Fishman Loudbox PRO-LBX-300

[gibson17]

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]

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

[Mick Bailey]

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]

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?

[gibson17]

[gibson17]

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

[Mick Bailey]

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]

This is the yellow I am talking about.

[gibson17]

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

[gibson17]

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.

[gibson17]

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

[gibson17]

And another:



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

[Enzo]

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]

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]

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.