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Thread: Another GK 800RB With DC Output!

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    Member parentheticalfact's Avatar
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    Unhappy Another GK 800RB With DC Output!

    I know I'm adding to the pile of threads but I figured I should reach out here.
    I have a GK 800RB on the bench that I had rebuild (new stock ON MJ15022G and MJ15023G and new stock MPSA56 and MPSA06 where existing units tested bad). The rebuild had gone well as far as I could tell, biased it as per the service diagram and ran it hard for 30 minutes, both 100w and 300w outputs tested great and clear, no crossover distortion.

    The customer reported that it hummed and didn't work at all when he brought it to his rehearsal space. He brought it back and sure enough it has negative DC voltage on it's outputs (-61vdc 100w and -70vdc 300w respectively) so I checked all transistors and diodes in the output section, checked the power supply voltages while separated from the power amp board, replaced remaining stock .33 resistors, C5 and C12, tested bias trimmers R38 and R14. Transistor and diode tests were performed with an inexpensive ebay MK-328 component tester, this has usually given me the info I need in the past but I am certainly open to hearing that it hasn't given me sufficient data to determine any solid state parts aren't at fault.

    I also checked for any debris in the heatsink holes where the output device leads pass through and cleaned them just in case. Only thing I will mention is silpads were used in the output transistor replacement and the Berquist SP400-0.007-00-92 is hopefully a suitable choice. I've used them in the past on these transistors and if they failed I never heard about it nor have had failures on any of my equipment with them but this is a new shipment.

    Here's some spots that are off voltage wise.

    Pin 2 of U1 is showing 13vdc (!) where .65v is specified, pin 7 of U1 also reads 13vdc, 71vdc +- on the power rails (dropped from 80 something voltages when output board is connected), -71vdc at the center of R38 trimmer, -71vdc at the junction of R40 and R41. Happy to post the rest of the test points as well but thought I'd start there.
    gallien-krueger_800rb_sm.pdf

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    Supporting Member The Dude's Avatar
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    Are your +&-15 volt rails good?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Member parentheticalfact's Avatar
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    Quote Originally Posted by The Dude View Post
    Are your +&-15 volt rails good?
    It appears -15v has broken down. I'll investigate that section of power supply. Odd as I did actually have voltage here before unloaded, now nothing even with no load. Perhaps a short somewhere along the -15 rail.

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    Supporting Member The Dude's Avatar
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    ... or a poor solder connection (open) that reared it's head when the amp was moved around. You can check rail to ground to see if there's a short.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Member parentheticalfact's Avatar
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    D2 in the power supply reads as an 82 ohm resistor. I think we're getting close, but I never celebrate too early these days.

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    Member parentheticalfact's Avatar
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    I'll order these and install, then we'll see where it's at. Much appreciated The Dude! Sometimes it's good to just have someone tell you to go to basics and triple check the power supply.
    There could always be failures down the road from this I'm sure so I'll post once I get that zener back in there.

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    Success!

    Quote Originally Posted by parentheticalfact View Post
    D2 in the power supply reads as an 82 ohm resistor. I think we're getting close, but I never celebrate too early these days.
    Replaced D2 (finally got my mouser order today) and all is well again! Biased per service manual instructions and blasted it for a while and everything is running fine and fairly cool.

    Many thanks!
    g1 and The Dude like this.

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