Mac SE/30 won't read floppies (with a good drive), bad SWIM chip?

[willmurray461]

I've been working on repairing a Mac SE/30, and I've run into an issue. The computer won't read floppies, even with known working drives. A few months ago, it would read them fine, but suddenly it won't read them anymore.

Whenever a disk is inserted, the drive will spin and occasionally seek, but nothing happens. Eventually, Finder will report that the disk in the drive is either unreadable or uninitialized, even though that is not the case. If I try to format disks with the drive, the format will fail.

I've looked on the board and found no corroded traces anywhere. I'm now thinking I might have damaged the SWIM chip somehow (perhaps static electricity or hot-plugging in floppy drives?). Anyway, I have a IIci parts machine that I can grab a replacement SWIM chip from (I checked, the part numbers are the same). I'm thinking of doing a transplant, but before I spend all the time and effort, I was wondering if anyone here had any advice.

Has anyone encountered an issue like this, and were they able to fix it?

 

[bigmessowires]

Check your +12V and maybe -12V supplies. They're not used for much else, but they are needed for floppy drives.

 

[willmurray461]

The motherboard seems to be getting all voltages fine (+/- 5, +/- 12). The floppy connector also has 5v and 12v present. I didn't measure any -12v on the floppy connector, but according to the schematics it's not hooked up on the SE/30. Speaking of schematics, I guess I can use a multimeter to verify each pin on the SWIM is properly connected, although I'll be quite surprised if anything is disconnected.

 

[zigzagjoe]

-12 is only used by the audio amplifier afaik.

Check the floppy drive spindle spins when a disk is inserted. Also, it's very possible to have a bad ribbon cable - I ran into that on one of mine. No 12v, no floppy spindle spinning. It'd seek around a little before giving up.

 

[bigmessowires]

Sorry for my bad suggestion. You said the drive does spin, so that shows the 12V is working.

Some other thoughts:

• bad cable like @zigzagjoe suggested
• bad socket that the cable plugs in to
• damaged SWIM like you were speculating
• the Bourns filter on the floppy I/Os (not sure if the SE/30 logic board has this, but some other compacts do)

If you have a logic analyzer, you could capture what's happening during startup when the Mac tries to search for a floppy drive and boot floppy. You should see the /ENABLE signal go low briefly, and some changing values on HDSEL and PHASE0 to PHASE3. /WREQ should remain high. You can find the connector pinout online.

 

[willmurray461]

Update: I fixed it! TLDR: It was the SWIM.

Here's a summary of what I did:

I downloaded a copy of the schematics for the SE/30, and verified with a multimeter that every pin on both the floppy connector and the SWIM was properly connected. All of the pins checked out OK. I also did the same for the floppy cable. No problems. After seeing that, I decided to do the previously mentinoed SWIM transplant. Using a hot air station, I desoldered the SWIM from the SE/30, as well as another SWIM from a parts IIci. I then soldered the IIci's SWIM onto the SE/30, and voilà, it works! Not sure how I killed the original SWIM, but fingers crossed it isn't due to a fault elsewhere that may someday kill this new SWIM. Perhaps I plugged in a floppy cable backwards once? Seems unlikely though, as all the floppy cables I have are keyed. Anyway, all that matters is that it works now.

Also just FYI, if by "Bourns filter" you mean a dampening resistor pack, there doesn't seem to be any in the floppy circuit for the SE/30.