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SA800 floppy outta whack

GanjaTron

GanjaTron

Experienced Member
Joined
Jan 11, 2007
Messages
201
Hi folks,

thought this might find a larger audience here than in the CP/M forum.

I've got two 8" SA800 floppies in an Altos box (yeah, the ones mounted upside-down!), one of which (the original boot drive) is giving me headaches. The drive gives consistent "bad sector" errors, or just plain refuses to boot. The other drive performs flawlessly, so I've been using it as reference to check the proper behaviour.

Having gone through the maintenance manual, I checked the read data test points and compared them with the good drive -- both drives give an avg. amplitude of 200mV, which is well within spec. The bad drive exhibits noticeable more fluctuation in the amplitude... bad head alignment/azimuth?

I next checked the head load mechanism; the head load buttons were ok on both drives (felt still intact), and the head load foam pads were also of similar consistency (i.e., not YET turned to goo). I readjusted to head load screws to apply slightly more pressure, but that didn't help.

Other things I tried: swapping pulleys between both drives and swapping logic boards: bad drive remains bad!

One difference I did notice is that the bad drive's stepper lead screw has noticeably more resistance and it NEVER returns to track 00 if I move it by hand, even after power-on reset! I checked the three stepper phase connectors, and all measured 20ohms, so the motor appears to be intact. If the logic is ok too (swapping boards didn't help), what's left to check?

Any 8" guru wisdom appreciated. :)

Have a great week,

--GanjaTron
 
Have you cleaned the lead screw? Sometimes gunk fouls the threads. Did the drive suddenly start going bad while in your possession, or did you acquire the drive in that condition? With no power applied, you should be able to easily rotate the leadscrew throughout the seek range. I've seen cases where the ferrite core in the stepper windings has cracked and now obstructs free movement.

Bottom line is that the stepper should rotate easily when done manually. Look for misaligned or binding bearings, lubrication problems, etc.
 
Hi Chuck,

I acquired the system with the drives in this condition. I believe it has been in disuse for a very long time.

The lead screws on both drives require considerable force to move, and noticeably even more so on the "bad" drive. I tried lubricating the lead screw with teflon spray (which I understand doesn't degrade the plastic), but that seems to make little difference. Is there a better lubricant? (Vaseline, the more obvious choice, does degrade plastic if I'm not mistaken)

Should I lubricate or otherwise first inspect the stepper motor itself? I hesitate to remove it from the drive frame because that will obviously require realignment, but I guess that's inevitable at this point anyway, right?

On another note, could a stuck TRK00 signal prevent the drive from seeking back to track 00? Probably unlikely the sensor failed, but I'll check the appropriate test point.

Thanks for the hints,

--GT
 
I'd avoid removing the stepper unless you're willing to go through the realignment exercise.

Since this is a CP/M system, it should be easy enough to write a little program to attempt to read from track 0 and check if the TK00 signal at the connector is coming active. If not, you at least have a lead on the problem. I believe that there's at least one disk editor for CP/M out there (written by Vern Buerg maybe?) that you could also use to do this.

The ACS8000 did come with a diagnostic disk that also might help--if you can locate one.
 
Hi Chuck,

I have the Altos diags disk. Didn't know it includes a floppy check. So far I didn't need it since the error is pretty easy to reproduce simply by attempting to boot from the offending drive.

But lo and behold, my hunch was right; track0 is *always* asserted on the bad drive, no matter where the stepper is! Checking the LED at the J2 connector (and directly at the leads after removal from the drive frame) confirmed the LED is shot. :-(

Ok, so the SA800 parts manual simply lists a "detector assembly, track 0". Not much help in finding a replacement. Any ideas what LED type I should be looking for (wavelength, power)?

See ya,

--GT
 
If it is the sensor, it's easy enough to replace. It's a more or less standard phototransistor+LED assembly. You should be able to figure that out very quickly without even having to move the carriage--just use something to interrupt the beam and watch the output per the Maintenance manual.
 
Hi Chuck,

it's an Optek OPB2255 photointerruptor. Can't find a datasheet, but if you say it's a fairly generic part, it should be easy to find a replacement. I'll keep you posted.

--GT
 
Well, I'm happy to report that the drive works with a generic replacement photointerruptor (Vishay TCST 2103). I didn't even have to readjust the detector tab on the carriage!

BUT, while checking the drive out and figuring out the Altos Diagnostics (without docs...), the OTHER (formerly good) drive made strange rubbing sounds while reading a disk and subsequently failed to read any others! This time it's NOT the TRK00 detector, but I noticed the read signal is weak, so maybe dere's sumptin' skewy wid da head load pad or button. I'll have to pull the drive and look what's up with THAT now... :huh:

One of the annoying things about the Altos design is that it's tough as nuts to test the drives in situ; the cables are extremely short and stiff, and the case pretty cramped for such big drives. Testing a drive turned on its side to get at the head mechanism is a royal pain...

Will keep you posted. Have a great weekend,

--GT
 
GanjaTron said:
Btw, I just discovered an earlier thread dealing with a failed track0 detector in an SA800: http://www.vintage-computer.com/vcf...20-Shugart-SA-800-floppy-bad-Track-0-detector
Apparently the issue wasn't resolved... :|

--GT
Click to expand...
Heh, actually I did resolve that! It was indeed a bad source diode in the pair. I ended up using a carbide cutter on a Dremel tool to make two holes through the old source-sensor pair assembly and stuffing an IR LED in one hole and a phototransistor in the other, held down with a bit of 2-part epoxy (they were Motorola devices intended for fiber links...gold plated TO-18 cans with lenses in the top...found a pile in the warehouse, so that's what was available). IIRC, I had to adjust the cutoff on the comparator that determined if the tab was interrupting the pair (can't remember if it was changing a resistor or just turning a pot). Still works today, but it's good to know that there's a good enough replacement available that doesn't involve drilling and cutting!
 
GanjaTron said:
BUT, while checking the drive out and figuring out the Altos Diagnostics (without docs...), the OTHER (formerly good) drive made strange rubbing sounds while reading a disk and subsequently failed to read any others! This time it's NOT the TRK00 detector, but I noticed the read signal is weak, so maybe dere's sumptin' skewy wid da head load pad or button. I'll have to pull the drive and look what's up with THAT now... :huh:
Click to expand...

Sounds as if your disk is shedding oxide. Take a look at the floppy--can you see grooves carved into the oxide surface? Toss the disk and clean the heads.

It happens a lot with some old media.
 
Way to go, glitch! ;)

I took the original detector assembly apart and considered shoving a flat replacement LED in there, but it soon dawned on me that even those come in all sorts of dimensions. When I realised photointerruptors with the exact same mounting and gap width are still readily available (and cheap at ca. 1 EUR!), I simply went for that option. Looks cleaner too, despite the fact it's hidden from view anyway (though I take pics for the sake of posterity and documentation). I like to get a job well done. Make's me feel, um, accomplished 'n' stuff. Of course the OTHER drive failing pretty much shattered that for the moment!

--GT
 
Well, that was a no-brainer. ;)

When I pulled the now wonky drive #2 I immediately noticed a small plastic thingy lying in the bottom of the case; the head load button! The clip broke and it just $*#@ fell off!

The felt still looks okay, but before I superglue this bit on for all eternity (precluding any further replacement), I'd appreciate some alternative suggestions (I read a sliver of cork would do the job), or -- dare I ask? -- where I could get original replacements. I guess it's only a matter of time before drive #1's button gives too...

See ya,

--GT
 
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