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Thread: 8” CDC Floppy Drive Repair

  1. #21
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    You're going to sell a drive with a bad PCB? Why not keep it for spare parts?

  2. #22

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    Quote Originally Posted by Chuck(G) View Post
    You're going to sell a drive with a bad PCB? Why not keep it for spare parts?
    Yeah, that’s actually a good idea, especially now that I’m getting more into computers with 8” drives. I’ll see what I can do with the parts.

  3. #23

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    it sounds like you don't know much about drive subsystems... my apologies if i'm telling you what you already know.

    have you ever seen your zenith system or one like it actually work with an 8" drive? i'm asking if you know what to expect the drive to do when it's working properly, what it will sound like? is your media any good? 8" media today is pretty iffy, it's old, was never meant to last this long... did you clean the head on the drive? be very careful with lubrication. it may help now but can cause you big problems down the line especially if the worm gear that moves the head starts spraying it all over the place. yeah, i've seen this happen.

    these drives can fail for a number of reasons. i fixed a lot of CDC drives many years ago and they go out of alignment easily. this includes the head alignment--making sure that the head centers on the proper track, and the track zero switch which indicates to the FDC that the drive is on track zero. track zero provides the FDC a starting reference for when it begins to step the head in to find the directory, or whatever.. the very first thing the fdc will have the drive do is to find track zero. if the track zero switch is way out it'll never find anything.

    head alignment was done with an o'scope and an alignment diskette. these diskettes were written on special machines that wrote the tracks in specific patterns so you could use a scope and alignment program to zero in the drive (using a "cat's eye" pattern). probably unobtanium now. they are not reproducible on a floppy drive. track zero you can probably do with a known good diskette made for your system and a scope, but not without understanding how the drive system (FDC and drive) functions.

    seems like there was a micro-switch and push-rod assembly in these CDCs that sensed when the drive door was shut and i keep thinking there was a problem keeping those things working properly--like the push-rod bent. some CDC drives locked those doors when using the drive. surprised lots of users when they went to eject their diskette and the door would not open!

    if you can find drives that were made for your system and are configured for it, that would be the best thing. get one that works along with a known good diskette with something on it, paying more if you need to, get your system working and then you can start to learn about and add additional drives, fixing or re-configuring as necessary.

    i never used a zenith system so i can't tell you any more about what it expects. CDC, Shugart, Tandem, and other drive manufacturers provided units to many different companies. often, media was not interchangeable because of how the format was written, number of sectors and such, varied between operating systems. jumpers/dip switches were not always there to set drive unit numbers... sometimes they reconfigured the drive to work in different ways. been a long time for me and i don't recall exactly on the CDC. some used pull up resistor packs on the data/select lines in the last drive on the cable. this is called termination. not using termination when called for will keep a good drive from working reliably or maybe not at all. at a minimum get a service manual for your zenith, preferably one that has a theory of operation in it and read about the floppy subsystem and what it expects. you'll get hints there on how to set up a drive. get a service manual for whatever drive you end up with, and read about the different ways to set it up, how the switches are set and such, termination... don't go flipping switches if you don't know what they do. this is the fastest way to blow something.

    these are not complicated to work on, but you have to know how they work to fix or configure them. or, be really, really lucky!

  4. #24

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    I just looked at the CDC manual Chuck(G) gave a link for. there is a theory of operation as well as descriptions of what the switches and jumpers do on the drive. also, it touches on various alignments, including the index alignment that i didn't mention. the FDC needs to know from where to begin counting sectors on a track and the index indicates that. the theory of op section is quite good. i didn't mention hard vs soft sectoring in my original post, but the theory of op explains what this is. be sure that you configure your drive to use the sectoring you zenith system needs.

    i also found a reference to the drive assembly you have: https://www.scribd.com/document/7602...-February-1980 it's in an ad on page 7. not a lot of info there, but it gives you a starting point if you want to chase more data about how RNB used their drives.

    nothing was plug and play back then. everything had to be configured by hand, especially drives. that's one thing that makes the old gear harder to work with, but i think it also makes it more interesting and more gratifying when you get it working.

    in my experience the Shugart 8" drives were the most reliable and easiest to work with.

  5. #25

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    Wow, all this is pretty in-depth. I’ve been pretty preoccupied with other things, but I’m currently reverse engineering the power supply board in the chassis. It was almost falling apart and the voltages were questionable. Simple fix using some 78xx ICs. After that I’ll be sure to clean the extra lubricant, thanks for the tip! I have in fact seen a Z-100 using an 8” drive. The port on the back is specifically made for 8” drives and connects directly to the FDD controller, which I found the datasheet for. Once I have all of the power and stuff working and I test the head movement manually, I’ll try and interface with the Z-100. Most of the drive seems fine, the track 0 switch works, the micro switch for the door works, motor spins fine, new belt, etc. I do have an oscilloscope and the maintenance manual has oscilloscope images in it. If I have to I’ll check those out. The only thing I’d be worried about is the alignment of the switches. Did this drive specifically need an alignment diskette? As for actual media, I don’t have any software on 8”. All (at least most) of the Zenith Z-100 software was on 5.25” disks, as 5.25” drives were already in the system. I do, however, have 2 boxes of NOS sealed diskettes. One of the boxes (both of which I opened) is for IBM Diskette 1. Very cool stuff.

  6. #26
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    If it was my PSU, I'd be concerned about using a 78xx linear regulator, as they're normally rated for about an amp--and that's without a lot of headroom (i.e. with a much higher line voltage than the load voltage, the IC's going to make a lot of heat. It would seem that an 8" drive would put a fairly hefty load on one.

    There are better alternatives today--and you could well go to a switching regulator.

  7. #27

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    to align the track zero switch to spec you do need an alignment disk. same for head radial. with a known good diskette you can wing it, but with blank disks you can't since there's no format on them.

    if you aren't interested in interchanging a diskette between drives, then it doesn't matter so much since your drive should be able to read what it wrote assuming everything else is working. the alignment diskette was used to make sure that what you wrote in one drive could be read in another drive, or when you wrote on a diskette from another drive you didn't damage the data on it because the head was not exactly where it should be.

    look in your z-100 manual to see whether it needs hard or soft sectoring and then make sure the appropriate dip switch on the drive is set accordingly. check the other dip switches to make sure they're set properly for you computer.

    just because you have NOS diskettes doesn't mean they're going to work anymore, or if they do that' they'll work for long. FYI.

    when you complete your power supply rebuild hang a scope probe on the output/s and make sure that the DC is clean. there should be a spec in the CDC manual for the voltages.. e.g., +/- 10% or like that.

    good luck with your project!

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