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  1. I ordered some TDAs. At this stage as it is impossible to trace where the problem is so removed every semiconductor on the board and will get them tested on a curve tracer. If none of the fail I will mode to inductors, then ceramics, then resistors. I will put it completely apart till I find what is wrong there.
  2. Dp3 and Dp4 signal diodes are fine on mine. Reverse biased up to 100vdc and no leak. CNY17 is also good. I think my culprit is the signal out of IP1 pin 5. Any other ideas?
  3. They are there but do not align. If I adjust the POT I cannot get them all balanced. Find the +12 has a bit of a ripple. I did test Dp11, Dp5, Dp7, Dp4, Dp8, Dp12, Dp2 on a good curve tracer and they are good. The feedback Mosfet seems ok under basic testing. Haven't got it to the curve tracer. Only weird behaviour I found on the RP6. It is leaking at around 70v when reverse biased. Not sure if that is a problem as that wind does the 5v rail. Also none of the signal ones leak reversed and they all share the same peak voltage. At pin 5 of IP1 I am not getting the suggested 100Khz/5vpk square. I am getting the same that goes in pin 6 which is the plot attached. And that is really confusing because IP1 is the feedback controller. If it was faulty I would not be getting feedback therefor no 5,12 volts at the secondary and my SMPW would be completely nonfunctional. Will the SMPW work and provide secondary voltages is IP1 is faulty? Here is a video of the system booting when the digital gets external feed. So digital board seems totally fine. https://www.youtube.com/watch?v=Rgj3HN0fEqE
  4. I have replaced some leaky capacitors on the analog and digital board of this mac. I did check some of the key voltages on the switching power supply block that seems to be inline with that Bomarc schematic. One test I did was to disconnect the +5 and +/-12 lines from the analog board and feed the digital board with clean power from my HP power supply. Computer worked just fine. Definitively something wrong with my analog board. One thing that I noticed was the positive end of the main bridge rectifier has a massive positive sine. As if the CP1 coupling capacitor was doing nothing to clean it. I did test the rectifier and replaced CP1 and got the same result. Any ideas?
  5. gkmaia

    Duplicating Mac Plus OS disk

    Is it possible to duplicate the Mac OS system 6 OS disk using the Mac Plus itself to do it? Will DiskCopy 4.2 be able to do it?
  6. gkmaia

    Mac plus probing signal at J8 pin 1

    Did a few more probing. Lifted R30 to isolate pin #20 (Video out) of BMU2 from the analog board and everything else. Nothing is damping it... it is pretty much weak on it's own. Also did some logic probing on the BMU2. All Is and I/Os are pulsing apart from I14 as it is nodded to OE. Would you narrow down to a faulty BMU2? Any other suggestions? https://media.digikey.com/pdf/Data Sheets/Texas Instruments PDFs/TIBPAL20L8, R4, R6, R8-25C.pdf
  7. Anyone out there with an open Mac Plus who could send an oscilloscope plot of J8 pins 1, 3 and 5? Specially pin 1 which is video out. Probing it I just got a noisy LOW signal. I was suspicious of my LAG IC but after probing V & H sync I got clear pulses at 60HZ & 22KHZ which seems to be ok. That will help me heaps to rule out a motherboard issue and focus on the analog board.
  8. gkmaia

    Mac Plus screen issue

    Yes me too. I am reluctant to go and buy a logic board without covering all possible issues with the analog board. All books mention about replacing U2 but none mention which gate of U2 controls the horizontal signal. If I knew that I could trace all between U2 and CRT and see if I find something.
  9. gkmaia

    Mac Plus screen issue

    Just found it!
  10. gkmaia

    Mac Plus screen issue

    I know what could be then... I am reading the Mac Plus logic is grounded to chassis and I discharged the CRT to chassis without a resistor. Apparently the logic board is sensible to that. Makes no sense to be the analog as I replaced the ICs and tested all transistors. Are you able to show me which one on the board the LAG chip? Or just its marking will be enough.
  11. gkmaia

    Mac Plus screen issue

    No silly question. Any comment is welcome. That is a tricky one. Wish I had schematics with voltages and waveforms to test against. If the brightness you refer if the one in the front under the screen. It works as intended as brightness control. But messing with it does not remove the horizontal lines.
  12. gkmaia

    Mac Plus screen issue

    Could be but I have redone all the joints on the analog board. Fluxed and added a bit of new solder. Unlikely it is that. I will check continuity on all cables and see if I find anything.
  13. gkmaia

    Mac Plus screen issue

    Did some more extensive checks. Replaced the LS38. Checked most of the transistors around the LS38 and nothing. Still the same issue. Also re-soldered the large connector on the logic board just to make sure. Nothing. Any ideas?! Other things to try?
  14. gkmaia

    Mac Plus screen issue

    I just replaced the LS38 logic and nothing. Still the same issue. will try page 169 as may be a transistor.... keep you posted!
  15. gkmaia


    Hi I bought one of these and can really get it to read or format a 720k/800k disk on my mac. I am running mac os Mojave. Is there anything I am doing wrong? Cheers, Gui