I have an old Shure M64 stereo preamplifier that is acting up. I'm hoping that what it's doing will be a pretty good indication of what the problem is. When I power it up, it produces normal output (stereo sound that goes into an auxiliary input to an amplifier [usually for a turntable]). But within 5 seconds, the output (sound from the stereo system) begins fading, and within 20 seconds, the output is down to zero. If I switch it off and back on, the cycle repeats. The preamp is from the 1970s and utilizes transistors, I assume. Would appreciate any help troubleshooting it. The circuit diagram for it is available here: https://pubs.shure.com/view/guide/M64/en-US.pdf
Thanks for any help!!
Thanks for any help!!