Berhringer BX1200 Ultrabass - no output

[bigdrums925]

I have a small bass amp that has no output. Fortunately the voltages seem to be correct. The output devices are 2ea LM3886T's and measure +/- 18V on pins 1 and 4.
Behringer+bx1200+Ultrabass.pdf

I get no output to the speaker or headphone jack.
I do get signal through to the preamp out jack.
I see a very small signal on the oscope through the preamp out jack with a signal going into the tape in jacks but only with the power off

The output IC's test good.

Scratching my head on this one.

[bigdrums925]

I've checked most everything I can track down. I am not getting 5v to the preamp board. 1 of the 6 pins does have 2.3 v on it but I can't tell which pin number it is. Looking at page 3 of the schematic, near the center of the drawing, next to R47 it says 27V on pin 5 at D19. Don't know how they can have 27V there when all we have to start with is +/- 18v. Maybe if that 27V were there I would be seeing 5V at the IC's on the preamp board. As of now I can hear a little signal getting through but none of the controls change anything.

Anybody help with this?

[Enzo]

Most Behringer stuff has part numbers silk screened on the boards. The inline connectors usually have pin 1 identified on the board near one end.

Also if you have a connector with ground on it, use your meter to find which pins are grounded, then see the drawing to find which pin numbers are grounds. Usually that tells you which way the pin numbers go.

The 27v is identifying D19 as a 27v zener, at least that is how I read it. And why do they only have 18v to start with? The main power rails would likely be much higher than that. I would wager more like 35v and -35v. I could be wrong. The main output is sampled back through R46 to D19, and that passes through a connector and becomes the signal LIMIT. Leads me to think if the output goes over 27v, it triggers LIMIT. LIMIT could be just a peak light, or it might be some actual limiting circuit. I don't spot where it goes right off.

Page 2 lower left is D9, a 5v zener. Is there 5v across it? If not, is it shorted? Or C31 shorted? And to get 5v acros the zener, you must have V+ atop R14, do you? Is R14 open?

Just on a whim, and especially if the mains fuse was blown at some point. Does this amp have the IEC mains power inlet that includes the little fuse carrier right below the connector. Rectangular thing. And look closely at it if so, does it say on it 120v on one side and 240v on the other? And if any of that seems to be true, any chance the fuse clip is put in with the 240v setting facing the arrow? That would cut all your voltages in half - like the 18v where I expect twice that.

[J M Fahey]

^^^^^^^^^^^^^^ what Enzo says.

Preamp will still work with lowered voltages (hey, Op Amps can work well with puny 9V batteries in pedals) ... and Power amp "should" ... but it has a built-in Mute circuit ... *maybe* extra low rails are not enough to turn amp on

And please repost schematic, maybe changing name or zipping it helps, because it comes out as the infamous "clear.gif" white dot to me

[bigdrums925]

Found the IEC fuse holder in upside down, when reversed resulted in the rails doubling to +/- 36V and a feed of 15V being sent to the preamp board. Across D9 I get 4.9V now.
Definitely is working better yet I still have a 60Hz hum from the speaker out. Nothing much from the input jack with either a signal generator or a guitar plugged in.
But thru the amp in jack I do get a signal to the speaker
Tape in also is now working controlled by the master volume control, which at this point is the only knob that is working

Not certain but I think the feedback is handled by the optocoupler because when I bypassed it previously the hum got louder.

As far as I know the "LIMIT" only affects the limit LED indicator

So the hum and the preamp are what I need to deal with now

I have checked the filter caps. OK

Here's a zip of the schematic:

Behringer+bx1200+Ultrabass.zip

[Enzo]

Well at least we saved you a bunch of "fixing" due to the voltage selector.

The opto is not feedback in a noise sense, it is a limiter for the power stage. Note the output is sampled - same sample line as the LIMIT we discussed - and that sample feds the LED in the opto. If the signal gets too large, the opto lights up, and that lowers its resistance which drags the signal INPUT down. Thus it limits.

So your preamp isn't working.

Does the tuner out work? I see several TL074 ICs in the preamp. Are any of the output pins stuck at voltage (usually +15 or -15)? On the TL074, that is the four corner pins.

[bigdrums925]

I get nothing out of the tuner out jack.
None of the tlo74 corner pins are above +/- 1v except 1 is at 6v

[The Dude]

Check your +&-15V supplies directly out of the regulators (IC1 & IC2).

[Enzo]

I get nothing out of the tuner out jack.
None of the tlo74 corner pins are above +/- 1v except 1 is at 6v

That means there is no signal after the first stage. That narrows it down to two sections of IC7, a couple JFETs, and various connections.

WHich pin of which IC has the 6v?

Are the input jacks plugged into the preamp board? Little three-pin guy. Did we try both input jacks?

Is signal even getting onto the preamp board?

[bigdrums925]

The -15v is absent even directly at IC2. When I remove it, measure -35v on center leg pad. but installed it's only minus 2-3v and headed to zero volts.
IC2 checks good. Even when everything is unplugged except the transformer the -15v is missing.

Plug everything back in and:
the 6v actually measures 7.2v on pin 9 of IC9. sorry not one of the tlo74's

Input jacks are plugged into the preamp board. Tried both input jacks

I think the -15v being absent needs solving first, but I can't find the problem. Something dragging it down I believe.

[Enzo]

Be more clear. IC2 does mot make -15v. OK, but then removed leaves -35v on pin 2. That is normal. My confusion is here: When a 7915 is installed WHAT is -2v? Pin 2 or pin 3?

Yes or course fix the -15v first, no system works without good power supply.

You say IC2 tests good? How did you test it? It sits there with 35v in but no 15 out. That sure sounds like a bad 7915 to me.

Does R6 measure close to 220 ohms? Does the output of the IC2 measure shorted to ground? (As if C4 or D5 were shorted)

[bigdrums925]

IC2 with 7915 installed: pin 2 -3.66v pin 3 -1.65v

I tested it in circuit with a VOM, then removed I have a transistor tester from Radio Shack.

R6 = 218 ohms. C4 & D5 appear to be OK.

Output of IC2 measures 15.5 ohms to ground. I followed it thru X25 to C40 (very difficult to see the traces or find on schematic)

That's all I have for now but will keep looking.

Hopefully once the minus voltage is there everything will start working properly.

[Enzo]

OK, the 7915 is not a transistor, it is an IC. Your meter can find if it is shorted, but cannot test its function. I might test the 7915 out of the circuit and measure resistance from pin 3 to pin 1. If you still get 15 ohms, then the part is bad.

Just for fun, measure resistance to ground of the 7815 to compare. I'd expect them to be similar. 15 ohms sounds real low to me. That would draw 1 amp from the part just sitting there.

[bigdrums925]

IC2 pin 3 to ground measures 15 ohms but its because of a short on the preamp board I believe.
When I pull the cable between the P/S and amp board and preamp the short goes away
None of the other 5 legs of IC1 and IC2 give a reading to ground.
Resistance between legs 1 & 3 are same on both IC1 & IC2
So I have to find the short on the preamp board somehow,
but following the traces and reading this schematic are very difficult.
Anything else you might offer is appreciated

[Enzo]

I'm sorry, I thought you said you had unplugged the amp boards from the power supply. Do you have both 15v rails then when teh connector is off?