Excessive hiss above 1/3 vol or so. Change U1 first opamp at input, LF353, to something low noise or some other known cause. When trying it out, the hiss peaked for a while, then dropped back, not returning on twizzling. Not tried heating/cooling yet
Get a scope and check for oscillation. Your symptoms describe just what you heard/saw.
Nasty short bursts of high power did not show any spurious ringing or unwanted oscillation of the Hi-amp anyway. Crude visual pk-pk of noise on scope was about 80mV on 8R load on hi-amp output with shorted input and main volume at max and Hi-Master volume at max, other controls at mid. Warming U1 ic,localised, to about 80-100 deg C made the noise level jump to about 200mV . Squirting freezer spray could make a minimum of
50mV but would then jump up and down abruptly between about 80 and 150mV, between discrete levels, which is probably what was most intrusive in use proper, the hiss varying in level. Localised heating of the other ICs nothing observable on scope. So out with U1. What did we use before freezer spray?, I suppose alcohol on a cotton ball and blowing breath through a straw.ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.