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This looks like one of those deals where OldMacGuy disappears and this thread disappears and its all best if no one asks any more questions.

May or may not be helpful to your exact problem but pretty good series right now on troubleshooting several Mac classics....go to YouTube and search Adrian’s Digital Basement

This is after I removed capacitor and cleaned up with wick. isolation is over and had to clean up for a while....will take the laser apart again hopefully in a couple of weeks and look at u26 as suggested and post follow up

......and right on cue that's what I get for giving a definitive answer, I am measuring these but blew this one (maybe got it mixed up with the C127 change).....anyway, was going to take one more look at C286 this afternoon and just replace it and turn on, but when I took it off...yes there is a...

no

Replaced C286 (not 266...schematic ink close together) 68pf......not sure if it is just the size of the cap or if I charge the cap by measuring, but it still looks like a short across the new cap. Replaced Cap 127 between pin 14 of U26and gnd  because it looked burnt after I replaced U26...

Pin 7 is ground and pin 14 is at 5.02 volts. Will go to find a 68 pF cap to replace C266...with U13 and D14M checking out, is this cap bringing down everything still a possibility?  What about the reset circuit?

I did measure pin 8 above,  not pin 18. These pictures are of pin 18 of U13....14.35 frequency is there.

When I turn the machine on and measure first with voltmeter then with oscilloscope..........pin 1 of U13 shows no voltage and no frequency detected, at the same time PIN 2 of U13 shows 1.34 volts and frequency 14.35;  PIN 8 of U13 shows 4.76 volts and no frequency detected, Pin 9 of U26 shows...

Well, took some work but finally got U26 off and replaced.....same behavior, no S14M signal, still stops between pins 9 and 8. Replaced U26 first then U13, still the same.

Once you get it set up ADTPro is pretty convenient.

parts on the way.....will be several days; at least U13 is socketed; work to pick up so may be a while; just hope it doesn't uncover more issues; my concerns are the "designed" chips for the video, MMU and disk control. Thank you. 

Learning and measuring here. I think we've narrowed it down. Sorry about the JPEG format on this post, but moving between computers and file types.

So the manual lists as part of U26, but XTAL1  looks pretty well labelled, big and silver just below U26 on the board and it gives me the only patterned oscillations I've found on the board, one pin a pretty smooth sin wave and the other regular and a little more stepped pattern......14.35...

On U34, pin 10 and pin 3 both read as steady 5 volts. I DO NOT have keyboard or internal floppy disk drive connected because it will be much more difficult to access the board. Even with magnifing glasses its difficult to hit the pins on MMU and then get a good look at oscilloscope, but  not...

These are the probes....can I even pickup/measure a 1mHz with these?

Probes here: on CPU pin 3 steady 5 volts, CPU pin 39 cannot find active signal, CPU pin 37 cannot find active signal, CPU pin 40 no delay and immediately jumps to 5 volts

Alright...so the power voltages on the CPU as above checked out; also went to obvious components on the schematics and checked out voltage and grounds which seemed ok....5 volts all around. . The onboard ram showed a plus 5 volt at pin 8 and grounded at pin 18....different than the ram on...

OK....lots of screws and sheildng to remove. Will check the CPU voltages and wait for oscilloscope probes. Everytime I flip the solder cable over or remove the sheilding , feel like I'm on borrowed time waiting for something to give. Just to be clear....for instance...ground probe to ground and...

I just obtained an oscilloscope in the last few days and currently waiting for the probes. When the probes arrive, it will be my first time using an oscilloscope. I will have the equipment "to check cpu clock and reset line", but not sure what the phrase actually means.  One end of the internal...