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Gallien Krueger 800rb repair - No power

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  • #61
    I've retested every output transistor out of circuit and they all test ok.

    I've removed the other transistors mentioned: Q9, Q7, Q2. Q15, Q16. etc and they all test good OUT of the circuit. But they don't in the circuit. This is with the board not connected to the PS board or anything else. Should that be the case?

    I removed the metal transistor mounting plates and cleaned them.

    I had also read about an issue with the 800RBs shutting off do to an issue with the effects loop and inserting a patch cable can fix the issue. Could that be the culprit here?

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    • #62
      Transistors can test OK with a DMM or tester and fail in circuit, as the testers use very small voltages (maybe 5v). The saying I've seen is that a tester can tell you if a part is bad, but not that it is good.

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      • #63
        I think you are talking about testing the transistors with diode check in circuit but unpowered.
        If so, you can not expect them to test the same as out of circuit.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #64
          Originally posted by g1 View Post
          I think you are talking about testing the transistors with diode check in circuit but unpowered.
          If so, you can not expect them to test the same as out of circuit.
          I removed each output transistor from the circuit board and tested it with the diode check that way.

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          • #65
            Yes. But when you put them back into the board, they will no longer give the same readings on diode check. That is normal.

            To see what they are doing in circuit, you really need to power up the board and do DC voltage measurements.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


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            • #66
              What DC voltages should I see if the transistors are good? Do I need to do the same combination of measurements as in diode check?

              That is: B to E, B to C, E to B, etc.

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              • #67
                For the voltage measurements you measure each pin to ground. So leave black probe connected to ground and check at E,B, and C.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #68
                  Originally posted by g1 View Post
                  For the voltage measurements you measure each pin to ground. So leave black probe connected to ground and check at E,B, and C.

                  Thanks. Yes, I understand that, but what voltages should I see on the DMM that would indicate normal or abnormal voltages?

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                  • #69
                    That will be different for each application. That's why it's easiest to post them so they can be analyzed here.
                    The only general statement I can make about 'normal' is that there will be around .5 to .7VDC between emitter and base for a transistor that is turned on. Polarity will depend on whether the transistor is NPN or PNP. And for darlingtons it will be double that value.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


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                    • #70
                      I had new transistors I bought and waited to install. Still getting a short - power board caps are shorted when everything is connected.

                      Can someone explain to me how this amp is supposed to work with the Collector of the output transistors being tied to ground and the Positive and Negative power rails also being in contact with some of the Collectors? It all connects together with the mounting plate for the chassis.

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                      • #71
                        Did you remember the insulators under the transistors?
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #72
                          I replaced all the insulators. Now getting shorts on 85- side. The 60w side hooked up doesn't trigger the light limiter.


                          EDIT*** I found one of the smaller brass transistor heatsinks was touching the through-hole bolt. I added a insulator I got for the smaller transistors and that fixed the issue.

                          Amp is now running without short and not tripping the limiter. I plugged it straight into the wall and it's not blowing the fuse - voltages are good on the power board.

                          The power indicator LEDs are not lighting up though. I only have a small crappy squier amp with likely a blown speaker at my shop. I hooked up the amp to test with that and I'm not getting any signal coming out.
                          Last edited by beedoola; 01-01-2020, 11:59 PM.

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                          • #73
                            Originally posted by beedoola View Post
                            The power indicator LEDs are not lighting up though.
                            Are the +/-15V supplies both working?
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #74
                              They weren't. I went and got some replacement diodes for D1/D2 and also replaced some resistors.

                              Got amp to work 100%.

                              Though I forgot to bias using the trimmers for the Hi-Amp. I couldn't get the 5MV across the stated resistors in the service manual adjusting the trimmer - was able to for the Lo-Amp. Amp blew a fuse and now I'm getting a short again with the light bulb limiter back in the circuit. Blew Q-14. I don't have a replacement tranny so I'll order some and see if I can track down the short - the filter caps are all shorting to ground again.

                              I tested the power transistor Collectors to make sure it wasn't them again - that one of the insulators was off - and it wasn't. I also tested the various voltage leads from the power board to ground and it's not them so I'll have to poke around some more.

                              So close!!

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                              • #75
                                Generally, if an amp won't bias, there's still a problem and you shouldn't continue to run it. It's time to shut down and continue hunting. Lack of bias is an indication of a problem. Check any associated resistors when you have a shorted transistor. A shorted transistor will often burn open a resistor.
                                "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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