C64 With Black Screen

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rmzalbar
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Re: C64 With Black Screen

Post by rmzalbar »

Swapping the head is something you want to avoid doing, because you will then need to do alignment. Better to swap the entire mechanism as a unit.


To format, you open a command channel to a disk drive, send it the NEW command, give it a disk name, and give it a two-character ID. Then you CLOSE the command channel.

This does all of that in one line:

OPEN 1,8,15,"N:EMPTYDISK,01":CLOSE 1

..formats disk in drive 8 with label EMPTYDISK and disk id 01



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Re: C64 With Black Screen

Post by C64Person »

Ah, good to know. Unfortunately, the other drive doesn't work, as the spindle doesn't actually move. I was just going to use it in case I needed parts.

I tried using my Fastload cart to format the disk, and that reported no problems, although it still gave a 21 READ ERROR when reading in from the disk or trying to format using the command that you recommended. It also gave a 74 DRIVE NOT READY error when trying to save to it.

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Re: C64 With Black Screen

Post by rmzalbar »

It's possible the fastload cart does a "blind format" with no verify, for performance.

Make darn sure that head is clean, use a magnifier and a light and give it a good look. I just bought a 1571 that acted like yours when I got it. I had already cleaned the head pretty good, I had thought, but I basically did it without looking closely at it, figuring the thorough alcohol swabbing I gave it should have taken care of anything. I unlugged the head and measured it, and it measured normal resistance OK across all coil pins. I was about to try replacing the logic board when I happened to give the head a closer look with flashlight and magnifier and, lo and behold there was a bunch of schmutz on it that didn't clean off and that I missed seeing when glancing in at it from a distance. A more careful clean while looking at it did the trick.

I have also had a newtronics 1541 with a bad head that measured open on one of its pins; that drive had to become a parts drive.

Anyhow, go through Ray Carlsen's 1541 diagnostic rundown, he covers the errors your getting and also talks about checking the head for open coils.

http://personalpages.tds.net/~rcarlsen/ ... ix1541.txt

And for the drive where the spindle doesn't move: do you mean it's mechanically frozen (can't be turned) or the motor doesn't turn on? It's easier to swap the spindle drive motor than the head, because no realignment needs to be done. It's just a belt drive motor sitting off on its own. I did have to replace a rectifier on the 12V side of a 1541 logic board. Drive responded OK electronically but no 12V power for the spindle motor.

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Re: C64 With Black Screen

Post by C64Person »

Yes, I've been looking that guide quite a bit. When he discusses measuring resistance on coil pins, does that just mean checking the resistance between all the points on the head wire to the board? Doing that, I'm getting some wildly ranging values on both heads, some in the low to high kΩ range, and others between 15 and 25 Ω.

Just in terms of continuity, on the dead motor drive, all pins are open, so I'm guessing that means it's a terrible head. On the other one pins 1 and 2 seem to be continuous, but nothing else is. Does that mean both my heads are dead?

The story just continues. Now the computer is just giving a DEVICE NOT PRESENT error with the drive that was sort of working, even though the drive resets when turned on and when the computer starts. I suspect bad 7406 on the drive, because both computers are doing it, and I only have one left, so I have to swap it between both drive boards when swapping those. Both drive boards are having the same error.

As for the bad spindle, I can manually turn it, but it just doesn't run at all. Whoever worked in these drives before me left the spindle motor wires disconnected from the little board on the drive itself, but I reconnected them and nothing changed. However, this makes it seem likely that there were problems with the spindle before.

What would the dirty head look like under a magnifier? It appears to have a clear splotch on it, but it looks like residue from the alcohol. Should I be concerned with this?

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Re: C64 With Black Screen

Post by rmzalbar »

I don't recall exactly what you should be seeing, I believe it's around 10 to 20 ohms per coil. Certainly not kiloOhms.

The layout of the head is like this:
m8Up3YZ.png
You can see three coils: R/W 1 and 2, and Erase. There is also a wire that is common to all of them.

If you measure from any coil to the common wire, you should get somewhere around 10 to 20 ohms.

You can see that there is a diode across the erase head however, so some types of multimeter might show a different result on that head depending on the polarity of your probe placement - use resistance, not diode check mode to avoid this. Also there is what I believe to be a resistor in the KOhm range across RW1 and RW2, so an open head will still measure as KOhms of resistance through that resistor even though the coil itself is open.

If you happen to be measuring between any two of the head lines, well they are all joined at common so you will be measuring through two coils and you would get double the resistance (perhaps dependent on probe polarity if the erase coil's diode is involved.)

Still. nothing should be in the kiloOhm range. That would indicate an open coil.

The three conditions you are trying to fix by cleaning a dirty head are:

1. Magnetic debris which will attenuate the signal from the disk. This would generally be magnetic particles shed from the disk surface itself. It shows up as brown to black smudges, and is very effective at preventing the drive from reading the disk.

2. Debris or contamination that is physically large enough to increase the distance between the surface of the head and the disk. You'd probably see that as well. Caked on crap or a blob of grease for example.

3. Abrasive grit that will scratch disks, wearing them out and shedding debris which will shortly lead to conditions #1. and #2.

A little clear spot left behind from dried up cleaning fluid is likely not a concern. I guess maybe #3 in the extreme case, thinking hard water mineral deposits here, but likely not.

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Re: C64 With Black Screen

Post by C64Person »

Sorry, I was thinking completely backwards - the one with all the coils open is the bad one. This also happens to be the head on the drive with the working spindle, so that's probably why it's throwing read errors. I'll swap the good spindle motor into the other drive, since all the coils on that head are reading in the 10 to 20 Ω general area.

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Re: C64 With Black Screen

Post by C64Person »

Update:

I swapped the spindle motor, and nothing changed, so I checked the supposedly bad motor in the other drive, and it worked. So, the spindle PCB was bad on the other drive.

I now have one drive a good spindle, and what should be a good head. I swapped in another 7406 to the drive's mainboard, but the computer still can't see it. Interestingly, I tried running a format, and the computer just freezes, but once or twice just started printing the letter G indefinitely. No clue where that's coming from...

I tried swapping in another set of VIA's to the drive, but the exact same thing is happening.

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Re: C64 With Black Screen

Post by C64Person »

Okay, I have done some basic troubleshooting on the drive with the oscilloscope, and here are my findings so far:

Both the 5VDC and 12VDC lines are good on both my boards. The large capacitors are fine, but are leaking so probably should be replaced. The 1MHz clock signal is good both in and out of the 6502, as well as on the two VIA's. The reset line is also okay, going low and running the spindle, and then going high. Still, the computer can't see the drive on the serial bus.

Probably not related to the DEVICE NOT PRESENT ERROR, but worth checking: The green LED on the front of the drive burned out - like literally turned bright orange before going dark. I checked voltage at the LED pins on the board, and I'm getting the full 5VDC coming straight into the LED. Is that bad? I don't really care if it works, but if it could be related to an overall electrical failure of the drive, I'd like to investigate.

Odd that I got head movement on command before, but now it seems the computer and drive aren't communicating at all.

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Re: C64 With Black Screen

Post by rmzalbar »

There's a 220-ohm current limiting resistor between the +5v rail and pins 1 and 3 of the power LED connector (pin 2 is ground.) With no LED providing load anymore, you'll still measure 5v, but with power off you should be able to see the 220-ohm resistance when measuring from pin 1 or 3 to any 5v trace.

Leaking capacitors could be failing electrically as well, and they can either act as a short (dragging your voltage down and introducing noise) or as low capacitance / high resistance (increasing noise.) You should be getting no significant ripple on the 12v and 5v outputs.

You could have a bad 6522 VIA, probably UC3. I wonder if swapping the two might change your symptom, revealing the cause. But also check to make sure someone didn't cut either of the two split-pad device number assignment jumpers to change the device number to 9,10 or 11 for use as a second drive. If someone did, you be able to access the drive through one of those numbers instead of ,8. You should solder bridge the cut jumpers to set it back to 8, as most multi-load software/games are hard-coded to only use device 8 anyway.

Image

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Re: C64 With Black Screen

Post by C64Person »

I guess the LED just died, because the resistor is good and I swapped in another LED which is fine.

I swapped all 4 of my VIA’s around and saw no change. I did check the device number jumpers, but they all seem connected, so it should be 8. I’m not seeing any ripple, and I swapped in capacitors that are not leaking (however, I don’t know that these are good, as their values are too high for my multimeter).

Is there anything else I can check without a working board? Both of my boards are doing the same thing, so it’s possible the same chips are faulty on both, but I’m not sure.

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