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Need schematic [message #29287]

Thu, 11 July 2024 09:10

HutchAmpMan
Messages: 1
Registered: June 2022

Junior Member

I need a schematic for a Kustom J695 two channel amp head. S/N is 10762

The amp had one bad power transistor (replaced with a NOS unit) but has low volume and distortion when volume is set high. Without a schematic, I'm just reduced to guessing.

There are no markings on the PC boards as to what number they are. The amp uses three power transistors, the first of which is connected to the preamp board with the other two paralleled from that. The audio board is mounted on three points to the chassis bottom via 90 degree tabs.

Any help would be appreciated.

Thanks!
Tom

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Re: Need schematic [message #29288 is a reply to message #29287]

Thu, 11 July 2024 15:22

stevem
Messages: 4751
Registered: June 2004
Location: NY

Senior Member

Unfortunately up to this point in time I have never had may hands on a schematic for these .
These very early amps had only two output transistors.
The other (3rd) transistor was is the driver to the phase splitter/ driver transformer.
The transformer should check out as such.

The primary should read 9.7 ohms.
On the secondary side tan to green should read about 5.9 ohms.
Brown to orange is about 6.1 ohms.

Is your audio signal into the PI transformer clean?
Is your audio signal out of the preamp board clean?$)
All of the electrolytic type caps need to get replaced, and maybe even the main filters off of the power supply diodes.
How close the bias resistors and emitter resistors on the outputs need to test out very very close to stated values on them.

Note that these amps where all of 22 watts of clean power into 4 ohms.

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Re: Need schematic [message #29289 is a reply to message #29288]

Thu, 11 July 2024 15:33

stevem
Messages: 4751
Registered: June 2004
Location: NY

Senior Member

[quote title=stevem wrote on Thu, 11 July 2024 15:22]Unfortunately up to this point in time I have never had may hands on a schematic for these .
These very early amps had only two output transistors.
The other (3rd) transistor was is the driver to the phase splitter/ driver transformer.
The transformer should check out as such.

The primary should read 9.7 ohms.
On the secondary side tan to green should read about 5.9 ohms.
Brown to orange is about 6.1 ohms.

Is your audio signal into the PI transformer clean?
Is your audio signal out of the preamp board clean?$)
All of the electrolytic type caps need to get replaced, and maybe even the main filters off of the power supply diodes.
How close the bias resistors and emitter resistors on the outputs need to test out very very close to stated values on them.
The good thing about any early SS amps like these is the the PI transformer isolates any problems that may be taking place on either side from blowing up parts on the other side.

You can, if need be listen in on the audio drive signal coming into the primary with a pair of headphones protected by a 10 Uf 15 volt non polarized cap if you do not have a O-scope.
Note that these amps were all of 22 watts of clean power into 4 ohms.

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