You should also google dim light bulb tester and consider making one up.

If hard to get a good joint, then you can wirewoll the resistor legs to remove any oxidisation. Best approach is to desolder the part, clean the board with solvent like IPA (alchohol) and toothbrush to ensure good joint. You should measure those components and if they check out ok, then resolder. The toast marks on the board are not too bad.

If it was a bad connection to the FET that would tie in with a LOUD pop. Might be wise to dismantle ad check the fit. Maybe the socket is the problem, or part of the problem and not the FET. If they are in sockets then it would be good idea to investigate the pins and the fit. I can't quite work it out from the picture as to whether those FETs are in sockets or just mounted to a piece of chassis. Have you verified that the pin on the yellow wire has become detached from the mosfet. Thank you guys for your time & any help you can offer. Hopefully I didn't leave out any pertinent info. I don't want to make any rookie mistakes, so if anybody would like to offer their expertise in which components should be tested & why, It would be very much appreciated. Until I can get some opinions & advice from those with the experience I am lacking. I sourced some parts but have yet to dive into replacing anything yet, Perhaps with your help I can test these components & be sure to correct any problems the first time & not blow up new parts by rushing thru the repair. I'm not very experienced at following voltage across a schematic but I'd like to make sure the components near these dark spots aren't damaged from overheating. However, there were a few dark spots on the circuit board. I removed the board & inspected the solder joints which all looked ok. Obviously this part got really hot & needs replacing so I'll replace both mosfets to keep them closer together in spec. So please don't become impatient if I don't completely understand everything right off.ДЄt first glance after opening it up, I saw melted insulation on the wires connected to the K135 mosfet.ДЄlthough it was still producing sound the pin on K135 with the yellow wire was actually able to move in the mosfet. I'm a novice to guitar amplifier repair but I have a grasp on electrical components & the tools/ability to solder, When patched thru another amp it produces a nice clean sound so that should rule out the preamp correct? This amp powers on, but I'm getting loud intermittent cracking & popping noises thru the speakers. This is the schematic I'm using since the only other one I could find is too difficult to read. The quality control sticker inside is dated 06/06/86 & appears to be unmolested. Marshall Lead 100 Mosfet 3210 ss amp, pcb: jm88

Marshall lead 100 mosfet 3210 manual