C64 With Black Screen

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C64Person
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Re: C64 With Black Screen

Post by C64Person »

Success! Well, sort of...

I was swapping chips around between my two drive boards, and suddenly it worked! It turns out that UA1 in my primary board was bad, as swapping the chip in from my other board now allows the handshake to occur and the drive spins up when running a load command. Not sure why the secondary board wasn't working, but I suspect that it has something to do with the messy rework that someone appears to have done on it.

However, I'm now back to my previous problem in that I can't actually get the drive to format or save/read disks. Getting a response from the drive should at least make this part a little easier though. I'm going to try to clean the new head and check for open coils again. Hopefully that will help, but if not, I may be back to troubleshooting the read head (which I unfortunately don't have another replacement for).


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Re: C64 With Black Screen

Post by rmzalbar »

UA1 is involved in ATN, CLK, and RESET. Do you get a flashing red light error when you try to access a disk? When you try to read/write (not format/new command, head bang is normal for beginning of format) does it do a cycle or two of "head banging" against the stop before it gives up, or does it just do the red light after a few moments of quiet spinning?

When the drive is in a blinking red light state, this basic program will read the error and print it to the screen:
10 open 15,8,15
20 input# 15,a$,b$,c$,d$
30 print a$,b$,c$,d$

It will print out like:
error number, error text, track, block

If you have an open head, I'm guessing you will have
21,read error,something,something
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Re: C64 With Black Screen

Post by C64Person »

I do get the flashing light when trying to access the drive, and the error is 21,READ ERROR,18,00. When trying to load from the drive, it does two head banging cycles before the red light begins flashing and FILE NOT FOUND ERROR is printed on the screen. Is it abnormal for the head to bang like that when simply loading?

I thoroughly cleaned the read head and it looks much cleaner now - still no change though. I did check for open coils on the head, and it does appear that pin 1 has opened up from the rest of the pins, which I assume is bad.
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Re: C64 With Black Screen

Post by rmzalbar »

A head bump in normal use, except when formatting a disk, is NOT* normal.

A head bump cycle at the beginning of a format is normal. The drive has no sensor to tell it where the head is, it only knows how many times it's stepped it in or out. Each track is two steps of the stepper motor, and there are only 40 tracks possible, so it just steps it 80 times toward track 1 to make darn sure it's starting from a known position, which means it will knock against the head stop a little or a lot depending on how far away it was when it started. Then it formats the disk from track one out to track 35 (tracks 36-40 are not used for standard commodore disks) which takes about 2 minutes in total. Each sector that it writes to a track is tagged with the track number, which is how the drive normally knows what track it's on when you are reading the disk back later. Then it seeks back to track 18 and writes the directory information to finish the format.

Because the sectors on the disk tell the 1541 which track it's on, no head bumping is needed when normally using (reading or writing) a disk. A head bump would mean it tried to read a sector and failed. Since it failed to read the sector, it also failed to get information about what track it's on.. so to make sure it hasn't drifted out of position, it does the head-stop bumping described above, then steps out the correct number of steps to get back to the intended track and tries again. After a few tries it gives up and sets an error. The cause of the read error could be the format on the disk is bad, the read head is out of alignment (meaning the stepper motor "steps" don't center directly over a track) or the read head is bad.

If the cause is due to being out of alignment, then you should still be able to format a disk with the same drive and then read it successfully, because the disk will then be formatted such that the written tracks would of course be written out of alignment by the same amount.

If the cause is due to a bad head.. the drive will fail with a read error during format, because after writing a track it tries to read it back before formatting the next track make sure the disk media itself is good. If the read head is open, it won't be able to see the sectors it just wrote, and the format will fail with a read error at track 1, right after the head bump.

Pins 1 and 5 on the R/W head connector are the two read/write coils. These should both be showing continuity to the other pins, resistance should be in the ohms range, normally 10 or 20 ohms, not kohms/megaohms.

* There is one legitimate reason for a read to cause head bumping: An early copy protection scheme for commercial game disks entailed intentionally putting a bad sector on a disk. Normal attempts by casual pirates to use Commodore DOS to copy a disk would not replicate a bad sector. The game would try to read this sector, causing a head bump and sending an error message back to the game's loader which would then assume the disk was original and simply continue to load. This is not really that common even on commercial games because it was mechanically stressful to be bumping the headstop, which could lead to the alignment drifting out over time, especially on early models of the 1541. Occasional disk formatting, no problem. Every single day when your kid went to load Ghostbusters after school.. not OK. Game publishers soon came up with more sophisticated copy protection schemes that didn't need to bump the head.
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Re: C64 With Black Screen

Post by C64Person »

It runs two head bang cycles and then gives up when running the LOAD"*",8 command, which sounds like what you described as it failing to read and trying to reset itself. Running a format just brings the head back to track 0, where it does one banging cycle and then stops right there. The red light blinks and I get 21,READ ERROR,00,00.

I do not think that alignment is the problem here, because as you said, format should be successful (which it is not, I get the read error mentioned above), and it would then be able to read its own disks, which it cannot do because they are not formatted. It's possible that it is out of alignment, but I don't think that's the main issue.

Pin 1 of the head is definitely not showing continuity to the rest; it is in the kΩ range, so it's open. All the other pins to one another are showing in the Ω range, so unfortunately the one open pin is probably the culprit (or at least one of them). So, should I replace it? I know it is difficult to just replace the head because that requires alignment, but I wonder if it will also be difficult to find a replacement for just the drive mechanism. What do you recommend here?

This wouldn't be related to the copy protection you mentioned, as I am using blank disks, but that is good to know!
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Re: C64 With Black Screen

Post by rmzalbar »

Unfortunately there is no known source of replacement heads, even as take-offs from old mechanisms due to the failure rate on the newtronics type. An entire disk drive from ebay is probably your best bet, and if it's a newtronics type then only buy if tested.
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Re: C64 With Black Screen

Post by C64Person »

Alright, I will buy a new drive then. I was hoping to fix mine but I don't think that's possible.

I'm new to this stuff, so do you recommend buying another standard 1541 like the ones I have (Newtronics 1541 with internal psu) or "upgrading" to a newer/possibly more reliable Commodore drive (1541-II, 1571, etc)? Or the opposite, any I should avoid other than untested Newtronics? If I'm going to buy one, I might as well get something I'll be satisfied with.

Also, will my existing logic boards and/or drive mechanisms be compatible with a different drive model? It would certainly be nice to have some known working extras that are compatible with all my devices, but that's certainly not an absolute necessity.

Thanks for the advice!

The other thing I do have is a 1571 drive mechanism (no logic board, case or anything else). Its head doesn't have any open pins; is it worth trying to do anything with that? Clearly it's double sided and not perfectly compatible with the 1541, but I'm just checking.
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Re: C64 With Black Screen

Post by rmzalbar »

The 1571 mech won't work with the 1541 logic board, unfortunately. I don't think you can do anything there since the heads won't mechanically fit on the 1541 mech, and even if it did, it would need to be driven with a different current level than the 1541 board is set up for anyway.

Your 1571 logic board is only good for a 1571.
Your 1541 logic board is good for any 1541 or 1541C with either the push-in(ALPS) or swing latch(NEWTRONICS.) There's a jumper you set on the board to use it with an Alps. I don't guess it will fit into a 1541-II case however, and anyway there were additional mech variants for the -II model so I don't know about compatibility.

If you're replacing the drive, and are looking to avoid newtronics:

1541 with the push-in latch instead of the swing latch are non-newtronics.

1541-II has a newtronics variant. If the swing latch is partially recessed into a cut-out then it's a non-newtronics kind.

1571s are known to be solid and reliable, not prone to head failure. But if you buy one of these, make sure the seller ships it either with a shipping card, or a disk, inserted in the drive and latched closed or else it won't survive shipping. This is because the upper head has a lifter that props it up so it won't touch the bottom head if there's no disk in the drive. However, it props it up in an unstable way and if dropped in this configuration, it can break the head. Twice I've had 1571s shipped to me in an unsafe way, but got lucky. 1571s by the way have an optical sensor instead of a head stop, so they don't have any head-knock when formatting or on read errors.

Pay attention to mains voltage if you're buying a drive. If you buy a 1541 that needs a different mains voltage you can just swap the power supply over from the one you have.

Finally, a newtronics drive that is confirmed working isn't necessarily a bad deal. The head may never fail.
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Re: C64 With Black Screen

Post by C64Person »

Ok, this is all good to know. In this case, I will try to look for the best pricing options, but will favor 1571 if possible.

I am not necessarily opposed to Newtronics, but as you said it seems they often have bad heads, so I would definitely only get one that has tested good.

That's too bad about the 1571 mech, but I figured that would be the case.
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Re: C64 With Black Screen

Post by C64Person »

Good news! My new 1571 arrived and it works great! It is very nice to finally have a fully operational C64 setup! I have a few more kinks to work out, but overall things are looking good.

The stepper motor runs very loudly - is this something that can be fixed? As long as it’s not a problem I can live with it, but I figure it’s worth checking into.

On a separate note, my original C64 seems to have some bad contact in some of the chips, as usually when I first start it up it does not show a normal start screen, or sometimes it even has a black screen. I then have to go inside and push down firmly on the logic chips (as it is usually a different one each time), and then it works. Also, if the computer gets bumped, the system tends to crash and require a restart. I have tried bending the pins on the chips slightly out so that there is a firmer contact with the sockets (every chip in here is socketed now), but the issue continues.

My never-ending list of questions continues, but each is a step in the right direction :) .
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