As soon as the unit is powered on, there is a constant static noise in both channels. You can hear it through the headphone jack, as well as the output connection. It doesn't register on the VU Meters, but it also makes any playback with known good tape extremely quiet. I've been using an audio probe to "listen" to where the noise is coming from, but get stuck at the preamp chip, IC501. There's a constant buzz in my audio probe, and when I turn up the volume to hear anything before the preamp chip, it gets drowned out by it. I did try swapping out the chip itself with one from a working unit, and even tried swapping out the socket that it goes in as a last-ditch effort. Neither change fixed the problem. I've also tried unplugging as many wires as I can from the amp board in an attempt to isolate the problem, but it's constant unless I remove one of the plugs that supplies power. More notably, the noise is still there if I unplug all inputs and outputs, like tape heads, microphone, and line in. Unfortunately, I don't have an oscilloscope, so the only other thing I can think to do is to start pulling components and testing them, but I'm concerned that even if I pull the bad part, it will still test good. Can something like a noisy transistor show up on a multimeter or an all-in-one component tester? Is there another, possibly better, way of tracking it down? I do have a nearly-identical (same model) unit to compare with, but all the voltages I've checked so far are similar.