VintageKustom.com: Repairing Kustom Amps » Kustom Bass III Channel 1 Hiss

Re: Kustom Bass III Channel 1 Hiss [message #28698 is a reply to message #28697]

Sat, 23 July 2022 17:38

thespeakerport
Messages: 11
Registered: July 2022

Junior Member

The NTE725, ECG725 or GE725 are not available anymore. You may be able to find them here and there, but there are modern solutions to replace the original RC4739. I did misstate the name, it's not Black Dog, just BDog adapter. I am not allows to provide links yet, so you can google it. "BDog RC4739 adapter" Should be the first listing that comes up.

The replacement IC is not the issue. It works. If you look at the schematic for the board in question, C16 couples signal into the bass/treble control which feeds into the first half of the IC. In between the first the second half is the volume control. The output of the IC directly feeds into channel two and then to the main amplifier. I had pulled C16 completely out and when I did, all hiss went away, not matter where I had the volume set to. Wherever the problem is, its not being generated by the IC. I started reaching out for help on AudioKarma and got a few folks chiming in (who originally had me pull C16 to sort of isolate the part of the circuit causing issues. (Again, cant put in links yet, but if you go over there and search from Kustom III Hiss....you'll see it.) I have no problem replacing the transistors again with new OnSemi's but I fear I am going to have the same exact result as I did when I originally replaced them.

As far as the replacement resistors are concerned, I did not start replacing resistors until the issue with hiss was discovered. So what was there was all original. The only things that were initially replaced were the IC and transistors before I started going nuts replacing parts. No, I am not perfect and you are right, it can be easy to replace a resistor with a factor of 10 up or down if you are not careful. But again, this was a pre-existing condition. Now, is it possible that this board was repaired before I got it and someone had put in wrong parts...ABSOLUTELY!. I am just going by what i found on the board and did a 1 for 1 replacement. One thing I did not do is step through the schematic and see if there is a mistake somewhere....a resistor that was possibly replaced with a wrong size somewhere...again, assuming this was modified at some time.

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