Apple IIc Glitchy Graphics

[KevKelley]

I recently reacquired a IIc after many years and am a bit rusty. For the most part my old disks work great. Occasionally when I try to load a disk it doesn't read but that is not as much. But when I tried one program some of the graphics were glitchy. I found the game on a second disk and the same issue. 

 

I am not too familiar with troubleshooting the IIc but was unsure if this was an easily identifiable issue.

 

I am including a picture of the glitchy issue, which so far only affected this one game - Switchbox. It happens when plugged into a monitor or using a RF modulator into a TV.

[KevKelley]

Forgot to mention. There should be numbers at the top and the garbled horizontal bars should be switches.

[Great Hierophant]

Start with the built-in diagnostics, which can test the memory.  I believe you hold open and closed apple and turn on the power.

[Iamgroot]

On 6/10/2021 at 5:06 PM, KevKelley said:

Forgot to mention. There should be numbers at the top and the garbled horizontal bars should be switches.

The font and the vertical bars printed correctly.  Animation on a IIc though, is different than on a IIe.  It could be the game is not programmed to run on a IIc.

[KevKelley]

20 hours ago, Great Hierophant said:

Start with the built-in diagnostics, which can test the memory.  I believe you hold open and closed apple and turn on the power.

I will try that. Thanks!

[KevKelley]

11 hours ago, Iamgroot said:

The font and the vertical bars printed correctly.  Animation on a IIc though, is different than on a IIe.  It could be the game is not programmed to run on a IIc.

It used to work years ago, albeit on a different IIc. These were the same disks I had used then. It seemed the sound worked and the game runs as it should if I hit the numbers. It even starts up okay, asking to input the player names. It is only when it gets to the main screen. Unfortunately I am less skilled at troubleshooting Apples so I gotta get back into the swing of things.

[KevKelley]

I forgot. Originally I thought it might be an issue with the drive or computer because initially when I first loaded it, the entire game was glitchy. But after a couple tries it ended up just this.

 

Is it possible the drive could be dirty or malfunctioning if everything displays correctly? I tried other disks and every once and a while it doesn't see anything on a disk. I plan on taking this apart and maybe dusting it off and seeing if the drive needs any maintenance.

 

[Iamgroot]

Dirty disks won't cause a glitchy screen.  It will just freeze up or crash or the disk drive rotates forever.  If an ONERR routine is in effect and there was a bad sector where the data was stored, this usually causes a DOS error to happen and returns to the program without crashing, but the program should branch to an ERROR handling routine.

 

Scrambled graphics should also mean that the program loaded correctly as the program is not crashing, so it is more likely the pointers to the shape table or bitmap are pointing to random memory, which are displaying random bytes as graphics.

 

By the look of your picture, it looks like the game of SWITCHBOX.