Berhringer BX1200 Ultrabass - no output

[bigdrums925]

First off, the schematic is not correct at least on the preamp/input board, pg 1 of 3. I have found same labeled parts on the schematic that are called out as a completely different value then the silkscreened labels on the board.
For example I chased the 15 ohm short to ground to C63, a surface mount called out on the schematic as a 15n cap. Yet on the board its a 100n cap.
Still other parts on the boards I can't find on any of the schematics
Measuring across the cap C63, I get the same 15 ohms. I will try removing it and see if the short goes away.
Nope still get 15 ohms, will keep looking

When the connector is off between the P/S amp board and the input preamp board the +/- 15V are all over the place even with a ground jumper. I don't know why but I've tried this several times.

[bigdrums925]

First off, the schematic is not correct at least on the preamp/input board, pg 1 of 3. I have found same labeled parts on the schematic that are called out as a completely different value then the silkscreened labels on the board.
For example I chased the 15 ohm short to ground to C63, a surface mount called out on the schematic as a 15n cap. Yet on the board its a 100n cap.
Still other parts on the boards I can't find on any of the schematics
Measuring across the cap C63, I get the same 15 ohms. I will try removing it and see if the short goes away.
Nope still get 15 ohms, will keep looking

When the connector is off between the P/S amp board and the input preamp board the +/- 15V are all over the place even with a ground jumper. I don't know why but I've tried this several times.

[Enzo]

When the connector is off between the P/S amp board and the input preamp board the +/- 15V are all over the place even with a ground jumper.

Help me out here. With the PS to preamp connector removed, what does all over the place mean? 15v appearing where it does not belong? Or you just mean it is now strongly present? And ground jumper? Connecting what to where? To test the supplies all we need to do is unload them with the connector, and see if the 15v appear where expected.

I found C63 in the feedback loop of IC8B, is that where you are looking? That cap wouldn't load down your power rail. The value doesn't matter much anyway. My schematic is revision H. That means this circuit had an original, then revisions A,B,C,D,E,F,and G. A changing part value over revision levels is totally common.

I don't worry so much about finding board parts on the schematic so much as finding schematic parts on the board.

SO you have 15 ohms to ground at C63? I bet you also have it at pin 7 of IC8. If you do, I'd wager IC8 is shorted. IC8 has power pins in the middle of each side, is the 15 ohm short there too? I expect so. Does pulling IC8 free the short?

How good is your meter? Measure exactly the resistance. Fractions matter. If I get 15.1 ohm near one end of the board, and 14.7 ohms on the other end, that 14.7 is closer to the short. SO probe the shorted rail around looking for a small difference. Also measure from a nearby ground point, not just chassis. For instance pin 12 is ground on IC8.

[bigdrums925]

Help me out here. With the PS to preamp connector removed, what does all over the place mean? 15v appearing where it does not belong? Or you just mean it is now strongly present? And ground jumper? Connecting what to where? To test the supplies all we need to do is unload them with the connector, and see if the 15v appear where expected.


All over the place means that both IC1 and IC2 outputs start at 10-12v and drop to less than a volt in about 3 seconds
Ground jumper goes from chassis ground screw to the only screw thru ground solder pad on the P/S.
Yes I am perplexed as to why the +/- 15V is not steady when the 6pin cable to the preamp is disconnected

SO you have 15 ohms to ground at C63? I bet you also have it at pin 7 of IC8. If you do, I'd wager IC8 is shorted. IC8 has power pins in the middle of each side, is the 15 ohm short there too? I expect so. Does pulling IC8 free the short?


This one is difficult to measure, I don't know why. Give me a day or so to sort it out.

How good is your meter? Measure exactly the resistance. Fractions matter. If I get 15.1 ohm near one end of the board, and 14.7 ohms on the other end, that 14.7 is closer to the short. SO probe the shorted rail around looking for a small difference. Also measure from a nearby ground point, not just chassis. For instance pin 12 is ground on IC8.

Difficult to say how good a meter it is, probably a $30-40 unit. More of a problem is getting the probes contacting the contact points well for the meter to settle down. I looked at a data sheet for the TLO74 and it says that pin 12 is non-inverting input 4. Can it be ground?

A pic of the preamp board to show how miniaturized everything is:

[Enzo]

I am familiar with it, I was a Behringer authorized service center for a long time before I retired. On their preamps, I think replacing TL074s was the most common repair. That and broken jacks.

OK< I think I get you. When you power up, the 15v rails start at 12v or so, and after that initial start, drain back down to a low level. IF you have the amp assembly loose from the chassis, you don;t have to ground it as long as your meter is grounded to the circuit "ground" or common.

C3 C5 are your main filter caps, the ground between them is your ultimate ground point. If you can get at it , great, I sometimes ever solder a little piece of wire to an inaccessible point so I can clip a meter to it. It is from that point you want to take readings.

If your 15v regulated supplies are dropping outputs, how about their inputs? The preamp is disconnected, so nothing is loading the supplies, especially if both are afected. I recall we checked R6 to be 220 ohms, and if R5 is about 120 ohms we are OK there, but is the main rail voltage present on them? IN other words when you power up do you get like 25v there for a moment and it drops to little? Or does the regulator input side stay up there?

How about C3 and C5? I don't know what VCC should be, but something like 30v, so does that 30v come up and stay, or do you get a momentary spike at first and then fades away to nothing?

And that all comes from the AC from the transformer. From our circuit ground, measure AC voltage to either side of the transformer, pins 1 and 3 of X2. Does it come on and stay? Probably around 24vAC. And pin 2 is the center tap, verify with ohm meter while power off that pin 2 has continuity to the circuit ground.

[J M Fahey]

Ouch, many **little¨** thingies in there.
Definitely very good light+good eyes+good lenses needed.

That said, please divide and conquer.

When Enzo said

Do you have both 15v rails then when teh connector is off?

he means disconnect PSU from preamp under test, and measure at the PSU end. to check whether now proper value 15V rails reappear or not.

The idea is to check whether they had disappeared because of preamp short overload or misbehaving regulators, two very different things.

this:

the schematic is not correct at least on the preamp/input board, pg 1 of 3. I have found same labeled parts on the schematic that are called out as a completely different value then the silkscreened labels on the board.
For example I chased the 15 ohm short to ground to C63, a surface mount called out on the schematic as a 15n cap. Yet on the board its a 100n cap.
Still other parts on the boards I can't find on any of the schematics
Measuring across the cap C63, I get the same 15 ohms. I will try removing it and see if the short goes away.
Nope still get 15 ohms, will keep looking

When the connector is off between the P/S amp board and the input preamp board the +/- 15V are all over the place even with a ground jumper.

is NOT the answer.

[Jazz P Bass]

Here is the 333 schematic: 333.zip

[Enzo]

Hey Jazz P. Buddy, I think you are mixing the 333 and BX1200 threads. Get some coffee.

[Jazz P Bass]

Allergies.

Never had them.

Now I do.

[bigdrums925]

I now have the +/- 15v at the regulators without the 6 pin cable connected.
I pulled IC8 but still don't have the -15V at the regulator with the cable connected
Unplugging the 6 pin cable between the P/S and preamp I have -15V at the regulator

Hope its not another of the TLO74's, very difficult to get them unsoldered.

[Enzo]

There is a product called SOlder Quik, look it up, it makes removing the small parts much easier.

[pontiacpete]

Chip Quik -

[Enzo]

Oh poo, yes Chip Quik, of course. I had a senior moment, sorry.

The stuff works great.

[bigdrums925]

I lifted the negative 15v leg on IC7 and the -15v is back at the regulator
I assume that means the TLO74 is bad.
Thank you Enzo for pointing out these short out on these amps
I'll advise if and when I get it all put back together and working
I wish I had only pulled the -15v leg on IC8. Might have saved me some seriously small detail work.

[oc disorder]

First off, the schematic is not correct at least on the preamp/input board, pg 1 of 3. I have found same labeled parts on the schematic that are called out as a completely different value then the silkscreened labels on the board.
For example I chased the 15 ohm short to ground to C63, a surface mount called out on the schematic as a 15n cap. Yet on the board its a 100n cap.

Bit late to the party but.... eon's ago before there was a big warehouse fire here in Melb I contacted Behringer for a schematic and they sent me all the revisions they had and revision C lists C63 as a 10n.
It's at the input jacks going to the positive input of IC7D.
I didn't have to sign any non disclosures !

After that they started sending me unsolicited spares and no invoice !

Well it was a gift horse ... the agency changed after that and the gifts dried up!

So hereBx1200 Rev C,D,E,F,G.zip

Left out Rev H as it seems to be available.

BTW seems Behringer are no longer on ElectroTanya!