Commodore 8050 Disk Drive Issue

[SiriusHardware]

Good going - those CMOS switches (4066 and similar 4016) have a bit of a reputation for failing in a lot of things.

[Radio Wrangler]

Originally Posted by SiriusHardware

Good going - those CMOS switches (4066 and similar 4016) have a bit of a reputation for failing in a lot of things.

Their reliability isn't too bad used carefully, gating logic signals, but those parts do also get used very 'creatively' and that poses added risks.

Used as analogue switches they easily become meeting places where 5v control signals and supplies meet up with analogue signals coming from 12v and +/-15v supplies etc.

A CMOS input only needs to go one diode drop beyond the supply rails and the dreaded parasitic thyristor implicit in the chip design can get turned on, leading to general destruction.

David

[ScottishColin]

OK - two diskettes received but neither will format (HEADER) on my 8050. I get Bad Disk for both diskettes, no matter which of the two 8050 drives I attempt to format them in.

I'm using my original 3016 by the way but that should not make any difference (I really need to resptay that PET - it's very much sticking out like a sore thumb in terms of the colour I chose four years ago).

I will send back two of my 3M 96tpi diskettes that I know work here, one blank and one with useful programs on it to see where that takes us. I'll list the programs in a subsequent post.

Colin.

[SiriusHardware]

Hmm, good job you stepped in, Colin, otherwise we would have been forever looking for hardware faults - which I've no doubt there are at least a couple of - only to find and fix them and then still be thwarted by these non working discs.

Are fake 5.25" discs a 'thing' now?

[ScottishColin]

Well, perhaps. I got mine from Dave Duke and he is a reliable seller who I have used many times. They are 3M branded, 96tpi diskettes (albeit not cheap). The ones I received today were unbranded/no label.

Colin.

Originally Posted by SiriusHardware

Hmm, good job you stepped in, Colin, otherwise we would have been forever looking for hardware faults - which I've no doubt there are at least a couple of - only to find and fix them and then still be thwarted by these non working discs.

Are fake 5.25" discs a 'thing' now?

[centrespot]

I forgot to mention/ask, last week; the D1 disc drive suddenly decided to reverse direction when looking for the start point.

Instead of reversing towards the stepper motor it drives towards the disc centre and hits the end stop. I tried it in the D0 position and it worked fine. I don't think I got any cables or components the wrong way round during the recent tests.

What would be the likely cause and what should I be checking/testing.

[TonyDuell]

That's a very odd fault and I can't immediately see how it could happen.

I'd start by scoping the 4 drive signals to the stepper motor while the head is moving. I think these are pins 7,14,1,8 of U13 on the analogue board. I want to see if one is stuck high or low or looks 'different' to the others.

[SiriusHardware]

If this is something which wasn't happening before, is now, then I have to say your first move needs to be to go back over everything you did just before that new behaviour started.

If the head slider motor is just an ordinary two wire DC motor check that the corrections aren't somehow reversed and if it is a stepper motor double check the connections as well.

[TonyDuell]

The head positioner motor is a 4-phase stepper motor. The driver circuitry for the 2 motor is on page 14 of the .pdf service manual, For each motor there are 2 bits from the digitial board which are decoded by a '139, then inverted/level shifted by '06 gates and used to drive transistors in the FPQ3724

[SiriusHardware]

Yes, I didn't see your saying the head motor was (as to be expected) a stepper because I think we cross posted. I'm at a party of sorts just now and I don't have easy access to the diagrams.

[centrespot]

Originally Posted by TonyDuell

The head positioner motor is a 4-phase stepper motor. The driver circuitry for the 2 motor is on page 14 of the .pdf service manual, For each motor there are 2 bits from the digital board which are decoded by a '139, then inverted/level shifted by '06 gates and used to drive transistors in the FPQ3724

I was checking the stepper circuit during your #87 post, I am getting signals on all 4 lines at P5, I will hook up a PC based scope to compare all 4 signals over a longer time period to see if there are any differences.

[centrespot]

I decided to connect a PC based Logic Analyser first to check any differences on U13 & U14 outputs to the respective stepper motors (pins 1,7,8,14)

I did a "directory" command on the PET for each Drive test

Both chips stayed high on B,C,D
Drive 1 U13 pin 7 went low (stepper A)
Drive 0 U14 pin 1 went low (stepper A)

The wires & plug on drive D1 can't be dislodged/broken etc because the drive works correctly when plugged in to the D0 PCB position.

At the moment it doesn't make any sense to me.

[TonyDuell]

It makes little sense to me either.

But logivally, either the stepper motor drive circuit (page 14 of the service manual) has a fault, or it's getting the wrong signals from the digital board. The stepper motor signals are controlled by 4 bits of port B of the 6522 VIA chip which is driven by the 'controller' processor (rather than the interface one).

As you have a logic analyser, can you look at the inputs to the 'good' stepper driver (U18 pins 2 and 3) when it's doing a restore-to-track-0 on drive 0 and then at the inputs to the 'bad' driver (U18 pins 14 and 13) when drive 1 is doing the restore-to-track-0.

It looks like you can force a reset by grounding pin 6 of UN2 on the digital board (thiis is a open-collector driver, so it's safe to do that). It might be worth adding a pushbutton switch between that pin and ground so you can easily reset the 8050 without powering it down.

[SiriusHardware]

A reminder, the 6502 and 6522 were briefly in the wrong sockets. Could the 6522 have sustained some damage at that time?

[TonyDuell]

If you have spares or any way to test them fully it would be worth checking the 6502 at UH3 and the 6522 at UM3

[centrespot]

Originally Posted by SiriusHardware

A reminder, the 6502 and 6522 were briefly in the wrong sockets. Could the 6522 have sustained some damage at that time?

I did check this yesterday but I will try again with ones from other machines.

[centrespot]

Originally Posted by TonyDuell

It makes little sense to me either.

But logivally, either the stepper motor drive circuit (page 14 of the service manual) has a fault, or it's getting the wrong signals from the digital board. The stepper motor signals are controlled by 4 bits of port B of the 6522 VIA chip which is driven by the 'controller' processor (rather than the interface one).

As you have a logic analyser, can you look at the inputs to the 'good' stepper driver (U18 pins 2 and 3) when it's doing a restore-to-track-0 on drive 0 and then at the inputs to the 'bad' driver (U18 pins 14 and 13) when drive 1 is doing the restore-to-track-0.

It looks like you can force a reset by grounding pin 6 of UN2 on the digital board (thiis is a open-collector driver, so it's safe to do that). It might be worth adding a pushbutton switch between that pin and ground so you can easily reset the 8050 without powering it down.

All 4 pins are high at idle

My test is by doing a "directory" on each drive (D0 & D1)

D0 both 2 & 3 stay high

D1 goes low on pin 13, pin 14 stays high


The shorting of UN2 pin 6 doesn't seem to work, I need to reset by powering off/on

[TonyDuell]

I am not sure a directory command should move the heads until the drive has read something off the disk to work out what track it's on and thus the direction and number of tracks to move the head to get to the directory track. I think you need to find a way to force it to do the restore when both heads should move to track 0 (am I rght that in your 8050, drive 0 does this but drive 1's head goes towards the spindle?)

So let's work out how to force a reset. Start by measuring the voltages on all 8 pins of UM2 (NE555), both with the thing 'normal' and with pin 6 of UN2 grounded. This is the circuit in the bottom right corner of page 27 of the .pdf manual

[centrespot]

Originally Posted by TonyDuell

I am not sure a directory command should move the heads until the drive has read something off the disk to work out what track it's on and thus the direction and number of tracks to move the head to get to the directory track. I think you need to find a way to force it to do the restore when both heads should move to track 0 (am I rght that in your 8050, drive 0 does this but drive 1's head goes towards the spindle?)

Yes D0 moves correctly but D1 moves to the centre spindle

[TonyDuell]

When does it do that? At power-on? Is there any other time you can force it to do that restore?