Behringer BX300T dead amp

[malukaluis]

Hi all, i'm new to the forum and in need of some help.
I have a BX3000T in for repair and it has me completely stumped.
The preamp seems to be working as the VU needle indicates when injected with a signal, However the amp is stone cold dead.
I've checked all the O/P transistors, all fine, voltage regs, all fine -15/+15.
fuse is fine and AC from the secondary of the transformer all fine.
the fan does turn on as soon as you flick the power switch, but at what i assume is an idle speed.

BUT........ i have no voltage across either of the large 4700mfd caps and can't find where it's gone wrong.
So please any help would be greatfully received, schematics even better.

[Jazz P Bass]

Well, the voltage for the 4700uf caps is most probably a separate circuit than the +15 & -15 volt rails.
You will have to trace it back.
High voltage Vac from the transformer secondary, into a bridge rectifier, into the capacitor, is what I would think to look for.

[malukaluis]

I had a look earlier and found 97VAC at the header and to the fuse..... I will trace it tomorrow. Thanks

[malukaluis]

Just a quick one...there is an optoisolator at the back of the board. Would that have any thing to do with turning on the supply to the amp psu?

[Jazz P Bass]

You may be on your own there.
Unless a schematic can be obtained.

[Enzo]

Are you sure this isn;t a BX3000T instead of a BX300T? I have not seen a BX300T nor a listing for it.

The schematic doesn;t even include those main power supplies. I'd imagine you have a winding on the transformer just for that, and a bridge. That should be easy to trace. A lack of the main voltages to the power am pwill certainly make it silent.

[malukaluis]

Sorry Enzo typo on my part It's a BX3000T.

The transformer measures fine I assume it actually runs 40-25-0-25-40 i think.
Are you aware of some sort on power up delay on this model?

[malukaluis]

UPDATE......
I've fixed it just take a look at what i found "UNDER" the power header.


AND HERE IT IS

[Steve Conner]

Well spotted!

[malukaluis]

Thanks i was really pleased.... nearly gave up and certainly started doubting my abilities.

[dporto]

Hmmm, can't see "it"...I've got one of these on my bench now with similar problems (it's a friends amp that he asked me to take a look at). I opened it up yesterday to look for any obvious signs, but didn't see anything right off the bat. I haven't removed the whole heat sink/main board/power section yet, but that's my next move...

[Jazz P Bass]

What 'similar problems' would that be?
The original post listed a few.
No amp output?
No voltage to the main caps?

[dporto]

What 'similar problems' would that be?
The original post listed a few.
No amp output?
No voltage to the main caps?

No amp output! Sorry for the sketchy post...unfortunately it's the best I can do at the moment. My buddy gave me the amp and said "it was working intermittently for a little while and it would come back to life if I pounded my fist on the top, but then it stopped working all together"... Hmmm, no wonder!
So, I haven't even gotten to the point of checking voltages at the main caps yet - They're still buried under the big heat sink. The intermittent part lead me to believe that it may be a cold or broken solder joint so my initial plan was to just retouch all the solder points - clearly I'll have to remove the heatsink and output section before doing so - I just haven't gotten there yet. In the meantime I figured I'd pop in here to do a little "pre-search" and I found this thread. Thanks in advance for any help!

[dporto]

So even though it's been awhile, I figured I'd post my results. Once I got the big heat-sink off and got access to the main board I just re-touched all the solder joints. I did this on the control board(s) as well. I guess I got lucky because the amp has played several dozen gigs with no issues since being repaired.

[Jazz P Bass]

Been there many times friend.
Glad that you got it working.