warren@cbnewsh.att.com (warren.a.montgomery) (01/28/91)
My stereo receiver has developed an annoying problem. Frequently, when first turned on, it sounds a loud 60 cycle hum in the left channel only. Flipping the power on and off once or twice will generally cure it. Once or twice, the hum has appeared while the equipment was on. I have an undergraduate EE degree and have done my share of amateur stereo repair, but hate problems like this that are not repeatable and don't suggest anything obvious to me. I also have no schematic and have been unable to locate the manufacturer (Sherwood), but the unit is old enough that it's built with discrete transistors and other parts and therefore repairable. 60 cycle Hum suggests either a power supply problem (ripple getting through a bad capacitor), but that would almost certainly impact both channels, or noise on one of the input signals (loose or improperly grounded plugs), but that should be sensitive to the source of the audio (radio, tape, etc.), or the volume control, which this noise isn't particularly. Any suggestions? -- Warren Montgomery att!ihlpf!warren
jws@cica4.mlb.semi.harris.com (James W. Swonger) (01/28/91)
A SAM's may be available for your unit. Try the TV Repair/Electronic supply places in your area. A hum that changes volume with the volume knob would indicate an open/ intermittent contact etc. upstream of the main amp. A hum that is constant is more indicative of power supply problems. There may be a poor connection at one of the recitifiers or caps. Some amps and rcvrs have separate supplies for each channel to give better isolation. See if you can cause the problem by wiggling the filter caps and rectifiers.