DuoDisk -107 analog board repair, plus general questions about FDD functionality

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DuoDisk -107 analog board repair, plus general questions about FDD functionality

Hi all!

So I have a DuoDisk that was on the fritz that, through a comedy of repair mishaps, is nonfunctional at this point. I'm waiting on parts and I'm pretty sure I can fix it, but I've got some questions that I've not been able to answer through googling.

First, I have a -107 analog board. I know there's a schematic put there but I'm reasonably sure it's for the -101 or -102 board because it has the MC3469 write chip, whereas my -107 board has no specific write control chip. Best as I can tell, Apple reimplemented it themselves with other chips and components, I'm guessing as a cost-saving measure. My question is whether anyone has a schematic for the -107 board.

Second, probably applicable to all board revisions, is what the potentiometers on the board do and whether any component replacements will require adjustments. I'm particularly worried if they deal with the analog side of the MC3470 read amplifier (which I don't intend to replace). If they do require adjustment, does anyone know what the adjustment procedure is?

Third, probably related to the second point, does anyone know how to use the built-in test points? I'm sure they were used during quality control, and provide a means of verifying functionality. I'm curious if anyone knows what Apple's testing procedure and parameters were.

Finally, as you may know, the funny behavior of the MC3470 is apparently key to some copy protection schemes. And, as with many FDD-specific chips, it's no longer in production (yes, new old stock and NTE rebadged chips are still out there). I'm curious about what other chips or solutions are out there, particularly what clone drives used (I have seen at least one clone Disk ][ analog board that definitely didn't have a MC3470), and how they perform with that copy protection.

Thanks for any information you might be able to provide!

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I am not familiar with the

I am not familiar with the DuoDisk but just wondering if you visited this page: https://www.applefritter.com/appleii-box/H084_1_AppleIIDiskService1.htm

 

I hope it helps?

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tony359 wrote:I am not
tony359 wrote:

I am not familiar with the DuoDisk but just wondering if you visited this page: https://www.applefritter.com/appleii-box/H084_1_AppleIIDiskService1.htm

 

I hope it helps?

Thanks for the link! I'll take a deeper look at that page. I see there's a page at the end about the DuoDisk (though unfortunately most of the images are broken).

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I have a good deal of repair

I have a good deal of repair information, and I also ave DuoDisk analogue board cores. 

 

What precisely is yor problem>

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Timelord wrote:I have a good
Timelord wrote:

I have a good deal of repair information, and I also ave DuoDisk analogue board cores. 

 

What precisely is yor problem>

Hi Timelord, so here's what's happening: Drive currently boots fine from known-working disks. Attempts to initialize or write data to any disk fails. In DOS 3.3, INIT fails with an I/O error on both drives on known-working disks. Prior to the repairs I just finished, ProDOS would claim known-working notched disks were write protected in either drive; I haven't tried it since repairing since, in my initial catastrophic tinkering with it, I managed to corrupt my only copy of ProDOS (I loaded a file, then tried to save it, and instead of failing write protected, it attempted to write, failed, and the disk won't boot anymore; consequently I'm skittish about trying to save files to non-blank disks right now). Prior to repair, downloading files via ASCII Express and trying to put them on disks failed with an I/O error (I haven't tried it again yet).

After fixing the things I messed up (had to resocket and do some bodges), I also rechipped everything but the HA1127 (labeled CA3146 on some schematics) and MC3470 (I can't see how the MC3470 would affect writing, the HA1127 seems an unlikely point of failure, and both are out of production anyway), and also replaced the electrolytics on the analog board just in case. I was told the 74LS125 was a likely point of failure so I ensured that I replaced that chip.

After these repairs and replacements, the drive boots like new, but still fails on INIT in DOS 3.3 on both drives (but instead of almost instantly giving an I/O error, the drive sounds like it's writing for awhile, and then when you expect it to finish, it throws an I/O error). Loading and saving individual files to a disk in DOS 3.3 throws a "disk full" error (cannot confirm it's a working, formatted disk, since I don't want to risk corrupting another non-blank).

I have cleared the drives out with canned air, which did not change this behavior at all. I have not cleaned the R/W heads nor have I cleaned the notch sensors yet, in part because it's both drives misbehaving in exactly the same way (seems unlikely to me that both would fail in exactly the same way). I know writing on drive 1 worked perfectly about a week ago (which was the first time it had been used in about a year, and before which it had sat in the basement for probably 20 years). I'm planning on looking at the write protection sensors and, while I'm in there, cleaning the R/W heads though really they looked immaculate last time I opened it.

To recap: The drive will boot from working disks, throws an I/O error on DOS 3.3 INIT (both drives), and does not seem to be able to write data to formatted disks.

In terms of equipment, I do have a decent digital multimeter and know generally how to probe things, but I do not have an oscilloscope (I may get one, I've been wanting to get one for awhile and fixing/studying this thing gives me an excuse).

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And just an added note: I

And just an added note: I have now cleaned both R/W heads with 91% isopropyl alcohol to no effect. They were indeed immaculate. I also attempted to clean the write protect detectors at the same time, no effect.

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I've seen a few drives with

I've seen a few drives with this issue. It occurs when someone plugs or unplugs the drive while the machine is hot, or plugs it into the wrong device port. The issue could be either on the controller card, or the drive analogue boards. As both drives do this on a DuoDisk, it is possibly the controller card, but the analogue boards are the first suspect. The four ICs to replace, in order of likely failure, that commonly fail are:

 

74LS125

3146

3470

2003

 

The 74LS125 is the most likely culprit. 

 

However, as both drives have this issue, you should also chec the controller card. It is functionally identical to a standard Disk ][ card, so if you have a known working Disk ][ card, you can swap the ICs over from one of them. The only difference is that instead of two headers, it has a hardwired 25 pin DSub connector on it. 

 

While you are in there, check to see if any of the big caps are burnt. 

 

 

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Timelord wrote:I've seen a
Timelord wrote:

I've seen a few drives with this issue. It occurs when someone plugs or unplugs the drive while the machine is hot, or plugs it into the wrong device port. The issue could be either on the controller card, or the drive analogue boards. As both drives do this on a DuoDisk, it is possibly the controller card, but the analogue boards are the first suspect. The four ICs to replace, in order of like

Thanks for the response. I'd actually already replaced most of the chips on the analog board (just not the HA1127 and MC3470 because they're a pain to source; every single other chip and all three electrolytic caps on the analog board are new). Nothing changed initially.

However, a few days ago, the drive began to work perfectly for no clear reason. The only thing I'd done between rechipping the board and then was to probe a few contacts to see if there was something obviously wrong happening.

... and suddenly this afternoon, after the drive working great, it now neither reads nor writes disks again. On power-on the drive motor sounds anemic and doesn't seem to be seeking. I don't know what happened. I did absolutely nothing other than use it several times over the course of a few days.

I'm wondering if I might have some fractured solder joints on the analog board. Alternatively, I guess it's possible there's something wrong with the controller card. I'll have to inspect it.

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Probably not relevant but on

Probably not relevant but on one of my Disk ][ I had a similar intermittent issue and it turned out to be the crimped ribbon cable that had oxidated where the cable is crimped. It would appear and disappear when the drive was moved - always disappearing when the drive was opened as the ribbon was moved!!

 Now the duodisk does not have those ribbon cables but maybe the 25 Dplug are oxidated/corroded? Do you have some "wet" contact cleaner - something like DeOxit that leaves some lubricant behind and improves conductivity - to spray on the sockets and plugs? I would also make sure the loom that connects one drive to the other is not oxidated - I would use the same contact cleaner and unplug-plug the connector many times. 

 

Finally, of course, check all the solder joints and also make sure the chips' legs are clean.

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Actually that's a great

Actually that's a great thought about the contact cleaner. The fact that when the drive was just completely not working the motor sounded downright anemic (like there was no robust "chug chug chug chug" from the steppers during bootup). So there could be something up with the contacts.

Someone in another forum suggested there might be some weird probe loading issue involved given the drive magically started working the first time after I poked it a few times, that there might be an issue with decoupling caps dying or a chip floating out of spec causing a periodic desync issue with the 74LS logic, and suggested dropping in 74HC chips as a direct replacement. I haven't fully unpacked the reasoning yet though.

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tony359 wrote:Probably not
tony359 wrote:

Probably not relevant but on one of my Disk ][ I had a similar intermittent issue and it turned out to be the crimped ribbon cable that had oxidated where the cable is crimped. It would appear and disappear when the drive was moved - always disappearing when the drive was opened as the ribbon was moved!!

 Now the duodisk does not have those ribbon cables but maybe the 25 Dplug are ox

 

It would not hurt to check the continuity between the internal cables that connect the two boards inside the DuoDisk to the port on the back, and the I/O card cable connector to where the connections terminate after the internal cmalp/solder joints.

 

Did you re-chip and re-cap both boards, and did you do this before, or after you discovered this issue? 

 

Intermittent operation means generally intermittent connections, so it could easily be corrosion on the connectors either on the cards or the cables, or a broken joint or trace.

 

The drive motor control, iirc, is controlled by the 2003. Disconnect the internal link to D2 and limit your tests only to D1. It's safe to simply disconnect the internal ribbon between the two cards inside the DuoDisk unit to accomplish this. You absolutely want to confine problems to the primary controller card first. 

 

For clarification: On what Apple II mode are you using the drives? There is a known issue with the DuoDisk on the //gs and an internal Apple Service Memo on how to correct it. This only affects the use of the drives on the //gs, and only with some revisions, but in the event that you are having difficulties with a //gs mainboard running the drives, I thought I'd mention it. 

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