Help with my Behringer Ultracoustic ACX900...

[morgdan]

Someone just gave me a Behringer ACX900. It was free...for a reason.

It has very low, distorted output. This amp has two channels and the problem seems to be in both channels. All of the bells and whistles seem to be working (EQ, effects, etc).

Is this a preamp problem? Is there an easy way to isolate the problem?

Thanks!

[phatt]

Does it have a EFX send return setup?
Or preamp out poweramp input sockets?

If so,,, take a lead from either of these outputs and run into another *Working Amplifier* and if all sounds fine then the power amp section is the problem.

Phil.

[morgdan]

Thanks, there is an effects loop section and I'll try that. Can I assume then, that if it still sounds crappy that it's in the preamp portion then?

[morgdan]

Ok, so I ran my guitar through another pre-amp and into the effects return and it sounds the same, so I guess that means it's the power amp circuit?

Any ideas on whether or not it's worth trying to chase this down on a relatively cheap amp?

[J M Fahey]

Any ideas on whether or not it's worth trying to chase this down on a relatively cheap amp?

It's up to you.
If I had it on my bench, I would certainly give it a try, because I would either be able to repair it in less than an hour, spending less than, say, $30 in parts, *or* I would find that it takes much more than that and it doesn't pay out ... but I would need to have it in front of my eyes.
If you have certain experience and repaired similar complexity items before, good, we can help you; if not so, and considering Behringer does *not* publish schematics and most of their stuff uses machine placed microparts ... leave it alone.
There is a 3rd option: if you are into experimenting and building amps: clean its guts out, now you have a nice cabinet, speakers, (now) empty chassis , power transformer, plus minor but useful parts as power switch, fuse, cord, etc.
You can build there a nice, useful, simple amp such as Rod Elliott's, or a chipamp with some of ROG projects as preamp, or at least a "powered box" to boost any other guitar amp you already have.
You definitely have no junk there !!.

[morgdan]

Thanks, that's great info! I'll put my mind to it.

[Enzo]

Well, before we tear it apart...

Check the speaker.  What does the amp sound like through a different speaker?  ANd play some working amp through its speakers.

ANd let's not assume anything, play into the front of the amp and send the FX send to some other amp.  Does the preamp sound OK?  If the preamp also sounds crappy, we may have a power problem.

Try the CD inputs, how's that sound?

The power amp is just a pair of TDA2050 ICs, one per speaker, simple enough.  Check the power supply, and the 2050s getting BOTH power rails?  The power amp also makes +/-12v for the preamp as well as +5v for the digital stuff.  A collapsed 12v could do this.  I don;t know what the power amp voltages are, probably 20-25v.  A missing one would also kill the 12v for that polarity.

[morgdan]

Thank you. I will run all the different possibilities. That last paragraph might be a little out of my league, but I'm sure I can find someone who can help me understand what you said there if it comes to that. You give me hope that this might be quite fixable.

[bry melvin]

in short...the power amp is actually two chip amps one for each speaker. They are low cost ICs that have a tab that bolts to a heat sink usually with an electrically insulating washer and five pins soldered to a board. The chips plus installation kit (washer insulator and screw and nut) should cost less than 10USD from somewhere like Mouser.

Cheap enough to go ahead find them change them and see if it works, without diagnostic tools etc. You just carfeully disassemble enough to get to the chips replace, then reassemble.

[Enzo]

But since ther are two, one per speaker, I am not quick to decide both have the same distortion problem at the same time.  I worry about some common element like power or maybe an op amp upstream.