hobbit@topaz.rutgers.edu (*Hobbit*) (01/22/89)
Well, I managed to replace those mysterious "Annnn" and "Cnnnn" transistors in this amplifier I was playing with, and it's *still* trying to eat the finals. This is a Fisher CA-65 with one channel gone south, and it's one of those crazy split-power-supply push-pull circuits in which if one small piece breaks the whole thing gleefully smokes itself. Found dead were: The NPN/PNP output driver tree, the .22 ohm emitter resistors for same, the 300 ohm resistor between the bases of same, *both* pre-drivers for these, the diode/trimpot base-bias thing for *them* ... in other words, everything. The input network consists of three more small transistors in some weird back-to-back configuration I haven't tried to diagram out yet, but basically incoming audio is common-ground-referenced and its positive swings "switch" in the positive drivers, and negative swings essentially allow a resistor connected to the negative rail to take over and swing the tree negative. *No*where in this entire arrangement is there *any* capacitive coupling! *Everything* must be working, and biased correctly, or the whole thing swings solid one way or the other and either blows your speaker or itself. So, I replaced all the above-mentioned busted parts, and everything else tested okay, and the output still wants to sit at +50 volts or so. [I "saved" it for the moment by leaving the final emitters disconnected -- I know this is safe to do because I did it to the other channel with no ill effects]. At this point, voltages compare roughly equally between the two channels. But I'm completely stumped by this silly thing; I'd *like* to fix it because it's quite probably a nice amp, and I have no clue where to look next in it. Fisher wants $15 for their service book, which is more than I paid for the amp. Any ideas??? Leaky electrolytics? Magic? _H*