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Solid State Amp Static

8/30/2004 10:41 PM
Solid State Amp Static
Fender Stage Lead 100 watt solid state - 2 channels, with footswitchable channel switching/reverb. In the overdrive channel, I am hearing intermittant low static - swishy, like the kind you would hear in a tube amp with a preamp tube going bad. Audible with or without guitar cable connected, and audible with master volume up and volume and gain controls zeroed. Any help on where to look for a problem would be appreciated.  
8/30/2004 11:13 PM

Plain old vanilla troubleshooting. Isolate the problem. You already isolated it to one preamp channel, yes? By inference you said the master had to be up, so if master at zero kills the noise, we eliminate the power amp. If the gain controls at zero do not kill the noise, then it is coming in after them.  
ANy control that affects the noise is after its source. COntrols that don't are before it or not in its path.  
In a tube amp we think noisy tube first, and then we think noisy plate load resistor. In a SS amp, it is most likely a semiconductor. I suspect a noisy op amp. By now we should have it narrowed down to a small area of the schematic. Follow along the output pins of the op amps in this section and scope them for the noise. Also, look for DC on any output pins. SInce the amp works, I don't expect a lot, but whereas it usually sits at about zero volts, if one is drifting a few tenths of a volt, it is suspicious.  
SOmetimes heat and cold can irritate a bad part into identifying itself. A can of freeze spray and a heat gun. Or touch the tip of your iron to the top of an IC for a moment or two to heat it.  
The essence of troubleshooting is to isolate the problem, when you determine places where the problem is not, you narrow down the number of places it could still be.
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