Behringer ACX1000 schematic and/or info

[cattattac]

I am working on a Behringer Ultraacoustic ACX1000. One channel is normal. On the other , the level is down and has some unwanted distortion. I suspect capacitors. Does anyone have a schematic or has encountered this problem before?

Thanx...Cattatac

[Enzo]

Why do you suspect capacitors? In my experience, it would be way more likely to find a bad op amp. In fact, I usually go down the row of op amps and just check each output pin for unwanted DC offset.

[Steve L]

Schematic

I don't have an ACX1000 schematic, however this one may be close.

Steve

Behringer ACX1800 Ultracoustic Amp Schematics.pdf

[cattattac]

I was going on my experience with the vintage amps I usually work on, where a leaky cap will put excessive DC on the input of the next stage,causing distortion and/or volume loss. This is the 1st "newer" amp I have cracked open. I guess a defective chip could do the same thing.............Thanx, I wll start checking voltages 1st thing Mon. The schematic from Steve below should help.

[Bill Zucker]

Enzo, thanks for the schematic. Not close enough to pin it down, but it's a good start. Last year Behringer sent me the schematic for an Ultrabass amp after returning a release form, but not this time (after repeated requests).

Got the same problem as cattattac: INSTRUMENT channel really low and distorted. Appears the clip circuit up front is seeing good signal levels, but by the time it gets to the "INSTR SEND" it's corrupted.
Unfortunately I can't see any more than a few millivolts DC offset on any of the op amp outputs. One at 20mv, one at 36mv, but nothing approaching volts.

Any thoughts? Seems like without the schematic there's no good way to proceed...

Bill

[Enzo]

Thank Steve for the drawing.

The mic channel is OK? So it works, and thus the power amp and speaker work? SO the problem is ONLY the instrument channel? The instrument channel has an FX loop, yes? Plug a cord from FX send to FX return, a sort of external bypass, does that wake it up? Plug the signal into the FX return there. Does that come out? Plug into the regular instrument input, then run a cord from teh FX send to some other amp. Does it come out OK over there?

Is the DI output clear and strong or is it similarly bad?

Does pushing the phase button make a difference?

Is the tuner out OK?

Path: IC6d-14, IC6a-1 (gain control), TMB tone stack, IC6b-7, (attack control), IC7a,b channel out. Trace signal along.

The clip light sees signal from two places. Output of IC7 and also from IC6a, they are OR-ed together.


Since the whole channel is two ICs, it should be compact. The DC test is just one test done quickly. If it isn;t that, move on.

[Bill Zucker]

Yup, everything's okay except the greatly reduced volume and seemingly attenuated bass freqs from the INSTR channel only. Other channel's fine. I have to set the volume on the INSTR chan to 5 to get the same level as from the MIC/INSTR chan on 1 or 2. Trying to get sufficient volume on the INSTR chan causes the clip light to fire as expected and serious distortion. Because both channels clip at roughly the same channel volume control setting, I'm thinking the first op amp at least is functioning okay. Looping through the effects send/return makes no difference - the signal at that point is already corrupted (evidenced when SEND is fed to another amp). Other controls appear to work as expected.

Already bought several each of the two op amps implicated. Little leary of swapping out SMT stuff though - this will be my first. If these were DIPs I'd already be done...

Enzo, many thanks for the signal flow info - I was running out of options. Bought a Hantek USB o-scope for the laptop awhile back - be nice to use it in earnest. Can you recommend a cheap signal generator to use for this project? Something at Radio Shack or Home Depot?

Because the signal on the INSTR chan is not that distorted when set below the clip level, just mostly attenuated, I'm wondering if I'll be able to find a smoking gun. Hopefully I'll find a corrupted waveform. But I'm thinking it would help to get the equivalent test points for the other channel to compare signal levels. Would you be able to help with those also, Enzo (ol' buddy ol' pal)?

Bill

PS - After many years of bringing up all sorts of issues on various forums, I think this is the FIRST TIME I've not had to answer my questions myself. How cool!

[Enzo]

A cheap signal generator? Sure. Whatever you already have. I only reach for an actual signal generator for specific tests. In general I just use a line out from my cheap bench stereo receiver tuned to a music station. Or use a CD player or something. Just stick some music into an amp. It will be a useful test signal, and easily heard as clear or distorted, and full range or limited freq, etc. And I have to think you have some sort of audio signals nearby like those.

If none of the four corners of that TL074 have DC, so be it, but i still suspect the IC. It is mostly what that whole channel is. Follow thge signal stage by stage, just listen to it, you ought to be able to hear wher it fails.


SMT is a pain yes, but get used to is, it won;t be going away. get yourself a small Chip Quick kit, it works.

[Bill Zucker]

So as for using another amp for a signal generator / test probe:

Are you saying I could take the output from an mp3 player (line level but possibly low impedance) and connect it straight to the input (in this case, the In- and "A" ground) of each successive op amp? Any risk of damaging something (like the player)? Need a true line level output from something with higher impedance?

Or are you suggesting I feed something into the ACX1000 and then directly connect the INSTR input of some other amp to the output of each op amp?

Bill

[Enzo]

Just feed a signal into the regular input for the channel. Impedance schmimpedance, any signal that works in the other channel will work in this one too. These are high Z inputs, all they care about is signal voltage anyway. Then just follow the signal stage by stage. No need to move the injection point around. Scope or signal tracer. A signal tracer is indeed just the input of some other amp, but we add a series cap to block any DC that might be present.

[Bill Zucker]

I'm still pondering the repair. Have only needed to use one channel so far, so if I destroy the amplfier trying to replace the SMT TL074C (or a series of them), I'd be pretty disappointed with myself.

Here's another clue though: Awhile back, after I repaired the output section, but without the small white connector to the input jacks plugged in, I noticed that I could turn the gain knob all the way up on the good MIC/INSTR channel with no ill effect, but when I reached the high end (maybe 7 or so) on the defective INSTR channel, there was high pitched feedback from the speakers. Probably should have mentioned this before.

Without the schematics I'm again at a loss. Any ideas?

Bill

[Zac Ross]

I'm still pondering the repair. Have only needed to use one channel so far, so if I destroy the amplfier trying to replace the SMT TL074C (or a series of them), I'd be pretty disappointed with myself.

Here's another clue though: Awhile back, after I repaired the output section, but without the small white connector to the input jacks plugged in, I noticed that I could turn the gain knob all the way up on the good MIC/INSTR channel with no ill effect, but when I reached the high end (maybe 7 or so) on the defective INSTR channel, there was high pitched feedback from the speakers. Probably should have mentioned this before.

Without the schematics I'm again at a loss. Any ideas?

Bill

Dear Bill,

You may contact our technical support team and parts department for any questions or troubleshooting. You can contact us by phone at (1) 702-800-8290 for the US and Canada and (44) 1562 732290 in Europe.
You can also contact our CARE department via email at care@music-group.com. We try to respond to every email within 24-48 hours, but please be patient if it takes longer due to the volume of emails we receive.

You may also contact an authorized service center for any inquiries or repairs:

http://www.behringer.com/EN/Support/...formation.aspx

Kind regards,

Zac Ross
Jr. Admin, Tech Support
MUSIC Group

[Bill Zucker]

Well, found the problem. Wasn't IC6, but IC7, a dual channel op amp 4580 chip. Now if anything, the INSTR channel is hotter than the MIC/INSTR channel.

Learned how to remove SMD's in the process. Behringer glues the chip down, so you can't just lift the chip as you heat the leads up or push them aside without risking pulling off the pads. I found the best way was to Dremel the leads apart at the chip body with a thin grinding wheel, wedge the chip body off with a pair of dikes, and then slide the remainder of the leads off the pads one by one. SMD's don't scare me anymore.

Enzo and Steve, thanks for the support, but thumbs down to Behringer for the lack of schematic support based on lame reasons. This will be the last Behringer product I'll care to deal with.

Bill

[Enzo]

Get yourself some Chip QUick for smt removal. Don't try to lift individual chip legs or try to rely on the solder being melted all around.

[Enzo]

Sorry... "Chip Quik."