This is a new problem for me on these. The amp came in with fuse blown of the PS nearest the power terminals and speaker plug. Without that fuse, the amp powers and will likely play on the surviving PS2. With a new fuse in PS1 the amp draws excessively and those PS fets get warm fairly quick.
I removed the board from the sync. PS1 fets are all testing OK but Im not really sure what's going on. Drive signal to the FETs is about 1/4 the strength of PS2. I removed the MPSA06/56 drivers from PS1 and the amp powers up on PS2 but without PS1 operating. It would seem that Im not seeing enough voltage on the NPN collectors of the PS1 drivers, but not sure. I removed the 2n6488 which feeds the NPN collectors and that part is testing ok. Its hard to tell what and why there is a problem here and Im scratching my head a bit.
Usually I find these amps with completely blown PSs but not this one. Its possible the PS Fets really have failed but I dont want to risk removing them all.
Rectifiers are testing OK in circuit. The transformer could be shorted but Im not really sure. I measure a few hundred ohms from primary to secondary just like PS2.
Thoughts?
I removed the board from the sync. PS1 fets are all testing OK but Im not really sure what's going on. Drive signal to the FETs is about 1/4 the strength of PS2. I removed the MPSA06/56 drivers from PS1 and the amp powers up on PS2 but without PS1 operating. It would seem that Im not seeing enough voltage on the NPN collectors of the PS1 drivers, but not sure. I removed the 2n6488 which feeds the NPN collectors and that part is testing ok. Its hard to tell what and why there is a problem here and Im scratching my head a bit.
Usually I find these amps with completely blown PSs but not this one. Its possible the PS Fets really have failed but I dont want to risk removing them all.
Rectifiers are testing OK in circuit. The transformer could be shorted but Im not really sure. I measure a few hundred ohms from primary to secondary just like PS2.
Thoughts?
With the 6488 out of the circuit, what's the voltage on the NPN driver transistor collector?
DCV on the base of the (missing) 6488?
Power up just for a few seconds to test because the rail voltage may go too high without the 6488.
DCV on the base of the (missing) 6488?
Power up just for a few seconds to test because the rail voltage may go too high without the 6488.
I removed PS1's rectifiers and both PS1 and PS2 seem to power up OK.
I realize this amp has each power supply going to separate channels. I think there may be something wrong with PS1's channel, though none of the TO220 nor the drivers are reading problematic. I did find one output transistor mis-matching and I removed it but it measures ok. Not fixed.
Im thinking of removing all the outputs and test with RECTs in circuit now.
I realize this amp has each power supply going to separate channels. I think there may be something wrong with PS1's channel, though none of the TO220 nor the drivers are reading problematic. I did find one output transistor mis-matching and I removed it but it measures ok. Not fixed.
Im thinking of removing all the outputs and test with RECTs in circuit now.
In the end, a set of 07/57 drivers for the affected channel were failed reading about 700ohms across 2 legs. Also the channel did have some signs of over-heating that I could not tell until I removed the 6488/6491 BJTs. Amp is on its way to restoration now.
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