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 #28725 is a reply to message #28683] Fri, 29 July 2022 18:36 Go to previous messageGo to previous message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Why change the chip back? You have already isolated the hiss to the circuit ahead of C16 that feeds the tone controls.

The volume control R6A does two things as it is rotated. When the control is CCW the output of the Q1 and Q2 is grounded through cap C7. As the control is moved to the CW side, the output is ungrounded and the gain of the stage is increased by grounding the C5/R5 connection to the emitter of Q1.

If the hiss increases as the control is rotated to the CW side, then some of the hiss is being generated in this stage.

The Clarity control is supposed to be a low bass control that I will guess works as a big negative feedback loop to boost ultra low frequencies.

I would suggest that you try and isolate the noise source to see where it is being generated. Maybe used the BFV cap method to ground out the signal at various signal points to see if you can isolate the stages.

As for the 1% caps, Kustom started using these metal film resistors in the K200 series amps. Back in the day, the only metal film type resistors were low tolerance 1% types. Kustom started using them in the preamps to reduce noise, as they were much better than carbon comps. The first time I remember seeing these was in the late '60s. There were even mentions of these parts in the marketing materials as part of the High Tech engineering that they used to build these amps.

I don't really think that the actual value is that important, so it would seem that what you have done should be fine.

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