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Hi! My analog board is having dificulties reaching the 5V DC output. At the maximum the potentiometer only gives ~4.90V Dc and never triggers the crowbar circuit. The Logic Board has 4MB RAM upgrade and boots up altough the under voltage at 5V. On the Macintosh Repair and Upgrade Secrets book...

Thanks!

Ok! Should I regulate the 5V potentiometer to the minimum before turning it on with the new optocoupler?

Thanks! I'll order one in case the 4N35 does not work.

Thanks for the advice. That's usually the case. But unfortunately, in this case that is not the problem. I've recapped the analog board, refluxed all the connectors, that were indeed cracked, and the Mac worked fine for some time. I don't know if its a coincidence or not, but when I bought an...

Hi! I'm troubleshooting my Macintosh Plus power supply and I suspect that I have a bad optocoupler. My analog board is the international version 220V. The original optocoupler is the GE H11AVX515R which I can't seem to find online. I have a substitute optocoupler from Motorola (4N35) that I...

I've been testing the machine with half the memory modules (2 mb) and I get the same screens each time. It doesn't bong. When I turn on I get this screen while testing the ram: then I get this screen with a sad mac but cannot read the whole error code:

@Nixontheknight thanks for the tip. I'll connect the oscilloscope and measure the ripple at the three voltages. Should I measure it at the external FDD port, or is it better and more accurate to measure it at the J8 connector on the logic board?

Hi! Continuing my Macintosh Plus 4Mb total restore, I bough a BlueSCSI2 external module and installed a number of disk images. It was working like a charm. Then, I went to the CR1 diode mod, so that I could have TERM power on the SCSI port to avoid the need to power the BlueSCSI2 from a usb...

Major update: Macintosh Plus is working fine! :) After all it was ok since the beginning. :rolleyes: The problem was that since there was no startup bong, and the 4Mb RAM testing takes some seconds, I disconnected it before it ended the RAM test, to prevent "further damage". After...

Hi! @cheesestraws Thanks for the info. I have another donor logic board that I can use to swap components. I'll try swapping this chips. Is there a way to check them with an oscilloscope before swaping? Thanks!

Hi! Thanks for the advice and reference. I'll have to buy the EEPROM reader/writer to test. Either I'll buy or do a project like the one Ben Eater has on his Youtube channel Build an Arduino EEPROM programmer (youtube.com) (This is part of the fun :) )

Ok! I'll do that this weekend and post the high res photos. (I've already reflowed all connectors at the analog and logic board...)

Thanks for the helpful information. Not going to buy new ROMs yet. I'll recap the logic board and in the meantime I'll take the high res photos and post them here.

Thanks for the info. I probed CPU pin 15 (clock) with the oscilloscope and got the attached signal. Don't know if it's normal because I don't have another functioning mac to test. Anyone can confirm if this signal is normal?

Hi Hi! Yes I've tried with the floppy, keyboard, mouse and everything completely disconnected, but the outcome is the same....

Thanks! :)

Hi Thanks for the reply. Indeed I removed both ROM chips and RAM SIMMs. Cleaned all contacts with De-ox and contact cleanner. Same result. But, yes you're right, sometimes MAC ROMs can get defective and, after recapping the logic board, if the symptons persist, I'll go that way. It will be a...

Hi! It doesn't give the chime sound, neither the FDD moves. I only get this screen, no mouse pointer nothing. If I press reset it resets and goes to the same screen. I'm going to repac the logic board. The clock signal taken at CPU pin 15 is wavy at the low edge and perfectly square above...