Shugart 850 and Cromemco 16FDC

[Endersending]

I need some advice for a problem I am having. I have an 8" Floppy drive that gives me read after write errors when using DS/DD disks.
I have followed instructions on setting up the 16FDC to accept Shugart drives.
my drive setup:
A: 5.25 half height floppy
B: 5.25 Gotek
C: 8" Shugart 850/851 (the drive I have been tinkering with so may be out of alignment we won't worry about this one right now)
D: 8" Shugart 850/851 <-- This one gives read after write errors for DS/DD.

I have been testing with really nice condition Dyson DS/DD Floppies or a Wabash Single Sided.

Using CDOS I can INIT (format) Double Sided Double Density DS/DD.
Trying to WRTSYS or XFER gives me read after write errors.
The 8" drive can INIT and XFER fine with all other formats.
Trying to copy using RDOS gives errors 30 and 34 (CRC)

The 5.25 drives can use all formats, no problems.

Why can the 8" drive work with single sided double density, and double sided single density, but not ds/dd?
Is this a Shugart drive problem or a 16FDC problem?
Any recommendations on testing I can do or ideas about the problem would be helpful.

Attached is a scope reading from drive C: both heads test points 1 and 2:
Left side is read signal, right side is write signal.
Read signal is about 130Mv (which the manual says is nominal)

 

[Endersending]

An update:
I can write double density to the drive, but with a lot of read after write errors.
The errors always come from surface 1, which is the top head.
I think this rules out a 16FDC problem and it being the drive itself.
Maybe it needs an alignment?

 

[Endersending]

With more testing I have a little more information:
It seems I can write small files (1k - 2k) correctly. larger files will error out on the first transfer.
Attached is a picture of the probes hooked to TP1 and TP2. My ground leads are on TP5.
It is interesting to notice how high the signal sits above ground. probe 1 is over 4 volts above.

With power applied to the floppy drives, no computer running, TP1 (top head - but both heads act the same) sits at 3.5 volts and while the drive reads it is at 3.5volts with the signal dancing on the top.
This does not seem right. I would expect the signal to be small and close to ground. I believe TP1 is after the head amplifier.
Although it does read, just doesn't seem to write correctly on the one head. Remember I can write single sided double density just fine.
I am not sure what to make of this. Hopefully someone reads this and has some advice or questions that could help me solve this problem.

 

[deramp5113]

The same circuit is used for reading both the bottom head and top head of the drive. The same applies to the write circuit. Since top and bottom work in SD and the bottom works in DD, it’s most likely not the electronics.

I’d clean the heads with a q-tip and alcohol if you haven’t already. Could be the head pressure is too light, though that’s a difficult adjustment on the 850.

 

[Endersending]

I have cleaned the heads with rubbing alcohol (everyone uses IPA, whats the diff?).
I thought about head pressure. I pushed a tiny bit on the top head and it instantly scratched the floppy disk. I believe the manual shows using a jig and a dial indicator to adjust head pressure.
Does the heads have any type of camber or radial alignment I have to worry about? The heads float on the arm and maybe something got bent and the head doesn't sit right?
There are 3 things I can adjust: sector tracking, sector 1 sensor, and head pressure.
I will clean the heads again and see (use my eyeballs) if can see where the top head has enough pressure.
Thanks for the response. Any advice is helpful. It helps me use other perspectives to think about how to approach this.

 

[Endersending]

A few more observations and questions:
Should the heads be perfectly aligned to one another? on one drive, the top head is about 20% shifted over from the bottom.
The other drive, the head looks rotated about a few degrees - As I look down on the head, the metal the holds the head doesn't look square to the arm.
Since the top head is floating, I am wondering if it is possible they have been physically moved, maybe by a forceful user.
I have also noticed as I replaced the bearings in the main spindle, you have to push the head away from the head load actuator and maybe that was enough force to un-align the top head.

Has anyone encountered this? maybe have some stories about what not to do to these drives.

I am also curious about the waveforms I have posted although maybe my scope isn't set up correctly. its odd those signals sit a few volts above ground. I though they would be no larger that 150mv

 

[Chuck(G)]

A few more observations and questions:
Should the heads be perfectly aligned to one another? on one drive, the top head is about 20% shifted over from the bottom.
The other drive, the head looks rotated about a few degrees - As I look down on the head, the metal the holds the head doesn't look square to the arm.
Since the top head is floating, I am wondering if it is possible they have been physically moved, maybe by a forceful user.
I have also noticed as I replaced the bearings in the main spindle, you have to push the head away from the head load actuator and maybe that was enough force to un-align the top head.

They're supposed to be offset from side to side.

 

[cjs]

I have cleaned the heads with rubbing alcohol (everyone uses IPA, whats the diff?).

IPA is pure alcohol and water; rubbing alcohol is adulterated with what from an industrial point of view are contaminants, which may even include oils such as castor oil. It's also generally just 70% alcohol, whereas IPA ranges from 70% to 99%. I always suggest 99%, or at least 90%, IPA for cleaning both because it's pure and it takes less time for the water in the alcohol to evaporate from the cleaned surface.

 

[Endersending]

I guess I will start using IPA.
I hooked the 8" floppy drive up to a 486DX with Dos. Although 22disk will format, it will not copy complaining about sector not found.
I set the BIOS to 1.2M 5 1/4 drive and dos can format and copy onto and off of. It won't make a system disk saying there is not enough room for system information.
So the drive does work. Maybe it is my cable I made to connect the 16FDC to the shugart

 

[Endersending]

With more testing my cable seems fine as per Deramp's cable page 3.
I can write double sided single density just fine, which to me means the top head works.
Is it possible if the top head is rotated radially a few degrees that FM (single density) works but not MFM (double density)?
here is a picture of the top head (Top view looking directly down, top is towards front of drive):


A few things I notice:
- you can see some of the bottom head exposed, so the top head is sitting back further than the bottom. (It has an under bite).
It also looks like the head should be rotated counter-clockwise a degree or two. I drew 2 red lines and the measured difference from each end was 7 pixels. That might not mean much since it was a picture of a picture and the viewing angle is far from perfectly straight on.
-The square holes in the bottom corners are the floating metal that holds the top head and the plastic of the arm. the right square looks larger than the left square, which make it look like the head need to be rotated clockwise.
Now I am just rambling. I have no idea where to look or what to check to solve this issue.
More probing I guess. I will go back through the service manual and check TP1 and 2 and see if I can see something about the signals that might give a clue.
- I have also noticed as I press the top head onto the bottom, you can see the hop head's angle change as it sits flat to the bottom surface. I don't think this is a problem because it sits correctly when down all the way but goes to show the head is not in a perfect position.

 

[Endersending]

I think I figured it out: head pressure.
After gently putting extra finger pressure on the top head the drive was able to complete a successful write.
From what I can find, there is a screw that runs into the neck of the arm that holds the head, on the back is a spring. I tried to turn the screw but I was unable to get it to turn because of the angle of the screw head to the drive makes it very hard to get at. I moved the spring up the screw towards the head, and that seems to be working for right now.
I will try to get it properly adjusted and report back. The other drive is doing the same thing so hopefully that is just a head pressure problem too.
I will post more!