Fishman Loudbox Mini died mid gig

JCA2550

Hi,

I've had a Fishman Loudbox Mini acoustic amp for 3 years from new and it's given sterling service until a month ago when mid-gig it smelt "hot" and slightly acrid, then died. We were playing a small local festival gig in a pub with "dubious wiring" and I suspect this was the catalyst as a Yamaha powered monitor also cut out earlier in the day? I replaced the internal slow-blow fuse by the power socket but that blew straight away.

 I've been in touch with Fishman in the States who were very helpful and put me in touch with UK distributor JHS.
JHS were less helpful as they've just given up distribution of Fishman gear so, they put me in touch with 440 Distribution who are in the process of taking on Fishman gear for the UK.

I've forwarded all details to Tom @ 440 Distribution but there's not very much happening?

Who's my best bet to get it fixed?

I'm in North London South Herts and usually go to Malcolm Hine in Edmonton.

Daft question possibly but thought I'd ask on here first.

Cheers gang,

C

JCA2550

Seriously, nothing guys?

ICBM

I can’t speak for the others, but I’ve never even seen inside one - or any model of Loudbox - I’ve never come across one with any kind of fault.

Sorry I can’t be more help...

DJH83004

Like IC unfortunately I have no experience of them either, I have repaired a number of acoustic amps over the years, and they do tend to be surface mount technology / switch mode power supply design due to the compact designs, so not easily repairable I'm afraid. All that said the Fishman maybe completely different - so good luck anyway. 

JCA2550

Thanks chaps, I'm getting a bit impatient with the lack of response from the UK distributor and, fingers crossed hoping someone on TFB knows someone that services, repairs these?

ICBM

This is all I can find about what they're like - this blog shows some pics of the insides.

http://lumberjocks.com/projects/312938

The two important pics -

https://s3.amazonaws.com/vs-lumberjocks.com/or7q4wy.jpg

https://s3.amazonaws.com/vs-lumberjocks.com/or7q5qw.jpg

Which show a conventional linear power supply (with a rather cheap-looking PT) and what appears to be a Class AB power section.

If so it's probably repairable, even without manufacturer-specific parts. The usual suspects would be either the power transistors, rectifier or possibly the PT if it's had a mains spike.

Whereabouts are you based? One of us could look at it and probably tell you within a few minutes if it's likely to be economically repairable.

jpfamps

The only one of these I've seen was simply a broken jack socket.

Looks like it would be fixable without service documents.

JCA2550

You are both gentlemen and proper toffs,
I'm in Tottenham, north London. 

JCA2550

Oh and that Lumberjocks spalted maple build is a thing of beauty.

JCA2550

I suspect the PT got fried, as nothing looks burnt or browned on the circuitboards to the naked eye?

ICBM

JCA2550 said:

I suspect the PT got fried, as nothing looks burnt or browned on the circuitboards to the naked eye?

You can't tell - a shorted transistor or rectifier will usually look normal.

The exact smell of burning would have been quite identifiable to some of us though - transformers smell different from semiconductors .

The PT would actually be the simplest to fix, by the look of it - it's just a single centre-tapped winding on the secondary side (the yellow and blue wires), with a dual-115V primary (brown/blue/black/white), so if you can't get the OEM part any other PT of the right secondary spec will be fine.

Did it 'thump' through the speaker when the second fuse blew? If not it's more likely to be the PT or the rectifier than a transistor. Although the problem with shorted transistors is that sometimes they can take the speaker out as well, which then starts to get expensive...

DJH83004

To be honest IC my money would be on one or more shorted output transistors and then you are down to chasing the fault as we know only too well. It does look like the pre-amp and power module are separate, so could well lend itself to fitting a new MOSFET output module, like the one below, have used them before and they are great bits of kit for amps that otherwise would be uneconomical to repair.
  
https://www.ebay.co.uk/itm/EBB-150-Mosfet-Power-Amplifier-Module-125W-RMS-Audio-Disco-PA/121457218844?epid=682645701&hash=item1c476a151c:m:mTacdPYiR_nhl51hWAaTJXg

JCA2550

"Did it 'thump' through the speaker when the second fuse blew? If not it's more likely to be the PT or the rectifier than a transistor."
No thump, just smell then noticed it wasn't working. 

ICBM

JCA2550 said:

No thump, just smell then noticed it wasn't working. 

Not when it first died - when the *second* fuse blew after you changed it.

Not conclusive either way - as DJH says a blown power transistor (or both) is still the most likely cause, and if both are shorted it wouldn't thump either.

But if it *did* thump, you can probably rule out the PT at least.

JCA2550

jpfamps said:

The only one of these I've seen was simply a broken jack socket.

Looks like it would be fixable without service documents.

Are you still upstairs with Angel Guitars, Denmark Street? I'm tempted to drop it in to see what you could do with it.

Cheers,

C