Results 1 to 31 of 31
Like Tree7Likes
  • 2 Post By Enzo
  • 1 Post By Enzo
  • 1 Post By Jazz P Bass
  • 1 Post By Enzo
  • 1 Post By Jazz P Bass
  • 1 Post By oc disorder

Thread: Berhringer BX1200 Ultrabass - no output

  1. #1
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156

    Berhringer BX1200 Ultrabass - no output

    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.

  2. #2
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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?

  3. #3
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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 and nevetslab like this.
    Education is what you're left with after you have forgotten what you have learned.

  4. #4
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    9,321
    ^^^^^^^^^^^^^^ 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
    Juan Manuel Fahey

  5. #5
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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

  6. #6
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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.
    Education is what you're left with after you have forgotten what you have learned.

  7. #7
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    I get nothing out of the tuner out jack.
    None of the tlo74 corner pins are above +/- 1v except 1 is at 6v

  8. #8
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    4,439
    Check your +&-15V supplies directly out of the regulators (IC1 & IC2).
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  9. #9
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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?
    Education is what you're left with after you have forgotten what you have learned.

  10. #10
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  11. #11
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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)
    Education is what you're left with after you have forgotten what you have learned.

  12. #12
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  13. #13
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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.
    J M Fahey likes this.
    Education is what you're left with after you have forgotten what you have learned.

  14. #14
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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

  15. #15
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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?
    Education is what you're left with after you have forgotten what you have learned.

  16. #16
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  17. #17
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  18. #18
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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.
    Education is what you're left with after you have forgotten what you have learned.

  19. #19
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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:
    img_0448.jpg

  20. #20
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    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.
    Education is what you're left with after you have forgotten what you have learned.

  21. #21
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    9,321
    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.
    Juan Manuel Fahey

  22. #22
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    11,860
    Here is the 333 schematic: 333.zip
    J M Fahey likes this.

  23. #23
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    Hey Jazz P. Buddy, I think you are mixing the 333 and BX1200 threads. Get some coffee.
    J M Fahey likes this.
    Education is what you're left with after you have forgotten what you have learned.

  24. #24
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    11,860
    Allergies.

    Never had them.

    Now I do.
    J M Fahey likes this.

  25. #25
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  26. #26
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    There is a product called SOlder Quik, look it up, it makes removing the small parts much easier.
    Education is what you're left with after you have forgotten what you have learned.

  27. #27
    Supporting Member
    Join Date
    Apr 2009
    Location
    NY, NY
    Posts
    1,084

  28. #28
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    28,460
    Oh poo, yes Chip Quik, of course. I had a senior moment, sorry.

    The stuff works great.
    Education is what you're left with after you have forgotten what you have learned.

  29. #29
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    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.

  30. #30
    Old Timer oc disorder's Avatar
    Join Date
    Jul 2006
    Location
    Land Down Under
    Posts
    1,264
    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!
    Last edited by oc disorder; 12-06-2017 at 10:10 PM.
    Jazz P Bass likes this.

  31. #31
    Senior Member
    Join Date
    Sep 2015
    Location
    Livermore, CA
    Posts
    156
    Thanks OC!

    The amp is now working thanks! especially Enzo
    It is curious though that when I use metal screws to attach the amp assembly to the chassis it won't turn on.
    I'll just use insulators

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Replies: 6
    Last Post: 10-22-2013, 12:11 AM
  2. help need on bx1200
    By selmax in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 03-05-2013, 03:01 AM
  3. need Behringer BX1200 Schematics
    By Odie in forum Schematic Requests
    Replies: 6
    Last Post: 11-10-2010, 12:55 AM
  4. Behringer BX1200 - no sound :(
    By terrapin in forum Maintenance, Troubleshooting & Repair
    Replies: 10
    Last Post: 10-26-2010, 12:06 AM
  5. Behringer BX1200 low Output
    By oc disorder in forum Maintenance, Troubleshooting & Repair
    Replies: 14
    Last Post: 08-05-2009, 05:55 AM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •