Tweet
Actually, I'm sure if I had the patience I could figure it out, or I could just replace *everything*, but I'm trying to avoid that.
Amp was originally delivered to a friend of mine with a couple of shorted output transistors, RCA #68271, in channel B. He replaced the entire set (all ten on that channel) with MJ15024, but the dim bulb test indicated the unit was still shorted so he sent it on to me for further diagnosis.
Eventually discovered that one of the pins of one of the op amp ICs on the driver board essentially wasn't connected at all, as well as one of the predriver transistors blown. Replaced both ICs and their sockets, and both predriver transistors -- Q5 with MJE5730 and Q6 with TIP48. This had the amp working but with intermittent "scratchy" noises (sounded exactly like a dirty pot, but even with the level all the way down). I recognized the sound as a leaky transistor (had a similar problem with a Lafayette receiver decades ago). Followed around with an oscilloscope, determined it to be the "Transient Supressor" (Peavey's spelling) transistor Q1, a P-channel JFET, which I replaced with 2N5461. At this point the amp seemed to be completely working, although still connected with a 150W floodlight in series with the AC line. Both channels produced clean sound.
That said, when the amp is first turned on there is a considerable DC pulse -- not long enough to trigger the triac board, but enough that, if a speaker is connected, it pulls enough current to light the floodlight all the way and prevent the power supply caps from charging up high enough to stabilize the amp and end that pulse. This alarmed me at first, but I happen to have another identical amplifier here that I personally own, so I know they all produce a considerable "pop" noise when the power is turned on. I plugged MY amp (which works fine) into the series bulb and verified that it behaved the same way, so I passed this off as normal behavior. For testing, I power on with no load and then connect the output after the bulb dims down.
So I run it for a couple HOURS, with the bulb in series, everything is fine. Sounds great, heatsinks not even noticeably above room temperature, I'm thinking this is great and I'm done.
Change the speaker wiring around a little bit to try bridge mode. Amp continues to work very well.
Remove the bulb from the circuit - full current now. Unhook the speaker out of habit, power it on, connect the speaker (still in bridge mode). It continues to work fine. Turn it off. There's the pop which I expected. Turn it back on. There's also the pop I expected, but then another pop. I give it a second thinking the channels just didn't come up at the same instant and channel A's pop had just been passed through by channel B which was already up. Wrong answer. Two seconds later there's another pop, then a hiss, then a sizzle and smoke.
Power off (quickly), pull it apart to assess the damage. The first thing I notice is R30, which is two parallel resistors between CR11 and CR12 (the thermal tracking diodes in the bias circuit) is charcoaled. Then I notice the emitter leg is blown off of Q7, which is the positive current limit transistor. So far I've got CR13, CR19, R24, R25, R30, and Q7 gone on the driver board, and R1 (the 100ohm resistor from the positive-side MJ6387 base to the output) on the power board.
I imagine PART of the cascade being caused by R30 going open, which would more or less bias the outputs straight into a short, which is probably what toasted the 100-ohm R1 which happened to be sitting between Q9 trying to drive it to +V and the output that was being pulled to -V by the negative outputs... I suppose a similar path of destruction occurred through R25 and Q7 I can't imagine what took out R24, since CR15 seems to be okay...
... but above all of that... beyond how the cascade failure happened... the reason I'm here... I'd like some input as to what caused the first failure. I haven't seen this one before.
Amp was originally delivered to a friend of mine with a couple of shorted output transistors, RCA #68271, in channel B. He replaced the entire set (all ten on that channel) with MJ15024, but the dim bulb test indicated the unit was still shorted so he sent it on to me for further diagnosis.
Eventually discovered that one of the pins of one of the op amp ICs on the driver board essentially wasn't connected at all, as well as one of the predriver transistors blown. Replaced both ICs and their sockets, and both predriver transistors -- Q5 with MJE5730 and Q6 with TIP48. This had the amp working but with intermittent "scratchy" noises (sounded exactly like a dirty pot, but even with the level all the way down). I recognized the sound as a leaky transistor (had a similar problem with a Lafayette receiver decades ago). Followed around with an oscilloscope, determined it to be the "Transient Supressor" (Peavey's spelling) transistor Q1, a P-channel JFET, which I replaced with 2N5461. At this point the amp seemed to be completely working, although still connected with a 150W floodlight in series with the AC line. Both channels produced clean sound.
That said, when the amp is first turned on there is a considerable DC pulse -- not long enough to trigger the triac board, but enough that, if a speaker is connected, it pulls enough current to light the floodlight all the way and prevent the power supply caps from charging up high enough to stabilize the amp and end that pulse. This alarmed me at first, but I happen to have another identical amplifier here that I personally own, so I know they all produce a considerable "pop" noise when the power is turned on. I plugged MY amp (which works fine) into the series bulb and verified that it behaved the same way, so I passed this off as normal behavior. For testing, I power on with no load and then connect the output after the bulb dims down.
So I run it for a couple HOURS, with the bulb in series, everything is fine. Sounds great, heatsinks not even noticeably above room temperature, I'm thinking this is great and I'm done.
Change the speaker wiring around a little bit to try bridge mode. Amp continues to work very well.
Remove the bulb from the circuit - full current now. Unhook the speaker out of habit, power it on, connect the speaker (still in bridge mode). It continues to work fine. Turn it off. There's the pop which I expected. Turn it back on. There's also the pop I expected, but then another pop. I give it a second thinking the channels just didn't come up at the same instant and channel A's pop had just been passed through by channel B which was already up. Wrong answer. Two seconds later there's another pop, then a hiss, then a sizzle and smoke.
Power off (quickly), pull it apart to assess the damage. The first thing I notice is R30, which is two parallel resistors between CR11 and CR12 (the thermal tracking diodes in the bias circuit) is charcoaled. Then I notice the emitter leg is blown off of Q7, which is the positive current limit transistor. So far I've got CR13, CR19, R24, R25, R30, and Q7 gone on the driver board, and R1 (the 100ohm resistor from the positive-side MJ6387 base to the output) on the power board.
I imagine PART of the cascade being caused by R30 going open, which would more or less bias the outputs straight into a short, which is probably what toasted the 100-ohm R1 which happened to be sitting between Q9 trying to drive it to +V and the output that was being pulled to -V by the negative outputs... I suppose a similar path of destruction occurred through R25 and Q7 I can't imagine what took out R24, since CR15 seems to be okay...
... but above all of that... beyond how the cascade failure happened... the reason I'm here... I'd like some input as to what caused the first failure. I haven't seen this one before.
Comment