• Please review our updated Terms and Rules here

TRS-80 4P Floppy Drive not reading disks

A

arrow_runner

Veteran Member
Joined
Mar 7, 2012
Messages
530
Location
Cincinnati Ohio
I have a 4P and one of the floppy drives does not read properly. Initially the problem was whenever I tried to read a floppy in that drive it would make an awful noise (I can't remember exactly, but it might have been grinding or clack clack clack). The 4P has sat for a few months and I just got around to disassembling it. Now that I have the drive out, the awful noise has gone away. Instead the drive head looks like it's not seeking or reading properly.

Here's what I've done so far.
1. Clean the drive head.
2. Swap the top circuit board with the one from the working drive. No change in either drive.

Any ideas? I don't want to just start swapping parts willy nilly much further because it looks like the drive head is held in by a tension bracket which could be difficult to get in and out.

Video : http://youtu.be/41IFXKA-uh8

These are Tandon TM-50-1 drives by the way. I meant to put that in the title.
 
Last edited:
Also, I can use regular 360kb floppy drives in this can't I? I'd prefer to stick with original parts with the original faceplates but if this isn't something I can't rectify by next week then I'll just throw a different drive in it and call it a day.
 
Arrow_runner,
I watched the video and noticed that your drives have different settings on the Programmable Shunt. To boot Drive 1 instead of Drive 0, you also have to swap the
Programmable shunt. Your Drive 0 has one less programmable shunt than Drive 1, but that probably doesn't matter. Also both drives appear to have the
Terminator Resistor Pack removed, which is correct.

I don't have a manual on the Tandon 50-1 Drives, but my Tandon 100-1 (100-2) manual has the following picture.

Shunt.jpg

The Shunt has the following Selections from Left to Right (looking towards the edge connector of the floppy)
HS DS0 DS1 DS2 DS3 MX SPARE HM

I believe I'm correct in stating from memory, the HS and HM jumpers aren't used, and should be open.
The MX jumper is only used in a Single Drive System.
So, DS0, or DS1, or DS2, or DS3 will be the only jumpers used. (If your Programmable Shunt has HS shorted
just retain that setting, as it's probably correct!)

Shunt1.jpg


Instead of using those Programmable Shunts, I've just inserted a DIP Switch in that Socket and made the necessary selections for Drive Select etc,
even if you have to let the last legs hang over the Socket. It works fine.

So, I'd recommend you make a note of the existing Programmable Shunt for Drive 0.
Then make a note of the existing Programmable Shunt for Drive 1, and compare the two.

When you get the settings that were in Drive 0, properly selected on Drive 1, and plugged into the cable for Drive 0, I'd bet it will boot properly,
assuming the drive can read the foppy. Just be sure to select the proper documented settings for Drive 1 when plugging it back into the
system as Drive 1, the second drive in your system.


Larry
 
Last edited:
On the 4p (unless the cable has been changed) the drive select is handled by THE CABLE... each position has ONLY the DS contact for THAT drive...
so no matter which ORIGINAL drive you are using, it will be right for its placement...

Now if these drives are not originals, all bets are off and the above info may be correct.
:D
 
In situations like this where you don't know if its the disk drive, cable, or FDC components on the motherboard causing the problem, I start like this:

Remove the disk drives and cable and put them aside
Connect a known good cable and 360K disk drive
Boot with a known good LS-DOS disk and confirm everything works
Then start looking at your Tandon TM-50 disk drives

Many times I've chased my tail on disk drives which worked some days and not others, only to find the problem lies in the FDC parts.

Hope this helps,

Ian.
 
arrow_runner,
Regardless if the Floppy Cable has Pins REMOVED, conductors cut, or conductors routed to different pins to do the Drive Select, the
SHUNT must be set corectly for the Tandon TM-50 Drives. I've searched the Internet and found a Tandon Drive Schematic for the TM-50
and verified the SHUNT (U19) must be set properly for the drive Select signal to function properly. I think the Schematic of the SHUNT (U19)
will remove all doubt.

https://archive.org/stream/Tandon_TM-50_Floppy_Drive_Schematics_19xx_Tandon#page/n0/mode/1up

Tandon_TM50.jpg

HS - Head Select - 1 to 16 = OPEN
DS0 - Drive Select 0 - 2 to 15 = Closed for Drive 0 (1st Drive in System)
DS1 - Drive Select 1 - 3 to 14 = Closed for Drive 1 (2nd Drive in System)
DS2 - Drive Select 2 - 4 to 13 = OPEN
DS3 - Drive Select 3 - 5 to 12 = OPEN
MX - Motor ON with Drive X Select - 6 to 11 = OPEN
SPARE - 7 to 10 = OPEN
HM - Head Load with Motor ON - 8 to 9 - OPEN


Larry
 
Would it not be easier to say
IF the one that DOESNT WORK
looks like
the one that DOES WORK

then they should both work correctly unless something ELSE is wrong?

Obviously you know more than I do.
I'll just sit here and learn.
 
The configuration of the shunts doesn't matter in this case.

Drive 0 Worked fine and still works.
Drive 1 did not work as drive 1. It did not work as drive 0. It still did not work as drive 0 when I swapped the top boards (which has the shunts on them). The original good Drive 0 with bad Drive 1s board still works as Drive 0.
 
I fear, from the comment about the noise, that you may have lost the head on that one... Thus, it couldnt boot from trk 0 and nothing happened...

Did you try moving the head inward before powering up 2c if it seeked trk 0? Just curious. :D
 
... but that is NOT true on the 'good one'?

The only other thing I can think of is that youve lost the stepper motor controller... not a good thing.
Couldnt tell on the vid... was the spindle motor running/spinning?
 
The good drive will seek to track 0 regardless of where I leave the head.

Which motor/motors are you referring to? The disk spins on the bad drive if that's what you're referring to.
 
Not familiar with the TRS80 4P, but here are a few thoughts. Swapping out the main logic board leaves only a hand full of things to test.

There is a manual that appears to be for this model of drive here: http://maben.homeip.net/static/S100/tandon/diskette/Tandon TM50 Service Manual.pdf

Given the buzzing noises, my guess would be that either there was a problem with the track zero sensor, or the stepper motor was jammed.

Try cleaning the track zero sensor and see if that helps.
Can you format a disk using that drive? If so then the head might be out of alignment.
Does the head stepper motor appear dirty or corroded? When the units are off can you manually move the heads without resistance? (Not all drives let you do that, but in this case you can compare the feel with your working one).
Is there any dust/dirt buildup at the ends of the rails that might prevent a full track zero seek?

A few other things you might check, but are less likely:

Is the drive spinning at 300 RPM? A visual comparison to the other drive should be sufficient just to get it to read, but it looks like these may have a timing disk on the bottom that you use get even more exact.
Are the heads making full contact with the disk? Gently pressing against the top head during operation may reveal if this is the issue.
Check the continuity of the wires running to the disk head with a volt meter. If there happen to be exposed connections on the head you can check each wire directly.
While the drive is running with a disk in it, is there an index pulse? (Check pin 8 of the connector with a logic probe)
Is the track zero output operating (check pin 26 with a logic probe when the head is both on and off of track zero)
Does swapping out the servo control boards make any difference?
 
arrow_runner said:
The good drive will seek to track 0 regardless of where I leave the head.

Which motor/motors are you referring to? The disk spins on the bad drive if that's what you're referring to.
Click to expand...

Then it sounds like youve lost the stepper motor/controller/motor connections. That's the one that moves the head in/out.
I couldnt see the disc spin but at least that says THAT part is okay.

You might want to look at the power connector... perhaps bad/cracked/broken leads/traces are preventing 12v to the stepper motor but not the spindle motor...

Or... just find one to replace it... :p

:D
 
At this point I'm pretty sure it's a bad head. The motor controlling the head movement seems to be good and I also swapped the bottom board which controls the head movement. I also tried applying varying amounts of pressure to the pad on top of the head. All that resulted in was a different error message.
 
I would say the head is bad as well by the indications you are getting. Go find another half height and chunk the bad drive. Just watch out for TEAC's they are some of the best drives but the edge connector is reversed. So get a Qume, Tandon etc.
 
You must log in or register to reply here.
Back
Top