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Peavey KB100 - overheating power transistor

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  • #16
    Thanks for the reply. There is no measurable DC at either output terminal (P112, P113). I measure about 90 mVAC at P113, which I also measure at C142/C134. When viewed on a scope set for AC, the waveform measured at either cap and at output terminal P113 appears identical. I hope this makes sense. I'm still trying to develop my scope chops, so please bear with me!

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    • #17
      Originally posted by dissonance View Post
      Thanks for the reply. There is no measurable DC at either output terminal (P112, P113). I measure about 90 mVAC at P113, which I also measure at C142/C134. When viewed on a scope set for AC, the waveform measured at either cap and at output terminal P113 appears identical. I hope this makes sense. I'm still trying to develop my scope chops, so please bear with me!
      Did you try testing the speaker and speaker wiring for shorts?

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      • #18
        Solved! (sort of)

        Well, I've managed to find the cause of my troubles on this one, and it's a bit embarrassing. Over on the output board there are two, two-terminal connectors, one of which is the audio output from the main board (P203,204), and the the other goes to the speaker (P201,202). It turns out that you can very easily connect them incorrectly, such that both connectors are turned 90 degrees in relationship to their normal orientation! In retrospect, the fact that it was due to my oversight during reassembly makes perfect sense, since the overheating transistor symptom was only apparent after I had taken things apart in order to replace the broken pots. I had even noticed on the component diagram that the silhouettes of these connectors were oriented differently from how I had them connected, but I just foolishly assumed that the illustration was incorrect. I also assumed that since one of the connectors was polarized, it would not be possible to hook them up incorrectly. Sometimes the most perplexing problems arise from overlooking the obvious. Thanks to Enzo for enduring my crazy questions, and to Bill for the suggestion to check the speaker itself.

        So, now the amp is working, but it seems it may have suffered some damage as a result of my foolishness. When powered on, it produces a fairly low-frequency hum that isn't very loud but still more than normal, and it's overall output volume is considerably lower that it should be. Is it possible that I've damaged Q105 as a result of it overheating?

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        • #19
          The amp has a hum you don't want. Start there. DOn;t prejucie your efforts by trying to link of solder heat. If the trouble turns out to actually be that transistor , then you might be onto something. But really, just troubleshoot it as if you just now laid eyes on it.

          And you are not the first and won;t be the last person I say this to: WHen something doen;t look right, your first reaction should not be that the factory made a mistake or a design flaw. it should be "what am I missing here?"

          Hum? Isolate the problem. Does it remain with all controls at zero? PLug something into the power amp in jack to disconnect the preamp. Still hum? IS there uneven ripple on the two high voltage rails? Are the +/-15v rails to the preamp even and clean?
          Education is what you're left with after you have forgotten what you have learned.

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          • #20
            My description of the "hum" in my last post wasn't entirely accurate. It is actually more like white noise, and it's present as soon as the amp is powered on, with or without anything plugged in to an input. The controls have no effect, even when at zero. In addtion, the amp also has abnormally low output volume. So, I opened this thing back up tonight, with the intention of scoping the power rails again. As I'm grounding my probe on the heatsink near Q101, the 3A fuse pops! Even though I was quite certain that I didn't short anything by accident, I removed my probe from the area completely, and snapped in a replacement fuse. Power the amp on, and it pops the fuse immediately now, every time. At this point, I needed to walk away from the thing for awhile, but first I started poking around with my meter in the hopes of finding a short. I happened to notice that while Q105 has a nice solid ground between it's case and chassis ground, Q101 does not. When I first probe it there is continuity, but it quickly goes open. Subsequent checks still reveal it to be open, but if left for a few minutes and tested again, there is continuity once again for a second or two.

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            • #21
              Q105 has a nice solid ground connection to its case because dissonance has left out the inslating mica washer between Q105 and the heat sink. The cases are the collectors of those TO3 transistors, and if they are grounded, then so it the power supply rail.
              Education is what you're left with after you have forgotten what you have learned.

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              • #22
                Actually, I've never removed/replaced either Q101 or Q105. I had mentioned in an earlier thread that Q105 had been overheating, during the time when I had the two connectors on the output board installed incorrectly. This short is something that seems to have just cropped up. Perhaps the mica washer was compromised by the overheat condition?

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                • #23
                  I doubt heat got to it, but it could be damaged. if you didn;t do that, then I apologize. In any case those cases are not to be grounded.
                  Education is what you're left with after you have forgotten what you have learned.

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                  • #24
                    No problem, Enzo. I appreciate your help. I guess I'll pull Q105 this weekend in order to inspect that washer.

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                    • #25
                      Does your amp have mica washers or those grey silicon pad insulators. The silipads can breakdown from heat and too much mounting pressure. I don't see many of the mica washers fail unless they are cracked from bending.

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                      • #26
                        Or chipped out around the hole.
                        Education is what you're left with after you have forgotten what you have learned.

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                        • #27
                          I finally pulled Q105 off the board this evening, and if my testing procedure is correct, it appears to be fried. The mica washer was present and intact, but the transistor is shorted between the collector and emitter. With my meter in diode test mode, I get a voltage drop of near zero in both directions between C and E. I guess the heat finally did it in.

                          I'll probably call PV for a replacement, unless anyone knows of a better source. The couple of times that I've called them in the past, the customer service seemed really great.

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                          • #28
                            Swapped out Q105, which has stabilized the thing so it isn't popping fuses. The hum issue still remains, however. As I mentioned previously, in addition to the loud hum, the output volume is abnormally low. Something else I've noticed is that it seems to clip very easily.

                            Any suggestions where to go from here?

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                            • #29
                              Originally posted by dissonance View Post
                              Any suggestions where to go from here?
                              Start by checking the power supply voltages as Enzo had suggested. Are they all up to normal levels?

                              Also try injecting the signal into the power amp in jack to isolate the problem: pre-amp or power amp.

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                              • #30
                                Power supply voltages look OK. I'm reading 33.5 VAC per side at P104/106 (red wires), and 18.8 VAC per side at P107/109 (orange wires).

                                Injecting the signal straight into the power amp in does the trick- no hum, normal output.

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