| Kustom Bass III Channel 1 Hiss [message #28683] |
Thu, 21 July 2022 22:11  |
thespeakerport
Messages: 11
Registered: July 2022
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Junior Member |
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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?
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