the numbers exactly, I came in at 35.8vdc and 14.5vdc. The outside legs of BR1 had VDC while the inside legs had VAC. Am I testing the correct points? Please advise.