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Another EPIC Blunder, this time a Mac 512K Motherboard

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Well, I'm in tears again over my own foolishness.  I've been shooting a video that focuses on recapping of 400K floppy drives.  I hadn't begun the actual recapping but was doing some preliminary tear-aparts and testing.  I had my 400K floppy drive mechanism connected via ribbon cable to my 512K motherboard, but I had the drive mechanism pulled outside the back (with the back case off) and sitting on some plastic floppy disk cases.  I had a single jumper wire with alligator clips at either end connected to the metal chassis of the Mac and to the metal chassis of the floppy drive.  All was going fine until I switched on power and noted there was no Bong sound!  I looked down in horror to see one end of my alligator clip wire had fallen off the floppy drive and onto the 512k motherboard, shorting one or two pins of the SYP6522 (about where the red dot is shown below) to Chassis ground (connected to Earth via wall socket).






With my heart racing and sweat on my brow, I immediately switched off the machine and removed the jumper wire.  When I switched on the machine, to my delight I was able to hear the startup bong!  And when I put in a disk, the 400K drive would spin the disk and move the head to try to read from it.  It spit out the disk, but the drive had been spitting it out prior to the Epic Blunder because I think the disk was bad.  Anyway, I then booted from my FloppyEMU (which was also attached at the time on the external Floppy connector), and it booted to the Desktop just fine.  Thinking that all was well,  I put in the same disk into my 400K drive, but the drive motor didn't spin.  Basically nothing happened, just as if I had the 400K drive disconnected and inserted a disk.  I then shutdown the machine and powered on, then put the disk back in, but the drive still wouldn't spin the disk or move the head!  And yet, when I power on each time, I do see the head move a few millimeters as it normally does at cold boot.  Even so, disk insertion results in no motor spinning and no head movement at all.  Yet my FloppyEMU works perfectly every time I try it.


I have another 400K floppy drive, known good, so with the machine powered off, I disconnected my dead 400K drive and I connected that other known-good drive via the same ribbon cable to the motherboard -- it too is an internal 400K drive mechanism.  That 400K drive accepted my floppy, tried to read it but couldn't, then spit it out.  So I booted from my FloppyEMU (still connected), and at the Desktop I inserted my floppy again. Sadly, this second drive has now stopped spinning the motor!  I powered off and on, then inserted the floppy again, but it still won't spin and the head won't move.  But like the other drive, when I power on, I see the head move just very slightly as it normally does (which happens even when no floppy is inserted).  


So why would the head move a little at power-on, but then give me no motion at all when I insert a floppy?  And why would both of these drives accept the floppy the first time, try to read it, then spit it out (perfectly normal when a bad disk is inserted), but then after booting to the desktop (via FloppyEMU) suddenly stop working?


If the above schematic is to be believed, it would appear I may have shorted one or more of the address lines (pins 36 and/or 37??) to Ground with that stupid jumper wire.  But why would a short there ultimately kill both of my 400K floppy drive mechanisms?


I have another 512K motherboard, but I'm a bit scared to test my 400K drives with it for fear that whatever now plagues the 400K drives could harm that motherboard too.


I would certainly appreciate hearing your thoughts and suggestions, especially yours, @Bolle.


Thank you.



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Let me see if I understand.


You believe you might have caused a short to your 512K logic board.

You also believe that although this logic board when wired up works as expected but drives the 400K drive to only move the head a short mm or so. A second working 400K drive exhibits the same issue when attached to this 512K board. You also appear to suggest some concern that the drives motors are not working as expected because a potential short on the board may be causing the motors on both 400K drives to burn out. Is this correct?


If you suspect failed motors on those 400K drives are you able to attach an external power source to understand if those motors can spin?


Normally, I would suggest attaching the affected 400K drives to a known good  512K or equivalent logic board as an isolation step.


From what I understand there may be a short on the 512Ks logic board that might be causing issues to those 400K drives. I mean have you reconnected any of those drives back to their original computers, well at least the one that you said worked before connecting to this motherboard that had the mishap?


I am inclined to believe the 512K board was impacted and the drives are showing symptoms but you could rule this out by externally powering the 400K drive's motors.

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Do you happen to have lying around one of those A9M0106 "universal" floppy drives that were sold for both Macintoshes and the Apple IIgs? Since you say the machines boots from a FloppyEMU but doesn't work with the 400k drives my wild guess would be that you did something that wrecked the PWM signal generation. Neither the FloppyEMU nor the A9M0106 rely on the PWM signal to regulate drive speed; they simply "spin" (spin is in quotes for the FloppyEMU for obvious reasons) at different speeds automatically depending on what track they're stepped to. I know a 64k ROM 512k can boot fine from an external A9M0106, so if your machine will then, well, that's where you should start looking.

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Gentlemen, thank you for your kind replies.



I do not believe the motors are bad because they are very robust parts AND the controller PCBs inside the drive are what connect to the outside world.  In other words, all power and signals flow to the motors via the circuit boards inside the drive.  The PCB acts as a filter of sorts between the connector and the motor. Hence any problem with the drive, assuming my drives were damaged (which I don't know), should be on one or both of those drive circuit boards rather than a problem with the motors.  I am just a bit leery to plug my drives into my last known-good working 512K motherboard to test whether or not they are good.  


Again, my now "bad" 512K motherboard isn't technically bad in that it still works when booting from the FloppyEMU.  But clearly, some pins on the floppy port (internal and external) are now not quite right.  The problem is that I don't know what the proper signals should be when drives are connected and when nothing is connected.



The second "known good" 400K drive I used in my testing with the "bad" 512K motherboard is the one shown at 6:09 in my video here.  I also just uploaded high resolution photos of the top and bottom of each controller board to my Flickr Album. (I put text under each photo on Flickr so you can clearly see what is what.)


Here is the floppy drive connector pin-outs, taken from Macintosh Repair and Upgrade Secrets by Larry Pina:




I highlighted the External Disk Drive DB-19 section in yellow above because I am doing voltage testing at that external connector.


I think the diagram below of the Apple II Disk Connection is easier to visualize though.  I lifted that from the FloppyEMU development page. Below is my modded version of that graphic, adding voltage measurements I made on my "bad" 512K motherboard last night.  




As you can see, some of the pin descriptions in the Apple II graphic differ from the Pina graphic, but the Pina graphic descriptions should be correct since they alone refer to the Macintosh. (For example, pin 11 on the Apple II graphic says PH0 but the Pina graphic says CA0.  Pin 14 says PH3 on the Apple II graphic, but the Pina graphic says Write Strobe.  And so on.)


My voltage measurements shown above in the Apple II graphic were made with the Mac 512K (64K ROMs) powered on but not booted and nothing attached to the internal or external floppy drive connectors.  Voltages were measured with a Fluke 8845A benchtop meter. The 60Hz noise on /DRIVE2 (not connected?) and pulses on WRPROT (SPEED?) above were measured using a scope.



My intention is to swap in my known-good 512K motherboard later today and then do the same voltage measurements under the same conditions (not booted and nothing attached).  Perhaps that comparison of voltages at each pin would enable us to better decide if it would be safe for my known-good 512K motherboard (which has 128K ROMs) to be attached to my 400K floppy drives for testing.  Feel free to share any further thoughts in the meantime.


Ultimately, I want to know if my two 400K drives are still fine, and I then will need to determine what chip or chips on the "bad" 512K motherboard would need to be replaced, and then how to obtain those chips.


Thank you for your kind help!


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Well, I finished my no-load measurements at the floppy connector on my known-good 512K motherboard (128K ROMs) and am not seeing any significant differences (highlighted in yellow below).  I therefore still have no idea what's wrong in terms of both my 400K drives not spinning when I put in disks.




I must say there is a confusing set of differences among the Pina pinouts versus that Apple II pinout versus the Mac 512K schematic vs my SE/30 schematics.  For example, PINA uses CA0, CA1, CA2 & WR Strobe pin descriptions while the Apple's 512K schematic and the Apple II Connection diagram and the SE/30 schematics say PH0, PH1, PH2 & PH3. Also, the SE/30 schematics say Pin-10 of the DB-19 is +5V-DISK (which seems like an error on that schematic) and that Pin-17 is ENABL2, which is confusing because the 512K schematic (see my opening post) talks about ENABL1 and ENABLE2, plus that 512K schematic is very hard to read too! 


What I did was examine all of those sources and compile the most accurate set of pin descriptions in the above diagram.  I also confirmed the above pin descriptions match my Chilton's Guide to Macintosh Repair and Maintenance, pages 68 & 69, which says those "PH" pins are "PHASE." It's bad enough I don't know what's wrong with my machine, so confusing labels needed to be corrected for clarity.  


But if indeed my stupid jumper wire (connected to chassis GND) shorted Address line A10 or A11 or A12 (or maybe two of those together) on the SYP6522 "VIA" chip, that would have passed through the IWM chip and then on to the 68000 CPU.  It obviously shorted something because when I switched on power with that stupid jumper fallen onto my motherboard, I didn't get a bong.  Yet, after removing my silly jumper wire, the machine bongs loudly and boots just fine from a FloppyEMU!  Even so, my 400K drives don't spin (when connected to the "BAD" motherboard) when I insert 400K disks into them. I'm not sure what to think at this point.  Is it safe to connect the 400K drives to my known-good 512K board?  If my "BAD" motherboard harmed my 400K drives, I don't want those drives in turn harming my last and only known-good 512K motherboard.

Edited by JDW
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I went through my closet and found a 400K external drive I had forgotten about.  I had not used it in many years, but I tested it just now with my GOOD 512K motherboard and found it to be working.  With the Mac powered on but not booted and with the 400K drive externally attached, I put a ribbon cable on the 20-pin floppy header on the motherboard and made the measurements below.  (I only had a Yellow striped ribbon cable here at home, having left my 400K drive cables at the office.  The Yellow striped cable disconnects -12V pin-9 & PWM Speed pin-20, hence I could not make measurements on those pins below.)




The following pins on the ribbon header go between 0v ~ 5v while booting from a good 400K floppy:

Pin-2 PH0
Pin-4 PH1
Pin-6 PH2
Pin-12 SEL
Pin-16 RD


And the following data was measured at the ribbon header during computer's resting state after having booted to the Desktop from a 400K floppy.




And with an 800K drive internally connected to the ribbon header via Yellow Striped cable, not booted, I measure values at the external floppy connector that are similar to the measurements with the external 400K drive connected:



Edited by JDW
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And here are measurements at the external floppy disk connector, with the 800K internal drive booted from the same 400K floppy, again using my GOOD 512K motherboard (128k ROMs)...




I don't have a scope at home, so I couldn't measure the SPEED pulses, but the voltage my Fluke measured is more than 100mV higher than previous tests.


I'm not going to connect any more drives to my "BAD" 512K motherboard until I hear more speculation from others on this subject, and I am still leery of connecting those two 400K drives to this GOOD motherboard.  I still don't know if the BAD motherboard could have damaged those drives in such a way they would in turn damage a good motherboard.  That's really why I created this thread, to gather more thinking in that area.  Who knows, maybe those drives are still good and will work find with this GOOD motherboard.  Or maybe one of them will kill it.  That's my concern.


Look forward to hearing your thoughts.



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Still no replies, so I will continue talking to myself.  :sadmac:


As I said earlier in this thread, my FloppyEMU boots the "BAD" 512K motherboard (with 64K ROMs) perfectly, but I only have tested it connected to the external floppy port.  I am thinking about testing the FloppyEMU with a direct ribbon cable connection to the 20-pin header on the motherboard.  Not sure if that will make any difference.  But that is the only difference in the way my two 400K drives were connected (via 20p header) versus the way my FloppyEMU was connected (via ext. connector).


Anyway, looking at the relevant section of the schematic again...




...it seems odd the machine would work fine if my chassis-grounded jumper did indeed short one or two of those Address lines A9 ~ A12, since those lines are used by all 5 chips shown above.  And if the address lines are externally shorted but the CPU tries to bring it high, that would be a 5V shorted to GND scenario that I would think would fry the grounded Address lines on the CPU.  But if that had happened, why then would removing the offending jumper wire then allow the machine to bong and boot normally? 


If my two 400K drives were OK, I probably wouldn't even be concerned now.  But something caused them to suddenly stop spinning the disk, and since that only occurred AFTER that silly short took place, I cannot help but think the short had something to do with it.  One of the 400K floppy drives was connected to the 20-pin header at the time of the short.



Edited by JDW
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JDW asked me to take a look at this thread. If the motherboard boots from the Floppy Emu but not from a real floppy drive, the first place I'd look is the Mac's power supply. Specifically the +12V and -12V supplies. Floppy Emu doesn't use these, but a real floppy drive needs them to spin the disk. If you have another compatible power supply, try swapping it in and see what happens. I think it's mainly the +12V supply that matters, and offhand I don't remember how the -12V supply is used.


I know you measured the +12 and -12 pins and they look OK, but those were no-load voltages. The floppy drive motor probably draws a current of something like 1 amp. If the power supply can't handle that anymore, the voltage will drop as soon as there's a substantial load, and the drive won't spin.


Problems with the PWM (SPEED) signal should not prevent the drive from spinning, but you would get drive I/O errors. Floppy Emu ignores the PWM signal.


Swapping the internal and external drives sounds like a good next step. The only difference between the internal and external floppy connectors should be the /ENABLE pin. To confirm, you can use a multimeter in beep mode to check continuity between all the other corresponding pins on the two connectors. I think there's also some type of passive filter (the Bourns filter) on the external floppy connector, but not the internal one.


I'm confused by the evidence here, but IMHO it should be OK to try the "broken" 400K drives with another 512K motherboard to help isolate the problem. If you don't want to risk another 512K board, you could also use a Mac Plus motherboard, which is much more common and easier to find.

Edited by bigmessowires
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I certainly appreciate those insights!


Based on what you said, I will take one of the non-spinning 400K drives and put that inside my external 400K drive enclosure, then connected that to my good 512K motherboard and monitor the 12V & -12V power lines and the PWM line at the 20-pin header.  If the drive spins the disk in that scenario and voltages look right and PWM signal OK, I will repeat the test with the other 400K drive in that same enclosure.  If that drive also works, then the mystery question will be why those drives do NOT work with the "BAD" 512K motherboard.  I will then connect my FloppyEMU to the 20-pin header, rather than external floppy connector, and see it still boots.  If the FloppEMU still boots even from the 20pin header, then I will connect one of the two 400K drives to the "BAD" motherboard from within the external drive enclosure and see what happens.  If it still does not spin, then I don't think I want to risk putting my last good 400K drive back in that case and testing it with the "BAD" motherboard. 


What continues to boggle my mind though is why both of my 400K drives accepted the disk, spun the disk and spit out the disk just fine ONE TIME, but then after booting from my FloppyEMU (attached to the external drive connector), when I put in a disk, the motor would not spin the disk and the head would not move.  The 400K drive acts like it is dead EXCEPT at power-on when I see the head move a couple millimeters (which is normal).  You'd think if the motherboard cause the problem, it would have killed the drive instantly.  But again, the drive accepted a disk, spin the disk, moved the head, and spit out the disk normally and then after that the drive decided it wouldn't work anymore (except for that couple millimeters of head movement at every power-on).  It's baffling to me!


Anyway, I will report back after testing further, which won't be today since I left the drives on my office workbench.  Thanks again for sharing your expertise!

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Well, I'lll be darned.  The problem seems to have resolved itself.  I do not know how, but it's clear after my testing tonight that my "BAD" 512K motherboard (with 64K "A" ROMs) is not bad at all.


Before testing with drives this evening, I checked the HYTEK filter (equivalent to BOURNS 4120R-601-250/201that sits between the drive connectors and the rest of the motherboard, verifying capacitance and resistance were correct. I then tried my FloppyEMU on the internal (ribbon cable) floppy port and verified it still booted the machine fine.  I connected what I thought was my "BAD" Sony OA-D34V-22 drive from within my external enclosure, and to my delight it would spin the disk.  But it then froze the machine with a Sad Mac error 0F0064.  But after a little Googling I was reminded of the fact that the older revision "A" 64K ROMs are only compatible with the "OA-D34V" drive, and only the "B" 64K ROMs (or 128K Mac Plus ROMs) are compatible with "OA-D34V-22."


Anyway, I then connected my "OA-D34V" drive from within the external 400K drive enclosure and it booted! I then tried it from the internal ribbon cable, and it booted again!  Why it didn't boot last Friday is beyond my comprehension, but I'm overjoyed it's working now and that my motherboard is clearly undamaged by my inept short-to-GND!  I guess the Address lines can handle shorts to Ground.


I wish to humbly thank @Alex & @Gorgonops who very kindly tried to assist me earlier in this thread, and special thanks to @bigmessowires, whose FloppyEMU product I highly recommend!

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7 hours ago, bigmessowires said:

Glad to hear it's working again. I'm still suspicious of your power supply (analog board).

I plan to do more testing tonight. The key difference in what I did last night and last Friday when I had the problem was that I did not have the FloppyEMU connected last night at the same time as my 400K drive. Not sure why that would make a difference, but that’s what I plan to test tonight.

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Well, my testing is finished.  Even with the 400K drive on ribbon cable and the FloppyEMU on the external drive connector, it boots fine with either device.  +12V and +5V and -12V are all spot on.  So I'm still unsure what happen the event of that short-to-GND.  It did happen round about Easter time, so perhaps whatever was dead resurrected. :-) If any strangeness occurs in the future, I will update this thread.  Thanks again for your help, Steve!

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