[sci.electronics] Massively DC-coupled amp

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*