by Bob Latino Fri Sep 28, 2012 6:21 am
Hi,
You said you updated the amp. I am assuming that you replaced the quad cap. The most common cause of a low level hum that does not change with volume AND is the same volume on BOTH CHANNELS is a leaky quad cap. Even if you replaced the quad cap 3 years ago, it is now possible that one section is bad.
To really test the quad cap you need a capacitor meter but you can do a pretty good job with a resistance meter. Before you work on the quad cap discharge all four sections to the chassis with an insulated screwdriver. Check the resistance from each of the four sections of the quad cap one at a time. (red probe on the section and black probe on the chassis) Normally the resistance will rise as the cap section charges from the battery in your meter. Each section should (eventually) show over 1 meg ohm by the time the meter stops. If any section shows a LOW resistance (maybe 10,000 ohms or less?) it usually means that this section is leaking voltage to the chassis. Sometimes two or three sections may show a low resistance because all the quad cap terminals are interconnected by the choke and resistors. The only way to test to see which one (or multiple) sections are bad is to remove all the wiring between the connectors and then test each section separately. If just one section is bad you can bypass this section with a separate capacitor of equal value but IMHO - I would replace the entire quad cap.
Bob
