Hi Mark and welcome to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, let's try a review of the basics:
Should none of the above above be corrective, then it would almost always point to an internal hardware issue. Would you then considering a DIY (do-it-yourself) repair approach? If yes, do you have access to and familiar with a multi-meter and even a soldering iron?
Great on the DIY!
Let's do a review and determine if the fault is in the amp circuits or before, the pre-amp.
For test purposes, feed the Sansui's 4-CH/NR L & R OUT jacks to the LINE INput of another amp/receiver. The idea is to determine if the Sansui would operate minus its amp/speakers.
Appreciate the update Mark.
With speakers correctly wired to the Sansui and the covers removed, please locate resistors R03 and R04 on the F-2856 (amplifier driver) board. Quickly/lightly touch either leg of these resistors and determine if there would be a thump/humming sound from the connected speakers.
A friendly reminder; the above would require that the unit be plugged in and turned on. It is therefore LIVE! Please exercise due care and caution.
Please locate relay RL601 at the F-2858 board and carefully determine if it would click on to engage when initially turning on the Sansui? ...click once? ...or would there be a second click?
Please be doubly careful as the F-2858 board primarily holds the voltage regulator circuits.
Great Mark! Good to hear that the issue has been resolved, congratulations.
Consider using compressed air or better yet, plastic safe / non-staining CRC 2-26, DeoxIT DN5 or LPS 03116 Contact Cleaner or equivalent to clean these switches but allow sufficient drying time
Good luck and thank you too for the opportunity to be able to share the information.
Try the cleaning first.
Then check for possible cold solders particularly on the wire-to-board/pin.
I will review the schematic diagrams/Service Manual for other check/test points.
I understand Mark and yes, remove and soak/clean out-of-circuit.
Do you have alligator clips with wires similar to the product shown HERE ? These can be used to jump the connecting points of the input switch. For test purposes, the other circuits then can be tested/checked.