Duodisk issues

2 replies [Last post]
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Joined: Apr 28 2017
Posts: 2

Hello,

Recently, I've gotten into retrocomputing by getting my hands on an Apple //e. It's been working fine while I've been having fun with it for the last few months, but I started to get some problems with the duodisk drive my computer came with. Initially I was getting "Unable to load ProDOS" errors while trying to read ProDOS formatted disks, but it would still read DOS 3.3 disks just fine. I tried to use some repair techniques I read about such as swapping disk drives, putting the card in different slots and reseating chips on the Duodisk analog board, but this didn't change the drives behaviors at all. Then, when I was swapping connectors on the disk drives, I accidentally replugged it to the computer while the it was still on, resulting in spontaneous drive activity and the possible death of the analog board. Surprisingly, the drives would still turn on, only now there was no R/W head movement, no drive activity lights, and it would spin indefinitely. However, the drive indicator lights would turn on for a second when I powered down the computer.

I decided to replace some IC's on the analog board (74LS125A, MC3470AP, and CA3141E) that were specified in the Duodisk repair manual, but that didn't do anything. I then replaced the analog board and controller card with some spares I had, but again didn't change the behavior. I finally experimented with different combinations of the analog boards and cards to no avail.

I believe the problem maybe with the actual computer itself rather than the drives, but I don't have the expertise to go through and debug the computer. Do you guys know what the problem might be?

Some technical information:
-Apple //e has been upgraded to the "enhanced" version
-The original controller card was a 655-0101 and the analog board
was a 676-102. The replacement card is identical while the
replacement analog board is a 676-101.
-Both analog boards had 2 capactors snipped off to prevent the
drives from chewing disks after power cycle.
-The replacement IC's for the original analog board are DM74LS125AN, CA3141E,
and MC3470N.
-All the accessable traces on the cards and analog boards have been
tested for conductivit

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Offline
Joined: Apr 28 2017
Posts: 2
Re: Duodisk issues

I began to test my controller cards by connecting wires from the the DB-19 connector to breadboard and a multimeter. It's not as accurate as probing with logic probes or an oscilloscope, but it was quick and simple. Good(?) news is that both cards seem to act the same, except that when booting with the replacement card, pin 11 (01) is outputting 4.30V while the original card is at 0.02 volts. The 12V Pins of the connectors and the motherboards slots are also about 0.20V higher than they should be.

Here's a link to the data if anyone's interested.

Also does anyone have any schematics or repair information about Duodisks and compatible controller cards? There seems to be little to no serious documentation out there on them.

Offline
Joined: Jul 31 2014
Posts: 34
Re: Duodisk issues

There is a schematic of the duodisk analog board here, might be useful.

http://bitsavers.trailing-edge.com/pdf/apple/disk/5_inch/

/Rob