Here's a '73 Twin Reverb we bought to restore to glory.
It had a ton of issues that we addressed: - cut off power cord - mismatched and missing tubes (power and preamp) - shredded pots - blown speakers - out of tolerance grid resistors.
Watch on YouTube for time stamps. Enjoy!
Here's a quick video of me knocking out a Fender Pro Junior Repair. Supposedly it just had a busted input jack, but I discovered dirty pots and that all the tubes were bad. The gunk on the amp ended up being two fold: WD40 and beer.