That preamp is old enough that I would replace the filter capacitors in the power supply before anything else! Electrolytic capacitors (which are used in the power supply) really are only good for about 20-25 years- after that probably about 50% of them are failing. You're well past that point!
When they fail they can start to draw current, and this can lead to power transformer failure. Plus, there's really no way the preamp is performing correctly; it will sound instantly better with new power supply filter caps installed.
When they fail they can start to draw current, and this can lead to power transformer failure. Plus, there's really no way the preamp is performing correctly; it will sound instantly better with new power supply filter caps installed.