Harmon Kardon 720 Power Output Transistors

[Geno Rice]

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

[Schizoid Man]

"Geno Rice" > wrote in message

> 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.

I have to ask this before anyone else does.

What was your son doing looking in your neighbor's trash?

[severian]

Interesting, I have no idea why someone would have wired it this way. I'd
suggest you post this on rec.audio.tech, this group has precious little to
do with audio outside snide insults and flame wars.


"Geno Rice" > wrote in message
news:3udge.1319$R13.1111@trndny09...
> 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
>
>

[Trevor Wilson]

"Geno Rice" > wrote in message
news:3udge.1319$R13.1111@trndny09...
> 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.

**Post the schematic, either to alt.binaries.schematics.electronic , or to
me directly. It is possible that HK used Germanium transistors in this unit.
Thus the choice of PNP devices. Unlikely, buy possible.


--
Trevor Wilson
www.rageaudio.com.au

[Geno Rice]

Trevor Wilson wrote:
> "Geno Rice" > wrote in message
> news:3udge.1319$R13.1111@trndny09...
>
>>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.
>
>
> **Post the schematic, either to alt.binaries.schematics.electronic , or to
> me directly. It is possible that HK used Germanium transistors in this unit.
> Thus the choice of PNP devices. Unlikely, buy possible.
>
>

Amazingly enough, none of the transistors are burned out. The
base-emitter and the base-collector diodes all measure about 0.55V
on the diode tester. That would make them silicon. Another poster
suggested that the unit had been rewired to use NPNs. My DVM goes
crazy when I try to measure the Vcc voltages (indicates overload)
maybe the filter caps are open or the rectifier diodes are caput.

[Trevor Wilson]

"Geno Rice" > wrote in message
news:tjxge.1480$1f5.990@trndny01...
>
>
> Trevor Wilson wrote:
>> "Geno Rice" > wrote in message
>> news:3udge.1319$R13.1111@trndny09...
>>
>>>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.
>>
>>
>> **Post the schematic, either to alt.binaries.schematics.electronic , or
>> to me directly. It is possible that HK used Germanium transistors in this
>> unit. Thus the choice of PNP devices. Unlikely, buy possible.
>>
>>
>
> Amazingly enough, none of the transistors are burned out. The
> base-emitter and the base-collector diodes all measure about 0.55V
> on the diode tester. That would make them silicon. Another poster
> suggested that the unit had been rewired to use NPNs. My DVM goes
> crazy when I try to measure the Vcc voltages (indicates overload)
> maybe the filter caps are open or the rectifier diodes are caput.

**POST THE SCHEMATIC!


--
Trevor Wilson
www.rageaudio.com.au