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IBM PC XT 5160 Tandon TM100-2 issue

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RetroSpector78

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Hi,

I have an IBM PC XT with a Tandon TM100-2 floppy drive that is having some issues.
C43 blew upon startup so I replaced that one, and I found another cap that was bad according to my ESR meter. I replaced that cap also and cleaned the heads but it is still not reading disks.

Upon startup I can see the drive motor spinning, and I also see the stepper motor moving the heads back and forth (as you typically see on a floppy drive) as illustrated in the video below :

https://drive.google.com/drive/folders/1OwIBybe4KfYp6EmhBh941P3CqeSK9hHk?usp=sharing

But when trying to access a disk (doing a directory listing, or trying to format a disk), i see the disk spinning but the head is not moving.
This is then followed by general failure reading drive, or a bad track 0 during formatting.

The floppy controller / cables seem to be ok as I've tested it with another drive. Drive termination and dip switches also seem to be fine.

What could be causing the fact that the heads move back and forth on startup, but they aren't doing anything when trying to access a disk ?

Thx
 
Not all PCs do a seek at boot. But since yours does, movement is at least a good sign. (What kind of controller are you using, and what exactly is the machine doing when the drive seeks? )

Most of what DOS needs to start reading a disk is on track zero. So if it can't read at all, it may just sit there.

Without knowing more, there are a number of possibilities, including read head failure, read head dirty, head alignment, read circuitry failure, read line failure in the cable, drive speed wrong, perhaps index sensor issues (although depending on the machine, DOS may read a disk even without an index).

I'd sanity check a few things first if you haven't already. Make sure the cable and controller works OK with another drive. Make sure the disk you are reading is known good, low (double) density, and was not written in a 1.2mb drive.

Try formatting a disk - if it formats but does not read other disks, that indicates an alignment problem.

Try running ImageDisk, and see what it reports. If it reports no interrupt from controller, that could indicate an index issue. If ImageDisk runs, try running it in alignment mode - you may be able to manually step the heads, and if the head is over the wrong track it may show you which track it is over.
 
the track 0 sensors are known to fail on those drives, check that

it's a mechanical switch not optical
 
SomeGuy said:
Not all PCs do a seek at boot. But since yours does, movement is at least a good sign. (What kind of controller are you using, and what exactly is the machine doing when the drive seeks? )
Click to expand...

Most XTs seem to do a seek at startup, and i am using the standard IBM floppy controller.

Most of what DOS needs to start reading a disk is on track zero. So if it can't read at all, it may just sit there.

SomeGuy said:
Without knowing more, there are a number of possibilities, including read head failure, read head dirty, head alignment, read circuitry failure, read line failure in the cable, drive speed wrong, perhaps index sensor issues (although depending on the machine, DOS may read a disk even without an index).
I'd sanity check a few things first if you haven't already. Make sure the cable and controller works OK with another drive. Make sure the disk you are reading is known good, low (double) density, and was not written in a 1.2mb drive.
Try formatting a disk - if it formats but does not read other disks, that indicates an alignment problem.
Click to expand...

I've already

- cleaned the heads with IPA
- replaced caps that were faulty
- checked the setup with a working floppy drive to rule out cable / controller issues. And the system can read / format floppies fine with the working floppy drive.
- tried different disks 360kb disks, attempting to read floppies that work on other systems, but also tried formatting floppies. (to rule out alignment issues)

SomeGuy said:
Try running ImageDisk, and see what it reports. If it reports no interrupt from controller, that could indicate an index issue. If ImageDisk runs, try running it in alignment mode - you may be able to manually step the heads, and if the head is over the wrong track it may show you which track it is over.
Click to expand...

Will give it a try but figure as it doesn't read any of my current floppies that changes are slim that it will read that one. But thx for the tips !
 
maxtherabbit said:
the track 0 sensors are known to fail on those drives, check that

it's a mechanical switch not optical
Click to expand...

That's the microswitch next to the drive motor that is pressed in when the head moves all the way back I guess ? I did spot that one and it did "sound" ok when I clicked it, but will check that one. So this is pressed in when the head moves into the position that it thinks is track 0 ?
 
maxtherabbit said:
the track 0 sensors are known to fail on those drives, check that

it's a mechanical switch not optical
Click to expand...

Took another look at it and noticed that the 2 terminals coming out of the microswitch don't have continuity when I toggle the switch. So this might be the culprit

2020-07-07 07.45.03.jpg
 
A better way to test the switch is to get on the Pullup Resistor(s) and check that it is being pulled "LOW".
If you are just measuring the two wires, there will never be any resistance from one to the other when
the NC or NO contacts are "MADE" (CLOSED), unless you are measuring the wire(s) to GND.

I've attached a .PNG of the actual Schematic.



Larry

Track0-switch.png
 
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