This one looks like a flood victim, the rear panel got soaked with water, see some green from the copper corroding away.
The radio is still salvageable - but whatever hammered the back end of that final and driver - let alone R236 and Diode combo - you certainly have picked a winner.
Start with the cap - by R236 and Rear Panel power supply and also gently start to rebuild the board - it's not bad, just rear panel and areas of water entry are the problems - you don't have mold - yet.
That green stuff appears to have been from a previous cap that was in that one spot the outline is different - so when it erupted - the green is the sulfite possibly from the copper sulfate or the electrolyte steaming out of it.
Some simple alcohol - could help but if properly used, some MEK (Methyl-Ethyl-Ketone) to get the stiffer stuff would help. The Green should flake off and some simple Isopropyl to help with the trimmer's rotation should be all that is needed to get that stuff off.
Note the rear panel support "Ears" or the riveted lugs used to hold the rear panel in place as RF ground return - you will have to drill out the rivet and replace with bolts and nuts to restore the rear panel RF backplane - else you may have issues with the Rf being produced back there. So, if initial tries and you still have problems - the next step is rebuilding that back panel to make the RF flow in the correct directions to work right.
Several spots by the AM regulator - looks like they did "Frankenstein" style surgery by removing the back cover and subbing in parts and values until they achieved an effect or made it work like they wanted to. Then kept parts in and then the radio wound up on a market and in your hands.
This is where an air station would help reflow the solder to take up the flux still left and bring the shiny solder to rewet and doing so gets the dirt out and float it to the top for easier cleaning.
There is a mod of some type done to the 4558 chip on that board so I'll have to leave these with you to help you sort out the work...
Not sure of the status of the Echo board, it is an early 50B type.
But aside from the age and the environment it was left in - it should be workable as a recovery - but gently first...see if you can get a heartbeat then work on the amputations and organ reattachments.
You have some work to do - and to be honest - I've seen worse arrive to my desk and have had to restore bigger messes - but that is an NDA for later...
