Home » VintageKustom.com » Repairing Kustom Amps » Kustom Bass III Channel 1 Hiss (extreme amount of hiss on channel 1 board (PC5127))
Re: Kustom Bass III Channel 1 Hiss [message #28693 is a reply to message #28683] Fri, 22 July 2022 16:58 Go to previous messageGo to previous message
stevem is currently offline  stevem
Messages: 4733
Registered: June 2004
Location: NY
Senior Member
The right way to trouble shoot this now is with a scope to look for which semiconductor is adding the hiss.

Short of doing that my best guess seeing all that you have done and with my own experience with these preamp boards is that your 14 pin IC1 is bad .
Interestingly enough a few years ago with my k250 I had this same IC go bad .
It was fine at one rehearsal and a week later when I powered it on it had a major level of hiss that almost made it impossible for me to use it that night!
If you do not have a scope then the fact that you have another working preamp will allow you to signal trace thru that bad preamp.
To do this just just make a open end cable to plug into the working preamps input.
Solder on a length of sure to the hot/ center conductor And then to the end of that wire solder on a 25 volt electrolytic cap with its negative end to the wire.

Using the + end of the cap as a probe will allow you to listen in to that preamp board wherever you want!
Since that IC runs on the + and- 12 volt rails I bet that when you had that bad Zener making for that high voltage it damaged that IC1.

[Updated on: Fri, 22 July 2022 16:59]

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