amplifier stage designed for a magnetic
phono cartridge, tone controls, and quasi-complimentary output stage with a differential
pair input to help keep distortion down.
This one was fairly dirty inside and the
2SC1030 output transistors were badly
corroded, but it survived in this condition for
quite a while. This was a good example of
Harman/Kardon not messing with a good
thing, too: The phono amplifier board and
final board show up in the model 430
receiver, and likely many others from around
the same time.
Except for the phono board, everything
can be re-worked without even removing it
from the chassis which made this one really
easy. The first task was to shotgun replace all
of the electrolytic capacitors on every circuit
board. These capacitors start to dry out or leak with age —
without fail — and can cause all kinds of havoc in the
circuit. Without known good capacitors in place, there's
no point in doing any other work. I replaced all the small
capacitors with Nichicon Fine Gold audio-grade capacitors
— all de-rated for longevity — and Panasonic snap-in
capacitors for the main power supply.
It's tough to find exact replacements for some of the
components used in these vintage amplifiers — especially
filter capacitors. It's easy to find something that's
July 2015 47
Partial schematic of the final amplifier board.
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A quasi-complimentary output stage is a type
of push-pull output stage made from two identical
NPN transistors connected between the supply
rails, in contrast to a fully complimentary output
stage which uses a mirrored pair of a PNP and NPN
transistors. Quasi-complimentary designs were
popular in the early days of transistor amplifiers,
when NPN power transistors were easier to
manufacture than PNP power transistors, but
continued to be found in lower powered and entry
level amplifiers for many years. They're almost
never used in modern construction as NPN and
PNP power devices are equally available.
This phono amplifier PCB shows up in several other
H/K amplifiers and receivers.
The tone and phono boards showing new Nichicon
Fine Gold audio-grade electrolytic capacitors.