Home » VintageKustom.com » Repairing Kustom Amps » Kustom Bass III Channel 1 Hiss (extreme amount of hiss on channel 1 board (PC5127))
Kustom Bass III Channel 1 Hiss [message #28683] Thu, 21 July 2022 22:11 Go to previous message
thespeakerport is currently offline  thespeakerport
Messages: 11
Registered: July 2022
Junior Member
I have gone 12 rounds with this animal and desperately need help. I had this unit come in because the unit was making a horrible buzzing sound when turned on. basic troubleshooting discovered that unplugging the 6 pin molex connector from channel 1 made the noise go away. Channel 2 was working ok. Traced the issue to the RC4739 chip being fried. All other transistors/resistors/caps seemed to be ok on the board, however, voltages were way off. Was getting -12V and +24V instead of +/-12. Found out that one of the rectifier Diodes were shot. Replaced it and re-tested the board and it still buzzed. So I got the Brown Dog adapter OpAmp to replace the 4739 and low and behold, it worked. BUT...channel 1 is putting out an enormous amount of hiss. I was suspicious of the opamp, but after pulling out the C16 capacitor to disconnect signal going into the Bass/Treble circuit (and subsequently the OpAmp), the hiss went away....so whatever it is is more up stream. My first instinct was noisy transistors. There were only 7 of them so I was able to get some info about suitable replacements and the good ole OnSemi KSA992 and KSC1845's came up (and i had them in stock). Replaced all of them and the the hiss remains. (Sigh) I decided to put back in the original transistors. Some additional searching and I found suggestions that these old carbon comp resistors may be getting noisy. Some were out of range anyhow so I decided to just update this board and replace them all. (Yes, I know way overkill, but I was looking for a quick victory.) No victory, hiss remained. Only thing left were the caps. So I replaced the electrolytics, films and ceramics. Might as well....when this far, why not. The hiss still remains! I dont get it. Every part was replaced and it affected nothing. All the parts are new on this board except the pots and the transistors (and i already tried swapped the transistors out.) I refuse the believe the pots are causing hiss as i have never seen that happen before. Crackle, static, dropout...yes....but not hiss.

I am at the end of my rope with this guy. Any thoughts/input as to what the heck is going on? Is this board that badly designed to where channel 1 is supposed to put out this much hiss?
 
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Read Message
Previous Topic: Kustom IV Bass Power Supply Caps
Next Topic: K200 Bright Channel is ALIVE...
Goto Forum:
  


Current Time: Thu Mar #d 14:25:30 EDT 2024
.:: Contact :: Home ::.

Powered by: FUDforum 3.0.9.