68kMLA Classic Interface
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| My IIfx progress | |
Posted by: David Cook on 2023-04-27 13:24:46I've found a transistor in the ADB circuit thats corroded away, schematics says it's a MMBT2904, I can't find any for sale.. Does anyone know what a suitable replacement would be? I believe you (and the Bomarc schematic) mean MMBT3904. You'll find plenty of them. : ) Edit: That transistor provides the output signal (low) to the ADB device. The specifications needed to do so are non-critical. Any small-signal NPN (bipolar) transistor will do. Given Apple's ubiquitious use of the 3904, the Bomarc schematic clearly has a typo on that page. | |
| Posted by: SophieRose on 2023-04-27 13:31:03 Ahhh darn these schematics.. thanx | |
| Posted by: SophieRose on 2023-04-28 07:51:11 Here's a video of what it's doing.. I replaced Q1 in the adb circuit but it does the same thing with or without Q1 even being present.  | |
Posted by: Phipli on 2023-04-28 07:59:10Here's a video of what it's doing..What happens if you try starting it without the floppy drive connected? You'll need to leave it for a few seconds longer because the icon takes longer to appear when there are no floppy drives. | |
| Posted by: SophieRose on 2023-04-28 08:04:34 It does the same thing.. left it for ages.. Still square grey screen with no mouse pointer. | |
Posted by: Phipli on 2023-04-28 08:14:51It does the same thing.. left it for ages.. Still square grey screen with no mouse pointer.Do you have any ADB devices plugged in? If so, what happens if they're plugged in? (I'd plug the floppy drive back in so that you see results faster) If you can't see anything wrong with ADB, SCSI and Sound were the next two things on the list. The sound chip is at least partly working because they usually control the reset line during power on don't they? Oh yeah, plus it chimed 🤣 | |
Posted by: David Cook on 2023-04-28 10:11:00I replaced Q1 in the adb circuit but it does the same thing [hangs with gray screen] with or without Q1 even being present. ADB pin 1 is a dual input/output. R66 pulls up the signal. Each device pulls down the signal to communicate. So, the transistor you replaced allows the Mac to output to ADB by pulling ADB pin 1 low. Without this transistor, the Mac can boot, but it can't talk to the keyboard or mouse. ADB pin 2 attaches the keyboard power key to the Mac's power circuit. It doesn't need that transistor you replaced. That's why the keyboard is able to power on the Mac without the transistor. (But, your Mac won't be able to read keys or mouse movements without the transistor in place.) ------ In your video, the memory is in the wrong bank. The IIfx requires bank A to be filled first. Move all four RAM sticks to the empty slots. | |
| Posted by: SophieRose on 2023-04-30 07:34:29 Cheers for the tips ☺ | |
| Posted by: SophieRose on 2023-05-01 11:36:51 Popping the ADB ASIC off..swapped with a spare which made no difference, I also tried swapping the SRAM that goes with it. | |
| Posted by: LaPorta on 2023-05-01 11:54:05 Nice video you have there! | |
| Posted by: SophieRose on 2023-05-10 11:13:24 Progress ☺ happy mac.. then starts to boot and stops.. mouse kb and nmi still work. Only boots at all if i drench the board in iso.. otherwise its sad chime and no video. more investigation needed. But so pleased its getting past the grey screen now. | |
Posted by: Phipli on 2023-05-10 11:32:36Progress ☺ happy mac.. then starts to boot and stops.. mouse kb and nmi still work. Only boots at all if i drench the board in iso.. otherwise its sad chime and no video. more investigation needed. But so pleased its getting past the grey screen now.Interesting... Poking time, poke areas of the board and chips one at a time with your finger and see if the behaviour changes. Narrow the fault down to an area if it is a dry joint or borderline trace. | |
Posted by: David Cook on 2023-05-10 12:54:14Progress ☺ happy mac.. then starts to boot and stops.. mouse kb and nmi still work. Only boots at all if i drench the board in iso.. otherwise its sad chime and no video. more investigation needed. But so pleased its getting past the grey screen now. Glad to hear about your progress! I admire your determination to fix this Mac. Your tenacity will pay off. BTW, getting to a sad chime means the diagnostic serial port is likely working. I briefly discuss this approach in a posting about repairing a IIsi. IIsi Repaired By Reheating Solder Joints and Replacing CrystalI recently bought two Macintosh IIsi computers. Neither worked upon arrival. The initial restoration steps are common: 1. Remove old battery 2. Blow out dust 3. Replace the capacitors on the motherboard and power supply. I decided to replace the big capacitors on the power supply as well. They...
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| Posted by: LaPorta on 2023-05-10 13:29:07 What was it you did to get it this far from the gray screen? | |
| Posted by: SophieRose on 2023-05-10 13:30:51 Replaced both SRAM ics ☺ oh and gave it a bath in iso.. | |
| Posted by: SophieRose on 2023-05-10 14:10:26 Oh and it did this once.. when i mashed the nmi button.  | |
Posted by: SophieRose on 2023-05-11 16:27:46Glad to hear about your progress! I admire your determination to fix this Mac. Your tenacity will pay off.Great idea.. I might try the diagnostics mode if I can do it mac to mac. | |
| Posted by: SophieRose on 2023-05-14 12:53:19 In other news ☺ Still something wrong but omg now it boots to the desktop if I apply a layer of iso thanks to a couple of bad traces I fixed.  | |
Posted by: Phipli on 2023-05-14 14:42:33In other news ☺Excellent news. For a while my 840av only booted if I pointed a hairdryer set to cold at it. | |
| Posted by: Phipli on 2023-05-14 15:36:25 @SophieRose Is it running well enough to boot from the Hardware test CD and run the Apple hardware tests? The 68k version is... second download here : MacTest Pro - Macintosh GardenMacTest Pro is repair-oriented diagnostic software that can test logic boards, video cards, monitors, floppy drives, hard drives, and CD ROM drives. It can also detect software corruption problems, and the existence of multiple System Folders on a volume. MacTest Pro is intended for use by Apple...
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