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[MicrocomputerSolutions]

Reading a disk is more difficult than writing the disk.

When the drive is writing, it isn't looking at where it's writing, it assumes that the head is in the right place, and lays down the data. If the head is out of alignment, or the index timing is off then the data is going to be laid on the disk, where it may not be retrievable when that same drive, or a different drive is used to try and read the disk.

The head on your third drive may not be bad. it could be that the index pulse is too short or too long, or not at the right time. The head could be out of alignment, to the inside or the outside of where the track is supposed to be laid. The head could also not be ready, or settled when the logic is trying to write on the disk, and the head load pressure could be too low, resulting in poor contact between the head and the disk. Unlike hard drive heads which fly just above the media, floppy disk heads are in contact with the disk when reading or writing.

You need to test the head output voltage on the inside tracks to see if the head output is high enough. If the head load pressure is good, the disk itself is good, and the head output is below spec, then you need to try swapping the logic board to verify that the problem is not the logic board, before scrapping the head.

When you replace the head carriage, make sure that the preload spring on the stepper motor is taking out any play in the carraige. And you need to get the head in the right plane with the disk. The manufacturer used a plate inserted in place of a floppy disk to set the head height when the drive was built. You can make your own plate, and use it to take note of how the original head was installed, before removing it, so you can duplicate the height when you install the replacement head carriage. After you get the replacement head carriage installed, you will need to set the index pulse width and timing, then the head radial, and check the head output voltage. You will need a calibrated drive alignment disk to make all of the measurements, except head output, AND a double or triple trace Oscope preferably a minimum of 100Mhz range.

I've been doing drive repairs and alignments since the early 1980s. While I prefer Shugarts, I also have the service manuals for other brands of 8" and 5.25" floppy disk drives. When you get to the point that you want your drives working properly, contact me, and I'll fix you up. I charge $100-$200 to repair, align, and test drives. I test on a S-100 computer both before and after servicing, and I use dedicated drive testing and alignment equipment with Genuine Dysan Alignment Disks.

 

[Mike_Z]

I agree. Reading is more difficult than writing. I was drawing at straws when I said alignment. Just not smart enough. I have three good FDD100 disk drive circuit boards, I say this because they all work in the other two good drives I have. Each one of these boards will not work in the 3rd drive. It is interesting to hear that the index pulse could be the problem. I looked at it, but only causally. It was full voltage and square, but I didn't check the pulse width. I'm using new disks that I purchased from Anthana and they work fine in the other drives. So, by process of elimination I arrived at the idea that the head was bad, because that is all that is on the base frame. Yet it could be the index pulse. I'll check that a little more closely.

I can check the 'head output voltage' on the inner tracks, but how much should I be looking for? I'd rather eliminate the easy prior to attempting the difficult.

I have a Tektronix 2230 dual trace 100mhz memory scope. I will keep you in mind. I'm also using a Shugart 800 drive that occasionally gives trouble. It turn out that, that one may need your service first.

Thanks for the help, Mike.

 

[MicrocomputerSolutions]

The peak voltage level and the shape of the wave form are not really important when it comes to the index pulse. What is important is the timing of the pulse, and length of the index pulse.

Which is why the index pulse timing, is adjustable, as well as the index pulse length (on most drives there is a pot on the index pulse sensor/receiver to change/control the pulse length).

Both my drive tester, and drive exerciser have circuitry to write on floppy disks. Step to Track 75 or 76, and write, then observe the voltage level of the data you can get back. You should see at least 100mv-125mv at the head output test point on the drive's logic board while on/at Track 76