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Floppy disk drive calibration?

JNZ

Experienced Member
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Sep 4, 2015
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165
Location
Utah
I have three Siemens FDD 100-8D drives that appear to read data only intermittently, and I suspect they need calibration. I have the technical manual for them, but unlike other drives it doesn't mention how to perform maintenance on them. From a manual for a Siemens 5 1/4" drive I read some of their procedures for measuring the amplitude of the magnetic flux off the head circuitry using a test point, pulse index timing, checking that the felt pad is pushing the disk into conformity properly, and that manual very helpfully gave specs for it. For mine, however, all I have is a detailed section on the theory of operation, without any remarks about how the drives often fail and what they need to ensure reliability.

So basically, does anyone have any tips on what I should do with these drives? Is it possible for someone to make an alignment disk that I'd use for checking for head alignment with sectors? I'm driving them with a Jade "Double D" controller card, and though I'm able to boot CP/M (reading from track 0 I think) and read the disk's contents, it almost always fails to load a program.

I'm not opposed to buying already-aligned drives, but no one had any when I asked recently on the marketplace.
 

hmb

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Aug 8, 2019
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You are looking for the "maintenance manual" for the FDD 100-8 which is found at:
ftp://bitsavers.informatik.uni-stuttgart.de/pdf/siemens/FDD100-8_V2.pdf

It is not possible to make your own alignment disk. For a long recent discussion on this exact topic, see:
http://www.vcfed.org/forum/showthread.php?46749-Aligning-Floppy-Drives

8" floppy drives are virtually always available on eBay, ranging in condition from "junk" to "new old stock". There is an allegedly "new old stock" FDD 100-8D listed on eBay right now (and no, I have no connection to that auction).
 
Last edited:

JNZ

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Gary C

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Often a drive only needs a simple head alignment which can be done by using a scope to measure the head output after the amplifier then adjusting the head position to get the highest reading on a known working disk.

It does mean you need a program to make the disk run continuously.
 

hmb

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Often a drive only needs a simple head alignment which can be done by using a scope to measure the head output after the amplifier then adjusting the head position to get the highest reading on a known working disk. It does mean you need a program to make the disk run continuously.

Or even without a scope, using the method described by Chuck in post #8 at:
http://www.vcfed.org/forum/showthread.php?46749-Aligning-Floppy-Drives

Of course for either method (scope or no-scope) you do need a "known good" floppy that has been recorded on a properly aligned drive.
 

JNZ

Experienced Member
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Sep 4, 2015
Messages
165
Location
Utah
Often a drive only needs a simple head alignment which can be done by using a scope to measure the head output after the amplifier then adjusting the head position to get the highest reading on a known working disk.

It does mean you need a program to make the disk run continuously.

I could write a program to communicate with my controller card and upload it via my serial terminal easily enough, as my monitor ROM allows for hex entry into memory.

When you say run continuously, what should I command the disk to do in that program? Seek to track n and then continuously read sectors?

I have a disk that was known-good a few years ago, and I suspect errors in magnetic flux that'd cause read errors wouldn't affect the actual track positioning on it.
 

Gary C

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ideally you should be able to step the head to any track so you can check its aligned on all tracks. So motor on, head loaded and in read and also be able to command to track 0 because when some (most ?) drives see the track 0 switch, they continue to step to an initial stepper motor energisation , so you can align at say track 20 then command to track 0 and find it steps past it. This I suppose was because the early track zero switches were actually a bit coarse.

I tend to step to 0, then fine align the head while watching the output, then step to mid track and make sure the output is about the same, then try a read.

Does that make sense ?
 

Dwight Elvey

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Jun 21, 2003
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Santa Cruz
You can manually control the drive with some jumpers to the inputs. All signals are driven from active low open collectors so you can just put jumpers to ground for selects and such. A push button and you can step to different tracks.
Be sure, if it uses a felt pad, the it is clean and even. Crud often piles up on the pad and makes it not track well.
Any way, some easy clips jumpers and maybe a push button and you have your own disk controller ( you don't even need to disconnect it from the computer, just connect to the input leads of the circuits.
I should note that a digital scope is almost useless if you have a real alignment disk. You need to get a full revolution of the disk on the screen at the same time as you are watching a 500K to 1 MHz signal on the screen( depending on the drive ). The aliasing of undersampling the 1MHz signal will make it look like shit. An analog scope is best for this used.
Dwight
 

Chuck(G)

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I agree with the DSO-vs-analog scope comment of Dwight.

Alternatively, if you're handy with code, just write a simple little program that issues a READ ID command and displays the result continuously. Simple. You know you're on-track when you can see the IDs for all sectors on the track.

As a matter of fact, that's akin to the way one of my Dysan digital alignment disks works--each track ID is recorded at a known radial offset. So you look at the ID returns and when they're centered around the one on-center, you're done. There are similar tracks to test skew, etc.

This one
 
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