Resolved Macintosh 512K Schematic

[m1maverick]

Does anyone know where I can find the schematics for a Macintosh 512K (or, I guess, the original 128K would work too)?

The internal floppy drive is non-responsive when I insert a disk. I found a pin out (with voltage readings) and discovered the /ENABL line is being held at 4.47 volts (the diagram states it should be 0 volts which aligns with it being an active low signal). I'd like to see if I can determine why it is measuring 4.47 volts. The external drive connector /ENABL measures 0 volts and the system does boot from an external drive.

In researching it looks as if this model uses the Integrated Woz Machine (IWM) as the floppy controller. Anyone know if this pin is directly controlled by the IWM? Are there any pull up / down resistors for this pin? Any other troubleshooting suggestions?

System is the original 512K with 400KB internal floppy drive. External drive is a 400KB drive. Substituting the external drive for the internal one does not solve the problem. The drive doesn't respond just like the original drive.

 

[dmr727]

This is a question for the 68kmla:

68kMLA

Forum software by XenForo

68kmla.org

If anyone knows, these guys will.

 

[MacTech68]

I suspect the (usually) yellow/orange colored RC Network hybrid IC packages located at board reference A10 or A14. That is if you've eliminated the floppy drive and cable as a cause.

/ENBL1 for the internal floppy passes thru A14 pin 2 to pin 19 and is derived from the IWM pin 20

EDIT: PDF ZIP (Higher Res):

 

[m1maverick]

Thank you for the schematic. I performed voltage tests on A14 pins 2 and 19 and they read the 4.47 volts. Do you know what A14 is? It is labeled "Filter" on the system board? At this point I am suspecting the IWM as the culprit. I'd like to remove A14 to see if A14 pin 2 is 4.47 volts or not. If it is then the IWM is likely the culprit. If not then A14 is likely the culprit.

EDIT: I have attempted to troubleshoot with an external drive which I removed from the case and mounted in place of the internal drive...same behavior with either drive. I have also performed a continuity test of all the pins for the cable. Unless the cable is not making solid contact with the connector pins I think it is OK.

 

[MacTech68]

It's an Resistor Capacitor network. Equivalent replacement is a Bourns 4120R-601-250/201

Internal circuit is thus:

https://www.bourns.com/docs/Product-Datasheets/601.pdf

EDIT: At a pinch, for testing you could use eight 47 Ohm resistors, leaving the outer 4 pins (1, 10, 11 & 20) with nothing connected.

 

[dmr727]

Impressive MacTech! I'm sorry I doubted the level of vintage knowledge right here on MR!

 

[m1maverick]

I have access to some non-functional 512K systems from which I can obtain these parts. If the individual approves I will replace the U14 and / or the IWM from one of these boards.

Thanks for the assistance MacTech. I'll update the forum with the results (should the individual approve performing the work).

 

[m1maverick]

UPDATE: I sourced a working 512K and swapped the system boards between the two systems and the problem remained with the faulty system. Having done this I have eliminated the system board as the problem. The two system boards read 4.47 volts (problem system) and 4.3 volts (working system). At this time I have no reason to feel the .17 volt difference is relevant.

I am now going to focus on the analog board. Perhaps I should have started there but everything but the internal floppy drive is working on the failed system. Combine that with the 4.47 volt reading on pin #14 led me to the conclusion the system board was at fault. I have reached the conclusion that the ~ 4.5 volt reading on this pin is normal and the documentation I pulled from the Internet is in error.

The battery did leak and therefore I am going to inspect for any damage (initial, cursory inspection didn't reveal any). I will also pull the analog / system board connector and measure voltages for each system. Does anyone have any feedback on doing this? Do's or dont's

 

[m1maverick]

UPDATE: After spending some time thinking about this I realized one thing I had not tried and that was...test the drive itself and that's what the problem ended up being. I got off track with the voltage reading that I completely overlooked the obvious.

With that said I have tested the voltage on two 512K Macs and the voltage on the floppy connector pin #14 (on the system board) reads 4.30 - 4.47 volts. While this is a simple sample of two it is consistent. It appears the information I found on the Internet is wrong. Live and learn.

 

[MacTech68]

I guess anything is possible with respect to causes, however, I can't reconcile these two:

Substituting the external drive for the internal one does not solve the problem. The drive doesn't respond just like the original drive.

...I realized one thing I had not tried and that was...test the drive itself and that's what the problem ended up being.

And after years of troubleshooting, this bothers me a little, but that's just me.

As for the voltage on pin 14, I'd love to know what documentation you have for the /ENBL1 pin voltage on the floppy drive, for reference.

 

[m1maverick]

I guess anything is possible with respect to causes, however, I can't reconcile these two:

The fact I had thought I had done it Too many things going on and mixed it up.

EDIT: Forgot to add the diagram I was using. It was just something I found while searching and all but the voltage at pin #14 matched the "Good" side of the diagram:

 

[MacTech68]

The fact I had thought I had done it Too many things going on and mixed it up.

Happens to the best of us.