An old stereo like that is also going to need new capacitors.
This. As capacitors dielectic fluids deteriorate over time.
With Shematics in hand, open up that stereo system. With a thin plastic/dry wooden stick (thickness of straw), gently knock on all caps while stereo is playing. You will find the culprit cap(s).
It is recommended that you replace all electrolytic capacitors, but more time and energy. Throw it on "future projects" basket.
Caution:
⚠️ Risk of electric shock ⚠️
****** BE CAREFUL *******
wear shoes (utility boots, eg. CAT boots)
wear gloves (a dry-cloth glove at least on hand that will hold the stick)
put the stereo unit on a table where you can stand while you test it. Make sure there's nothing behind you, in case you need to abruptly fall down. 'Cause accidents happen.
bonus: have plastic lamina paper cover areas where grid power gets stepped down to prevent accidental touch. 'cause, for most well-made devices, grid power is handled on a separate PCB board. You will clearly see it (huge capacitors and a big fat transformer)
It's seen around 300,000 miles, both coasts of the US, immense hail storms, a small tornado, a multi-car pileup, a few bullets, and multiple hurricanes.
True true