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Kustom K100-5B [message #29155]

Sun, 05 November 2023 18:25

retrorod
Messages: 5
Registered: June 2012
Location: Outer Banks, NC

Junior Member

I have the K100-5B on the bench. My experience has been primarily tube amps.
This amp powers up. It produces a loud swoosh sound. Not a hum as much as a dull roar. It is consistant and not effected by rotating any controls of the four channels.
I get output from all channels although channels 2 and 3 have a little less gain.
Reverb works.I have installed a grounded power cord. Measured +-38
vdc off the large caps and +-8vdc on the four preamp pcb wires. It appears to have original RCA transistors.
The sound is less like a hum than a rushing swoosh noise. My hopes are that one of you more experienced Kustom gurus can steer me to a logical area to test.

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Re: Kustom K100-5B [message #29156 is a reply to message #29155]

Mon, 06 November 2023 08:11

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

Hello and welcome.
Unfortunately our schematic and amp list in our technical section is non functional once again , but if you PM me I can get you what you need.

This amp consists of 3 different circuit boards .
It has 4 PC105 boards .

1 PC803 which is the reverb mixer / send and recovery .

1 PC900
Preamp voltage regulation and output driver board.

With what you have posted about I would think that your issue is a noisy transistor, and it could be on the 803 board or the input side of the 900 board.

All the blue wires in the amp carry just the audio.

The blue wires going to the 900 board carry the audio so if you disconnect that from the 900 board you can start the process of elimination.

Note that if you need to back off the 900 board to work on it that a long lead diode goes from the board to a metal clip between the two output transistors and that these leads are very fragile, so don't go moving that diode around any more then you need to.

Until you have made 5 post to this site I will need to approve your post, after that they will just go right up.

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Re: Kustom K100-5B [message #29159 is a reply to message #29156]

Mon, 06 November 2023 16:05

retrorod
Messages: 5
Registered: June 2012
Location: Outer Banks, NC

Junior Member

Thanks so much for the response and ideas. I disconnected the blue wire from pc803 to pc900. No change in the hiss noise. Are the 36892 transistors on low left corner of pc900 suspect? What are the 2 heat-sinked transistors in the center of pc900?
Sorry for all the questions. I know my way around vintage Fender tube amps pretty well but trying to understand solid state.
Thanks, Rod

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Re: Kustom K100-5B [message #29160 is a reply to message #29155]

Mon, 06 November 2023 16:53

stevem
Messages: 4774
Registered: June 2004
Location: NY

Senior Member

The 36882s on the 900 board are for the negative and positive voltage regulation to the 803 and 105 boards and will not be a source of your issue.

The other two 36892s on the floor of the chassis are the output transistors.
These are not the likely cause of your noise, just as unlikely as having hissing 6L6GC tubes in a Fender.

That blue wire that you unhooked goes to a resistor and then to a mixer transistor like what V4 does in a Fender with reverb.

I think that this transistor is the source of your noise

[Updated on: Mon, 06 November 2023 16:56]

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