Floppy disks mount duplicated on Mac 512K

[mitchW]

I am having an issue with my recently restored 512K. That 512K was upgraded (RAM, SCSI, Mac Plus ROMs, etc.) - so it is essentially a Mac Plus., but I am not sure if upgrades are the culprit.

Original 800K drive was beyond repair (bent heads), so I managed to find and repair a 1.44MB drive (MFD-71W), that should work in this Mac.
That drive works, ejects disks, formats disks, etc. But no matter what System software I boot (I used 1.1 and 5.5), I get duplicate mounted disks on desktop. So I get two System Software disks mounted on desktop, despite I only inserted one floppy. If I eject one disk, it will eject it, but the other will stay mounted.

When I checked the info, it sees one disk as it is in internal drive, and other in external. Of course I only have internal floppy drive hooked up!
Even if I boot to MacPaint 400K disk, it does the same thing, or if I boot off the HD20SC, it is also the same.
Did anyone have a similar problem? I would like to get it solved.

 

[jmacz]

If I remember correctly there are two enable signals, one for internal and one for external, both from the IWM. Curious if they might be shorted causing the internal to respond as both external and internal. Or maybe something’s wrong along these lines with the IWM. I think there is a shared filter pack between the IWM and the two floppy ports (internal ribbon connector and external port) that could perhaps be shorted as well.

BTW, you said the 1.44 floppy should work.. it certainly won’t work as a 1.44mb floppy since the 512k and Plus (and most SEs) had the IWM instead of the SWIM chip.

 

[volvo242gt]

Also, are you using the correct cable for the drive? Most 800K drives, and all MPF-75W 1.4MB drives use the red stripe cables.

 

[bibilit]

Yes, i will say the same thing, never seen that before but probabaly an issue with the board, cable or component on the LB.

you cannot complain, you have a second drive for free.

 

[mitchW]

Yes, 1.4MB floppy does work in Mac Plus, but only as 800K drive. The original cable has a red stripe, but previous owner cut the 2 lines (pin 9 and 20), so essentially this converted it into yellow strip cable. But if I am not mistaken, this modification was only required for some early 800K drives. Newer drives (also my 1.4MB unit) apparently work with either cables. I can try reversing the modification to see if it will work any better.

Next, as I have a spare Mac 512K logic board (stock, only has newer ROMs from 1985-ish), I hooked that one up to the same floppy drive with the same floppy cable and it worked flawlessly, just one floppy disk was mounted on desktop. So this will confirm my suspicion that both cable and the drive are okay.

As far as I checked the logic board schematics, it could be an issue with a filter, some select logic / buffer or a bad IWM. Or could simply be an issue with ROM revision and not using the right floppy cable.

 

[LaPorta]

I'd probably think a filter issue, there can be ghosting issues if they are not working properly. Best thing to do is to test the filters (basically a resistor pack) against states values in the Pina guides:

https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

 

[mitchW]

I'd probably think a filter issue, there can be ghosting issues if they are not working properly. Best thing to do is to test the filters (basically a resistor pack) against states values in the Pina guides:

https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

Thanks, but I don't see any values of the resistor packs in this PDF. It mostly covers analog board repairs / upgrades.

I did find schematics for 512K board and I am including a floppy controller part here. Any suggestions what I should measure first?

 

[LaPorta]

I must have had the wrong book, sorry:

https://vintageapple.org/macbooks/pdf/The_Dead_Mac_Scrolls_1992.pdf

 

[mitchW]

Anyways, I found the problem and actually repaired it.
One of the lines (ENBL1) from the IWM was reading just 150 ohms to ground. On the other board, this line measures about 3M ohms! As it passes through the RC filter (FILTER2 / 115-0002), I figured I should first isolate it first. I sucked the solder out of the pins 2 and 19 (input and output). But even when I just added solder to make desoldering easier, the resistance to ground changed, this time it was reading around 1K and jumping around. Removing both pins (in the end, I just snipped the legs from the top), made the resistance go up into megaohms.

In the end, I recreated the RC filter by using some 402 parts from a MacBook donor board. I used two 33 ohms resistors and a 220pF capacitor.
I think it is close enough, as this they just pass an enable signal, and will work fine if I just shorted it to the IWM (risking blowing up the IWM if something goes wrong of course, so don't do it). Originally the filter has two 25 ohm resistors, and a capacitor to ground between them.

So in essence, that one part of the RC filter had low resistance (short) to ground, most likely caused by internal 200pF capacitor. As this was on ENBL1 line, which enables and disables the floppy drive, it would make sense to cause problems like this.

I could replace the whole RC filter as I see they are being remanufactured, but for now, this is a good enough solution