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Harmon Kardon 720 Power Output Transistors

Last response: in Home Audio
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Anonymous
May 11, 2005 7:07:47 AM

Archived from groups: rec.audio.tech (More info?)

As a summer project with my son, we are restoring a
Harmon Kardon model 720 stereo receiver which he found in a
neighbor's trash.

This is a mid-sixties transistor unit, with a quirky
topology: the output pair is two PNPs transformer fed
from the driver transistor, and the speaker is
direct coupled without a capacitor.

Though the schematic is clear that the outputs are
PNPs (both from the direction of the arrows in the
transistor symbols and the polarity of the power supplies,
negative being connected to the collectors and positive
to the emitters) what's actually installed in our
unit is 2N3055's which are NPNs.

These transistors must be completely fried, if not vaporized.
The funny thing is, some sound does come out, not just noise
but recognizable music & speech. Also, the date codes on
two of the 2N3055's is 1968 and on one is 1972. The 4th
transistor is a plastic package with the leads bent to fit
in a T03 socket. I mention the dates and varieties of devices
because one might guess that the unit was repaired multiple
times and it actually worked for several years in between.
Hard to believe, I admit.

Any idea why someone did this apparently crazy substitution,
and what are the original HK PNPs? The service manual just
gives an HK number (H43020418).

Thanks for your help.

Geno Rice
Anonymous
May 11, 2005 11:50:41 AM

Archived from groups: rec.audio.tech (More info?)

Geno Rice wrote:

> As a summer project with my son, we are restoring a
> Harmon Kardon model 720 stereo receiver which he found in a
> neighbor's trash.
>
> This is a mid-sixties transistor unit, with a quirky
> topology: the output pair is two PNPs transformer fed
> from the driver transistor, and the speaker is
> direct coupled without a capacitor.
>
> Though the schematic is clear that the outputs are
> PNPs (both from the direction of the arrows in the
> transistor symbols and the polarity of the power supplies,
> negative being connected to the collectors and positive
> to the emitters) what's actually installed in our
> unit is 2N3055's which are NPNs.


Easy enough with that topology to rewire the output section
to use NPNs. Why not ask the neighbour about its history?

--
Eiron.
!