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audio bleeding between channels in the power amp stage of a mixer amp

Discussion in 'Troubleshooting and Repair' started by turbobridge, Jul 15, 2013.

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  1. turbobridge

    turbobridge

    4
    0
    Jul 13, 2013
    I am working on a fender mixer amp. I have done a lot of repair work on my own equipment but am not really that good at it. Just look it over for anything that looks bad and such. Usually if it doesn't work at all I can fix it. I have this unit working now but the left and right channels seem to bleed across. If I run music through one channel and pan left to right, at the connector from the pre section to the power amp I tested with a signal tracer, all is well. Pan right only the right wire has sound pan left only sound on left. So I think I have eliminated the mixer preamp section. When the unit is on and music is being played through it panned center, left or right master up, (doesn't matter which but one on and the other off), I have sound out of speakers both left and right. Sound through the side of amp that should be on is louder. Now when I turn up the other side volume goes down over all but both channels are equally loud. I'm thinking volume drops because of Phase cancellation? Long explanation I know, now here is the question. What would be the most common cause. The schematic Shows the two sides of the amp as separate sections and don't share any semiconductors or other parts twins if you will. Only the power supply and speaker jacks out share ground. On the speaker jack pcb there are two caps which appear to be ok. At the input there are several diodes and caps on the shared ground. I suspect these are the points of bleed. Anyone have this problem before and might share some hints. Thanks
     
  2. shrtrnd

    shrtrnd

    3,821
    519
    Jan 15, 2010
    Not familiar with your equipment or what you've done while working on it.
    First thing I'd check, is any 'grounding', or sharing of common grounds in your re-work.
    You may be bleeding from one circuit to another .
    I'd start with what ever work you just did.
    Then I'd look at the common between all of your speaker jacks.
    I would assume you have a more serious failure, a part of whatever problem you originally
    started working on the amp for. But before I did additional troubleshooting, I'd go back
    and re-verify everything that you did up until this point, first.
    I think you're correct to look at your diodes/caps on your input, as I suspect they're the
    most susceptible to damage, secondly.
    It's a lot easier to help with questions like this, when we can see the schematic.
    If you still see a problem, and you can do it, more people can offer suggestions if you
    can post the schematic here.
    Good luck.
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    It is possible that there's a switch or a jumper to "narrow the stereo image"? It could be deliberately selected. I haven't heard of this, but it seems possible...
     
  4. turbobridge

    turbobridge

    4
    0
    Jul 13, 2013
    Thanks for the replies guys. There is a bridge switch but it is on the mixer board and really has to do with routing of the signal through the master section selecting meters and makes the left master the overall volume. but that's all before signal travels to the power amp. So far I haven't done much to it Replaced a Cap And two power resistors that were getting real hot. the unit worked when i got it ( it was free) but not right. First thing I did was hook up two car audio speakers I had laying around because they were handy. The protect circuit kicked in right away. These speakers were 4 ohm each. If i unplugged one it would stay on. Each amp should run run with a Four ohm minimum load. Well I hooked up two pa speakers that were 8 ohm each and it worked fine. Now I'm thinking i should hook up another two 8 ohm speakers in parallel and see if it kicks out at four ohms again. There are two ceramic disc caps on the speaker jack output board 103p I believe. They are hooked the common ground and the other end to the right and left speaker jacks respectively. I don't have a meter that checks caps but the are not shorted but maybe not right either? If those caps were bad maybe that could cause bleed and impedance change?
    The mixer amp I'm working on is a Fender Audio 8500s. I have the schematics on my laptop and if you really wanted to see them I'll figure out how to post them later I have to go to work now though. THANKS for helping me think!!!! Somehow those questions open up new trails to explore.
     
  5. elebish

    elebish

    177
    12
    Aug 16, 2013
    Many amps use "op amp ic's in the pre-amp section. Usually, they use dual op amps to save costs. One of these could be internally or externally shorted or could have some solder flux connecting joining pins if the amp has been worked on lately. Some solder fluxes made today are substitutes for the old types that were used previously and are conductive. Clean that area with alcohol and small brush. Open or bad grounds could also be the problem and you might want to check the dressing and routing of all wiring, especially those that are shielded. Another problem could be a short or bad ground near the rca input jacks that couple audio to the amp. Too much signal at the preamp section could cause bleeding problems also. Ed.
     
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