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Have a first gen Fender Cyber Twin that has developed a hissing sound in the right channel (stereo power amps). Trying to figure out where to start. The fairly low level hissing occurs in the right channel speaker with or without a guitar plugged in, does not change with volume adjustment or in other controls. It will stop, in the main amp speakers, when plugging in headphones and does not appear in the headphone speakers. Left channel speaker is completely silent. Any ideas on where to begin ?
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OK, phones have separate amps, but run off same signal, L and R out. SO it points to power amps at first. So go to power amp page and see input resistors R301 R401. Ground that resistor and see if it silences the hiss. If it does, then the hiss is not in the power amp, if the his stays, then the PA is noisy. And my first suspect is a semiconductor.
If before the power amp, go back to the host board, and see if it is hissy leaving U20 or the op amp U27.Education is what you're left with after you have forgotten what you have learned. -
Cejay,
Your observation of hiss going away when the phones are plugged in, is super helpful. Note that the grounds are passed thru to the headphone channel, by way of connector. I'd suggest poking around with a multimeter in resistance mode and check continuity for all of these signals/grounds and see what happens pre and post headphone connection. I've learned to throw away what I THINK the problem is and only look at the data that the problem is showing me. The action of plugging in the phones, and recreate the problem, is a good base to start. I'd also suggest seeing if mechanical tapping or other, can cause the problem to come and go. If so, look at the phone jack solder connections. Cheers and good luck!Comment
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One other thing... this amp has a ton of connectors. Along with straightforward, looking for the obvious, I'd check every connector for proper engagement.Comment
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