SE Dual 800k - Issues with Lower Drive Port

[LaPorta]

Hi everyone,

I've been troubleshooting a somewhat bedeviling problem with one SE motherboard that I have. When a floppy drive is connected to the Lower Drive slot, it is unable to properly function. By this I mean you can start up the machine, and stick in a floppy. Sometimes I will get a happy mac, then it will read for a second or two then start doing the "searching" motion with the drive head, and then a System Bomb message will appear on the screen. Other times, if you insert the disk, the drive doesn't even attempt to read the disk and ejects it after two seconds of sitting there. Either way, it does nothing but give errors or not work.

The Upper Drive slot and external connector work perfectly without issue. It is not the drive, because I have tried multiple drives in the Lower Drive slot, and they all do the same thing. Now, I figure it is not the IWM chip simply because the other ports work fine. I looked on the back, and thought that under the microscope that there might be some dry/cracked solder joints, so I re-soldered them all, as well as the filter chip near it. No success.

Is there anything that someone can suggest that might point me in the right direction? Is there one filter that goes to that port specifically that might need to be replaced? I am pretty lost here.

 

[AwkwardPotato]

Those filters are known to cause problems similar to this in the SE and SE/30.

 

[LaPorta]

So is there any one of the multiple filters that is responsible for that port? How can I tell which is bad?

 

[AwkwardPotato]

Couldn't find the SE schematics so I checked my own board. Turns out that both of the filters next to the floppy ports are connected in some way between the IWM and both floppy ports, so I'd replace both. According to this post on ThinkClassic they can be replaced by isolated, 16-pin 47-ohm resistor networks such as these.

 

[LaPorta]

So I read some of my Pina Mac Classic and SE secrets. He pinpoints this sort of thing to U8B, one of those specifically. He gives specific values for resistance, and what is good and what is bad. I tested it vs. his values, and also another board that I have. Nothing appears amiss. I may try replacing them both with the scrap board that I have and see if that works.

 

[LaPorta]

Couldn't find the SE schematics so I checked my own board. Turns out that both of the filters next to the floppy ports are connected in some way between the IWM and both floppy ports, so I'd replace both. According to this post on ThinkClassic they can be replaced by isolated, 16-pin 47-ohm resistor networks such as these.

Unless I am seeing something off, I ordered these and then realized when they got here that they have 16 pins. The ones on the board have 20 pins...

 

[AwkwardPotato]

According to the post I linked, the 16-pin resistor networks will work if offset by 1 position (in other words you'll have 2 unused holes on one side of the network and 2 unused holes on the other side). The datasheet for the stock filters show that the two pins on each end (1, 10, 11, 20) are all connected, but this doesn't seem to be necessary if a resistor network is used as a substitute.

 

[LaPorta]

Interesting. Alternatively, I’m assuming these bridges could just be replaced by a bunch of 1/8 watt 47 ohm resistors across the terminals.

 

[jimjimx]

I was going to put something here, but it was wrong info.

Nevermind 

 

[AwkwardPotato]

I’m assuming these bridges could just be replaced by a bunch of 1/8 watt 47 ohm resistors across the terminals.

Sure, that would work fine too; it would be the same electrically.