Looking for schematic for MITAC Model AD-1 and SHUGART Model 390 Alternates for Apple Disk II Drives

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cfkerchner's picture
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Looking for schematic for MITAC Model AD-1 and SHUGART Model 390 Alternates for Apple Disk II Drives

Hi all,

Does anyone have a schematic or know where to find one online for either of these two DISK II alternate brand vintage floppy drives? Looking for schematic for MITAC Model AD-1 and/or SHUGART Model 390 vintage versions of the Apple Disk II 5 1/4" floppy disk drives.

In particular, I need the MITAC AD-1 schematic as I own two of them. I did one in by placing a new slightly longer cable on the wrong way the other day, blowing up the 74LS125. I mean it was literally in pieces. All the ICs in this beast are soldered in. That makes repairing even more tedious. And it has TEN (10) IC chips, all soldered in, instead of the only FOUR (4) ICs in the Apple Disk II drives, which for mine those ICs are all on sockets. Which makes repair of the Apple Disk II drives easy, relatively.

I desoldered the 74LS125 chip today and put in a 14 pin socket and plugged in a new 125 chip. But it still don't work. So more damage was done.

Without a schematic my only recourse is comparing voltages and signals on the remaining good MITAC Model AD-1 to the bad one. That and circuit tracing by vision and hand. A schematic would make the troubleshooting and fix so much easier. Plus having a good schematic for equipment you own and maintain in one's library is always a good idea. Especially as more and more years go by re these vintage drives.

I've all ready ordered a complete set of replacement ICs (those that I could still find as available) of the 10 IC chips in the MITAC.

Here is a listing on eBay of someone who is selling one of the clone drives.


Based on this fellow selling this he must believe the MITAC AD-1 and the SHUGART 390 are virtually the same thing. So I'm of the mind that the circuitry may be much more similar in these two 3rd party suppliers of Apple floppy drives, than they are to the DISK II drives which have only 4 chips. Maybe one company made both and sold it private label under the other brand name.

But I don't want to buy another MITAC aka SHUGART, or vice-versa. I want to fix mine. And I will. But a schematic would make it so much easier.

I googled tonight and could not find a schematic for any of those model numbers or brands. So, if anyone has one, let me know and we can arrange to get a copy. Or if you have a link to one, post it in a reply.

Thanks in advance if any of you can help with this repair and troubleshooting documentation item.

Last seen: 2 days 15 hours ago
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The Shugart 390 is the model

The Shugart 390 is the model designation for a stripped down mechanism consisting of the frame, stepper motor, spindle and drive motor and the speed control board at the back, the black fceplate and door, plus the read/write head and its sled.

That's what Apple bought from Shugart, who then added their analog board and shell to turn it into a "Disk II".

It was basically a striped down Shugart 400, which was far more complex and expensive.


That Mitac drive on eBay is a Shugart 390 mechanism with Mitac's own clone analog board.  Dpending on the era, full sized Mitac drives have clone analog boards with nearly exact circuits to the Disk II.  Four chips.

Your Mitac may be different -and you noted so - Mitac did make a family of half-height drives.  Their analog boards will be different.


As for schematics, you'll probably find you'll run out of luck finding any pretty quickly for any Mitac drive.


Show us some photos of you Mitac drive's internals.  I'm curious.

Here's some pics of the Mitac AD-1 drives that I have (with Shugart 390 mechanisms):

cfkerchner's picture
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MITAC Model AD- floppy drive pictures - Front, Top, Bottom, Guts

Hello Baldrick:

Here are some pictures of the MITAC Model AD-1 5 1/4" floppy disk drive - Front, Top, Bottom, Guts. 

In the Guts shot you can see where I have removed the blown the 74LS125 chip that was soldered in and I replaced it with a 14 pin DIP socket and a new 74LS125. But as I mentioned, it still don't work. So there is likely further damage from me plugging in the disk drive cable wrong a few days ago. Maybe the MITAC circuitry is more vulnerable to damage beyond the LS125 than the Apple drives are. That is why I'm looking for a schematic to further diagnose the problem without having to desolder all the chips and socket and replace them all, etc., or tracing out the circuit visually and making my own schematic to help deduce what else could have blown.

MITAC AD-1 front view:  http://www.kerchner.com/images/computers/MITAC_AD-1_Front.JPG

MITAC AD-1 back view (with brand label and specs):  http://www.kerchner.com/images/computers/MITAC_AD-1_Back.JPG

MITAC AD-1 top view:   http://www.kerchner.com/images/computers/MITAC_AD-1_Top.JPG

MITAC AD-1 Bottom view:  http://www.kerchner.com/images/computers/MITAC_AD-1_Bottom.JPG

MITAC AD-1 Guts view:  http://www.kerchner.com/images/computers/MITAC_AD-1_Guts.JPG

As you can see, the analog circuit board has a lot more chips on it than the Apple Disk II drives, and they are all soldered in. The one "chip" that is duller looking than the others and difficult to read is a "7406   PC".  It is not blown. It just looks duller than the others.  I think it is some kind of jumper IC to connect traces together using a chip instead of individual jumpers.  Not sure about that. 

I know I will probably get it fixed one way or another since I have a working one to compare voltages and signals to, etc. And I have ordered all the chips on this MITAC and I'll get them in a week or so.  But if I had a schematic it would save me a lot of time.

I also considered looking for an Apple Disk II analog board in working condition and just swapping in that board into the MITAC unit since much of the other parts look very similar. But I haven't found any of those analog boards offered for sale in working condition, or even at all, in my searches for one of those.  Hmmh, I wonder if that MITAC drive in your pictures had that swap-in of a Disk II analog board done to it at some point in time. Just thinking out loud as to why the guts of mine which I bought back in 1982 look so different as to the analog board than your MITAC guts view.

I'd be interested in your thoughts or ideas. 

P.S. Is there an easy way to insert the pictures into posts and replies like you did. Where is the image insert button?  Or what is the correct HTML syntax to do it for this forum. Thanks in advance for the help on that too.


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Inserting images is easy -

Inserting images is easy - look for the "media browser" button in the reply window's header.

You can upload your pictures to the server and hav them displayed.  If they're really large when they get inserted just double click on them and a parameter window will appear.  You can select the width of the image to display in pixels (800 is best) .  Don't put in a height - the system will calculate that to keep the image proportional.  The original image will still be there in full size if someone wants to "zoom-in" on an area but it will display smaller for casual viewing.

The 7406 is a hex-inverter chip: 



I bought my Mitac drives in 1985 or 1986 - that might explain the difference.  The analog boards on mine are far simpler so maybe it was a cost cutting measure (a smart one at that).


As for repair...What I'd do in absence of a schematic is follow the +5V and +12V lines.  Find out where +5V and +12V would have been routed to with the cable mis-plugged.  Target those ICs to test/replace.

Also, what's the symptom?  No read?  No write?  Doesn't spin?  Head doesn't move?  Those functions are all controlled from different areas on the analog board.


cfkerchner's picture
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Troubleshooting ideas

Thanks for the troubleshooting ideas.

I ordered a set of IC chips found in the MITAC from Quest Components. I should get them in a week or so. I will then proceed to try and troubleshoot it further since changing out the 74LS125 did not fix it.  I probably won't get to working on this drive for at least a week since at the moment I'm in the process of upgrading an old PC from a Pentium III motherboard to a working Pentium 4 that I got hold of for $37.00 Got it working on the bench.

The next step is to try and find a way to get my existing Windows XP PRO (which was installed as an upgrade on the Pent III machine of the Windows 98 which was on it when I bought it about 20 years ago from small scale, no-name local manufacturer) ... on the Pent 3 system over to the Pent 4 system now in pieces but in working order on the bench, without having to do a complete Windows XP PRO install on the new hardware with a cloned disk attached, cloned from drive C of the Pent 3 machine.

And I'm told I won't be able to do it and I'd have to buy a original full version of Windows XP PRO and do a direct install on the newer hardware HD and then port over all my apps after I get XP Pro running on the Pent 4 hardware setup on the bench, etc. So far all the tips and tricks to do a HD clone type transfer from the Pent 3 hardware to the Pent 4 hardware, with its attached cloned HD of the Pent 3 OS, that I found online have not worked. It just won't boot up past the BSOD error message window. What I did was temporarily intall an XP Home edition I had laying around and I now have the message saying I have 30 days to activate it everytime I turn on the on the bench Pent 4 "machine". So I know all the hardware is working.

Microsoft sure made it not easy for moving XP to a new hardware machine. So I'll likely have to buy on eBay a copy of XP Pro full version. This is my first time working on a PC motherbord swap.  Oh well, having fun at the ripe old age of 76 with the ripe old age hardware in my cellar. :-)

Please excuse the off topic PC ramblings here on the AppleFritter forum.

Getting back onto the MITAC version of an Apple Disk II, here is a picture of the guts, the analog board of my not working MITAC.  The 74LS125 that I replaced and is now on a socket is in the lower left corner, 2nd chip from the left.  Doing this image in a message thing here is as a test, practice of inserting a picture into a message, per your lesson. I uploaded it full size and then did the double click and set the width at 800 as you suggested.

Thank you for the image insertion tutorial for this site.

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XP was the absolute worst to try to move to different hardware.  Like you've seen, it just doesn't work.  A complete format and reinstall is the only way to get XP functional.  Vista and Windows 7 can be hit or miss but usually semi-works at least in safe mode long enough to do some tweaks and then get it to reinstall drivers although depending on the change in motherboard brands - sometimes it also refuses to work. Windows 10 is pretty easy to move around.  But not Xp...nightmares...

cfkerchner's picture
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You are correct about moving XP OS to new hardware - not easy

Hi Thellmer:

Re my interim PC motherboard swap and upgrade project while waiting for Apple II+ repair parts:

You are correct. After trying all the tips and tricks posted online, none worked. In fact one site tried to sneak me some malware in their "this will do it for you" freeware site. As I always do, I scanned it first with my virus checking software after the download. And seeing the warning pop up, I deleted the file.

In the end I re-installed an XP Pro full edition on the new old Pentium 4 system in pieces but working on my work bench. The new install went very smooth. I even got a dual 5 1/4" and 3 1/2" floppy drive hooked up to it on the bench. Other than LAN and internet connection its a fully working Windows XP Pro machine scattered all over my work bench but working fine.

All I'm waiting for now to finish my motherboard upgrade project is a PCI Ethernet card that is arriving Sunday from Amazon. I don't want to pull the card from my Pentium II unit since it won't then have LAN or internet access. And I want the two machines to be talking to each other and transfer some data that way, before I pull the Pent III all apart.  PCI ethernet cards are dirt cheap. When I get it,  I'll get everything up and running including home network LAN working and via that to my router and internet access.

Once everything is checked out and running flawlessly for a few hours of burn in time on the bench, I'll then gut the Pentium III motherboard out of my tower and put the new old used but working great Pentium 4 motherboard in that I bought for the whopping price of $37, which included the CPU and and 750k of RAM (I have lots of RAM that I can upgrade that too later if I wish), and away we'll go with being able to run some newer updates of some the software on that legacy machine (which has some specialty cards in a couple slots and legacy peripherals on it), ... that some of that said software will not run on a Pentium III, like my virus checker which no longer will do definition updates cause the Pent III can't execute an instruction they now use, and also my hard disk backup software for the same reason. It will be good to get the old machine up and running at full strength, and safe again. I want to keep that old baby running as long as I can. It's my tinker with machine. It will be the one permanently connected via a serial cable to the Apple II+. They are in opposite corners of my cellar.

The working but old almost obsolete (for much modern software at this point) Pentium III motherboard, and the spare boards for the various slots I now have, will go into a cardboard box on a shelf in my cellar. I rarely throw anything away. Never know when I'll want to go retro, etc., or salvage some item off an old board.

It's been a fun project and a great learning experience having never done a motherboard upgrade before, in all my 76 years on this earth. :-)

Re Apple II+ bring back to full life after 40 years on a shelf project:

Then I'll be going back to work on repairing the two blown items on my Apple II+ system ... the original power supply which I need to re-cap and I"m still waiting for that re-cap kit from Console 5, and the MITAC Disk II unit which I have to troubleshoot and find the other blown (likely) IC, beyond the known 74LS125.

Studying the board and the front end from the ribbion cable connector and its similarity to the Apple II Disk II unit at that part of the circuitry, and thinking about what would likely get damaged by plugging in the disk drive connector wrong, which I did, I have deduced it is likely the ULN2003A chip. That chip is the one in the extreme lower left of the MITAC analog circuit board, just to the left of the LS125 chip, in the picture I posted previously in this thread. And all the darn chips in the MITAC are soldered in and not socketed. Once I get the chip set I ordered from Quest Components, I will de-solder that one ... socket it, and plug in a new ULN2003A chip and see if that fixes it. If not I'll work into the more complex circuitry of the MITAC analog board until I find what's blown and fix it.

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Wanted to post this days ago

I wanted to post this days ago regarding the Shugart 390.  I have a pair of  original Shugart 390 drives with the boards that came with them.

From what I've read and the best I can tell, these are much like the drives Woz received from Shugart when he introduced the Disk ][ to the public.

The Disk ][ Drive Card in this photo is a replica I made that looks a lot like the original card that Woz hand-wired at the time.  I thought it'd be cool to replicate his effort.

The card shown in the photo is fully functional and has 13 sector fuse-link proms. I made up early DOS disks that go with it as well.  It was a fun project.

I've since put Disk][ cases around the drives with the Shugart Logo on the face. (that photo is not shown)


cfkerchner's picture
Last seen: 7 months 9 hours ago
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Nice work on that home-made interface card

Hi Macnoyd:

Nice work on the home-made interface card.

Last night after five beers and at about 2:00 a.m. in the morning I decided to fix my blown and not working MITAC flopply drive, without a schematic. I just went about it using an old analog ohmmeter R1 setting on an old analogy multimeter that I've had for decades and which I prefer to use when "ringing out" unknown circuitry PC boards. It uses a single plain 1.5V AAA battery. Enough to forward bias an in circuit transistor or diode if need be but not enough to hurt anything. And watching the needle move slowly or just ever so slightly gives me better insight as to what is going on, capacitor charging up, etc., than a modern digital VOM.   JMHO.   I started my electronics education in the Navy in the analog world and matriculated to digital while in college as that world developed. In college for a digital design project, I designed my first microprocess controlled device using the Intel 4004 brand new invention way back when. I started engineering college with a slide rule and ended it with a HP-45 digital scientific calculator, and they both still work.  :-)  And the 1.5VDC won't hurt anything in any Apple board I'd be "ringing" out.  But of course I use the modern digital True RMS VOM I have for voltage readings. Each tool has certain special purpose uses, etc.

My diagnosis methodology:  I took a known good board out of my still working 2nd MITAC unit and placed it side by side to the non-working MITAC board on my table and started checking the unergized boards with cable disconnected ohmic readings on the cable connector front end circuitry chips ... the 74LS125 and the ULN2003A. And since I knew I had blown the LS125 by putting the cable on the connector wrong, and had replaced it using a socket and new chip, I decided to read out those 14 pins on the socketed LS125 in the non-working board compared to a known good LS125 in a known good and working MITAC board.

I had one end of my analog VOM connected to the +12V connector pin (pin 19) using a small push on connector. And the other lead was then free to poke around and take reading. That would be my starting point reference point. I had the VOM set on the R1 scale.

I started with the LS125 on the basis that it was the chip I blew up in the first place and on the old "what did you touch last" when things don't work. Sure enough the one pin for the LS125 in the socket on the non-working MITAC board ohmed a very different reading from the working MITAC board with the soldered in LS125. And then when I pulled out the LS125 and read the pin without the IC in, it was totally open. Thus with the IC in I was getting an ohmic "Thevin equivalent circuit" reading of only the IC and not the parallel resistance of the rest of the board if the socket pin was properly connected to the board solder trace, which I now know it was not. So with the LS125 I was getting a substantially higher ohmic reading than compared to the good board since I was only reading the LS125 ohmic resistance since it was not properly connected to the parallel ohmic resistance of the rest of the board. So bingo, I knew I had a bad solder joint connection UNDER THE SOCKET where I could not see it.

So, I fired up my new handy-dandy vacuum desoldering device that I just bought which heats the donut traces and sucks out all the solder. It worked terrific. Much better and easier than using solder wick and a soldering iron which is how I pulled the blown LS125 out a week or so ago. It is so good to have the right tools. I should have bought one these electric vacuum desoldering devices years ago.

I pulled out the old socket and sure enough there was a donut hole pad broken from the trace to it under the socket on pin 14 (+5 VCC) of the LS125. It mut have been loose from the desoldering of the blown chip a week ago and when I put in the socket somehow it pushed the broken donut pad away from pin 14 and never took solder via the plated through hole from the other side of the board.

I cleaned up all the holes with the new vacuum desoldering device. jumper patched the adript donut hole, put a new socket in, soldered all the pins to the board, and then fired up the board in the MITA drive. And whoopie, it works fine again. I had to use a 16 pin socket this time since I did not have any 14 pin ones with long legs in my parts bin. And I wanted a connector with long legs so I could mount the socket and see under the socket and/or touch up solder there if needed. So I just cut off the extra two unneeded pin for now. If I re-socket any old Apple II boards after this I'm always going to use longer pinned sockets as a practice and leave them standing up above the board surface so I can see and probe under them if needed, etc. See below picture. You can see the LS125 in the lower right section on a new 16 pin socket with the last two cut off and unused. Continued below the picture.

So, just like the Apple II Disk II drives, it appears it is ONLY the 74LS125 that gets blown when you plug the cable in wrong. So I decided to find out why such a violent outcome from a cable connection error. I checked into the circuit by tracing things as to why, and found out its because the pins on the LS125 see both +12V and -12V on several pins when a connector is plugged in on a wrong row of pins and thus its internal circuitry sees a net delta voltage across the chip of 24 Volts instead of the usual +5V. Thus, pow!  They blow up.

So all is well and I no longer need the schematic for the more complex MITAC analog board.  The front end circuitry from the cable is virtually identical to the Apple II Disk II drives, and I have a schematic for them.

Thanks for all for the tips and feedback on this thread. Hope my long saga and solution will help some other newbie in fixing a blown floppy disk drive analog board, MITAC or APPLE.


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