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1970s Peavey Bass 300 - popping/shorting when load connected

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  • #16
    Originally posted by Drummer4gc View Post
    When the speaker pops, it corresponds with significant intermittent DC on the speaker out, up to about -18VDC. Back down to 0v or close between pops. No fuses blowing. Couldn’t get any reliable difference with freeze spray, although I did notice some of the power transistors getting warm (not hot).

    could one or more of the power transistors be breaking down? Worth replacing them all, or is there more zeroing in here?
    I'd like to tell you checks to make, but it's a little tricky to talk about due to the schematic. There are no component designations to refer to, and there are a bunch of errors in the voltages listed on the bottom half of the power amp. I'm attaching a clearer copy.

    Because it seems to be ok with no load, I think the output transistors are more likely the issue.
    The 4 power transistors furthest to the right are the outputs. The other 2 of the same type are the drivers. Not sure if they are socketed or soldered in. For test purposes you could remove a pair at a time and apply the load to try and isolate if any of them are bad.
    So you would remove 1 output from the positive side and one from the negative. If no change, put them back and remove the other pair.

    Attached Files
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    • #17
      Don't forget always check all the resistors associated with those power transistors. An open one can create havoc.
      Education is what you're left with after you have forgotten what you have learned.

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      • #18
        Finally got to try a few of these suggestions out.

        -replaced the power transistors one pair at a time until all 6 were replaced (with unused mj15003 that I had in my drawer). No difference.

        -all the cement 5w resistors measure properly in circuit, none are open.

        -I removed the power amp input capacitor to isolate any signal from the preamp. No difference.

        thoughts on a next step?

        thanks,
        matt

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        • #19
          Bump. Any other ideas?

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          • #20
            I would say move up to the next pair ahead of the output devices, but it's going to be tough subbing out those 40409 & 40410's.
            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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            • #21
              Actually I have a couple bags of those in storage, NOS. Most still in the original RCA boxes.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                So, fired it up again to see where we’re at, did some testing, here are the details:

                -Plugged in with no load, no bulb limiter, and fuse immediately blew. Replaced fuse, added limiter, tried again - bright glowing bulb. Moved the circuit board around a little (it’s unscrewed and balancing on a standoff) and turned it on again, got a stable dim bulb. Must’ve been something shorting.

                -Plugged into a cheap 4 ohm speaker, no bulb limiter, and decided to let it run for a bit and see what happens. Got some crackling right away, not too much, so just let it go. Crackling came and went, came and went, for probably five minutes. Then, big crackles and sudden constant DC buzz on speaker, cone extended (or sucked in, can’t remember). Sprayed freeze spray on the output and driver transistors during this time, no fix. Let it go long enough to start smelling the speaker coil, turned it off.

                -Pulled all six of the output trannys I had replaced, checked them one by one in a tester, all checked out - none had blown. Put back the original trannys which has all also tested fine when I pulled them, as previously mentioned.

                -Pulled the 40409 and 40410, each tested fine. Put them back.

                -Current state - negative DC on output regardless of if load is attached out not. About -22vdc with no load, about -8vdc with a 4ohm load.

                I hate this thing. Don’t let me give up.

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                • #23
                  Never connect a speaker to a SS amp that might have DCV at its output. Speakers can tolerate only little DC power. If your voice coil started smelling, there might already be some damage.
                  Troubleshooting doesn't require a speaker. And running the amp with a speaker or dummy load may take out even more components.

                  If you really need to listen to the output, wire a 100R/10W min. resistor in series with the speaker.
                  - Own Opinions Only -

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                  • #24
                    Yeah I hear ya. Thanks for the 10w resistor tip. It’s a speaker I don’t care about, and honestly I was hoping that if this issue was being caused by some intermittent component as has been suggested, leaving it in failure mode for 15-20 seconds would be enough to let the part fry completely and make the repair obvious from there. Not the most delicate solution for sure, but I’m feeling stuck. And far as I can tell, the output devices and drivers still all seem to have the magic smoke inside!

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