The Strangest LC Problem Ever

[Scott Baret]

My apologies for being away for a while...but I return with what may be the strangest LC problem ever recorded.

I got my LC out of storage on Saturday. All was well. I was using it without anything out of the ordinary until tonight.

The monitor, a 12" Apple RGB (the standard LC monitor) keeps flickering on and off, but only in one respect. Half of the blue pixels go out. The others are fine (all reds, all greens, and the other half of the blues). I tried unplugging the video cable and reconnecting it. No difference was made there. It has gotten progressively worse as far as how often this happens (it will come, stay for a while, then go away).

However, it DOES NOT DO THIS when I set it to "THOUSANDS OF COLORS". In fact I can go to the monitors control panel, select thousands, and it will fix the problem if it's present...and it returns if I set it back to any of the other four settings.

This is making me wonder if maybe I have a bad VRAM chip or something? (It's a 512K I pulled from an LC II five years ago). Or maybe there's a problem with the blue pin, although it looks perfectly fine and does fine when on thousands of colors mode.

I may try the monitor on another Mac tomorrow just to rule that out, but the monitor has no signs of anything wrong when it's on thousands of colors mode. Trouble is half the programs I have on the LC want either 16 or 256.

(The pattern of problem blue pixels, BTW, is pinstripes. It literally looks like a prison cell without those blue lines there. I am hoping it's not a capacitor issue.)

I am thinking it's not the blue pin though...put it on black and white mode and it's a white line pattern through the screen as in prison cell pattern with the bars further apart...this is proof that my blue pin is working fine.

I'm thinking VRAM off the top of my head?!?

 

[LCGuy]

Have you tried switching displays with another system?

To me it sounds like a problem with the graphics subsystem. Try maybe using a different VRAM module.

 

[Bunsen]

Wait .. you'd think if it was a VRAM problem, it would tweak out on higher colour settings (using more memory), not lower ones ...

Muh? :?:

 

[Scott Baret]

OK...I tried the monitor on a Quadra 700 and it worked fine. Ruled that out. Big sigh of relief since it's uncommon to find a 12" RGB in as good of shape as the one I have (plus I LOVE 12" RGBs).

Got my old 256K VRAM SIMM out. I put it in the computer and had it run for a while. At first all seemed well..but the jail bars returned.

I put the 512K SIMM back in. 16 bit still works perfectly.

Is there a possibility that the problem lies elsewhere? Perhaps it's the power supply or something on the logic board?

For now I will use it in 16 bit mode but I lose the functionality of Kid Pix...which only likes 256 color screens.

Anything else I can try? Are there any PDS slot cards I can get for video that would drive a 12 incher? Or is there something I can try to look for on the logic board?

 

[JRL]

Try cleaning the VRAM with a pencil eraser.

 

[Dennis Nedry]

He tried 2 different VRAM SIMMs, though, with no improvement.

You say it takes a little while before they appear; that would seem that heat could be a factor. Right when it starts, try pressing you thumb on each chip on the logic board to dissipate the heat and see if you can get it to momentarily go away. You could find a specific chip to suspect this way. What good that would do to find out is another question though.

 

[Scott Baret]

I'm going to try that on the chips next to the video board. And yes, it only happens after I've had the computer on for a while.

The monitor works fine on the Quadra 700 after it has been on for a while, clearly not the monitor here but something on the LC board.

I'm thinking of picking up an extra LC board to part out and am also looking for a video card for the empty PDS slot. However, I'm hoping to fix the internal video first since I recently got a IIe card that I want to put in the LC.

 

[Bunsen]

Try cleaning the VRAM with a pencil eraser.

He tried 2 different VRAM SIMMs, though, with no improvement.

Although, there could concievably be contact corrosion in the VRAM slot. Try cleaning it out with a soft paintbrush and/or blowing it out, and then repeatedly inserting and removing a SIMM a few times. This should clean the contacts a little.

 

[Dennis Nedry]

Try cleaning the VRAM with a pencil eraser.

He tried 2 different VRAM SIMMs, though, with no improvement.

Although, there could concievably be contact corrosion in the VRAM slot. Try cleaning it out with a soft paintbrush and/or blowing it out, and then repeatedly inserting and removing a SIMM a few times. This should clean the contacts a little.

VERY interesting idea. Another possibility of something that could be fixed.

Also you could try wiggling the VRAM SIMM a little during the problem for further clues.

 

[Scott Baret]

I'm going to try all of these sometime this week. I shall keep everyone posted on how things go!

For now I am going to use my Quadra 700 in the spot formerly occupied by the Mac Plus. The LC will knock out one of my two remaining Classics on my other surface.

 

[Scott Baret]

UPDATE on the LC...

I had it on for four hours today, got a lot of work done on it (yes, I DO use my LC for real work...and sometimes I believe I may be the last man left on this planet who uses MacWrite II on an almost daily basis for productivity).

I had it on 256 color mode the entire time. No problems whatsoever. I ran a bunch of different programs on it, from MacWrite II to Print Shop to Zoo Keeper to Carmen Sandiego. The only thing I didn't attempt was to change the bit depth to 4 bit color.

The only changes I made was removing After Dark from my system folder (which I did since I needed a little bit of hard drive space). The more I think of it I didn't have After Dark in my system folder before the problems. I know it's highly unlikely, especially because the jail bars remained after a restart of the Mac, but it's the only thing I can think of that I did today. (The only other change to my workspace is an awesome new chair from Ikea, but I doubt that would have an impact on the video system).

I'll be testing this Mac again the next time I work on it (likely Wednesday or Thursday night).

Perhaps this came because I just purchased a Mac LC logic board today?

 

[Bunsen]

(The only other change to my workspace is an awesome new chair from Ikea, but I doubt that would have an impact on the video system)

Pffnaaah... it's not like we're dealing with SCSI here.

 

[Scott Baret]

UPDATE on this problem...

The LC had been working fine from my last post (August 19) until now.

I had attached an 80MB SCSI hard drive to the LC and found the problem to recur when I removed the drive.

Interestingly this drive and cable had been used with the LC shortly before the problem began the last time.

The problem appears to lie in one of the pins that deals with blue frequencies. I'm wondering if there's just some static on the logic board somewhere?

I'll try the same procedures I did before with this LC. Once again, it works fine on thousands of colors mode.

I now have a spare logic board in case I need it. I'm hoping the problem just goes away on its own like it did last time.

Any more thoughts on this would be greatly appreciated.

 

[Scott Baret]

Latest Update...

The LC kept doing the same thing this morning. I wondered if perhaps time would be the correct medicine for the computer and turned it on (minus the monitor) before I left for a two hour choir practice tonight.

When I came home I flipped the monitor on and the 256 color mode was working normally again. Hopefully it won't relapse tomorrow (it didn't last time but given the age of the hardware there's always that uncertainty factor).

Time really does heal all wounds, including those on the logic board!

I'm still going with a theory that something interfered with the board when that SCSI cable was used. The SCSI cable in question is rather beat-up and the drive itself is somewhat questionable (it's in a 20SC case but the internals are something I threw together myself two years ago; the drive already has a few SCSI ID issues due to the lack of a usable switch for the number). I don't think I'll use that drive enclosure again in its current form on any machine and will likely play around with its internals whenever I get a free moment.