If you search the web you can find the Service Manual for the 7850. I did.
The service manual has instructions for reading internal diagnostics.
My 7850 problem: the unit ceased to put out drive to the speakers, yet supplies audio to the Zone 2 RCA phone connectors when zone 2 is set to Tuner , but not when "sync" was selected for zone 2.
My intuition is that the unit enters protective shut-down which disables power to the amplifier board.
My intuition also says semiconductor component failures are rare, but electrolytic capacitor failures are common in consumer electronics.
(from my early career as a service tech, I learned most problems are mechanical, and I count dried out electrolytics as a mechanical problem.)
(my later career was building electronics widgets for physicists at a national lab)
A shorted output transistor would trigger the protection mode. The data in the output transistor spec-sheet (perhaps sc5200-sc1943) suggests those output transistors are very robust, and operated nowhere close to their design limits; that accounts for my expectation they're OK.
Before taking the cover off and measuring, unplug the unit.
You can reach the Emitter, Base, and Collector leads of the output transistors from above once (only) the cover is removed. Complete disassembly would only be necessary if a part needs to be replaced.
The output transistors are all lined up next to the heat-sink in the middle of the unit.
An ohm-meter with a "Diode-mode" is the best choice for making these measurements.
If the meter is good enough it's probably not necessary to unsolder leads.
Measure resistance both polarities E-to-C, B-to-C and E-to-B. The outlier is the bad one.
Check the emitter resistors too.
Replace both transistors in the "totem-pole" with a matched pair. (after replacing, make sure the "Idle Current" is in range!!!!)
If the transistors are OK, then check for faulty electrolytic capacitors (the big cans).
When the capacitance drops, the power supply ripple increases, and that can trigger a fault-mode.
(sometimes bad electrolytics bulge. Sometimes they leak. Sometimes you can't tell without unsoldering them.)
In some cases a bad electrolytic can cause a switching power supply to fail to start.
Internal diagnostics may tell you which power supply to investigate. Study the schematics.
That's my 2¢
I have a 592 and an 7850 to fix.
Fixing these things is a labor of love when you HAVE the documentation. Maybe not economical to engage a tech.
If you don't have documentation, a used unit off of ebay or craigs list may be the best option. Otherwise upgrade to the next tier. (a good ebay unit is also useful for comparing with the broken one)
Without documentation, board substitution is a repair option. Experience and intuition helps.
Our efforts repairing these things helps keep them out of the landfill. imo a noble objective.
