My Model 4 Is Going Nuts!

[Computer Collector]

Something went wrong with my model 4 tonight. Heres what happened:

I go to play a game. So I turn the computer on and then put the disk in and hit the reset button. The screen comes up but goes off to the right and makes a high pitched noise. Its not supposed to do this. So I take the disk out and try to just load BASIC instead.
When I turn it on, it goes completely crazy like a mad dog!
It does different things. Its never the same. But heres an example:
Ill turn it on and see weird characters on the screen:

vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv
vmvmvmvmvmvmvmvmvmvmvmvmvmvmvmv

Then, it will start going nuts. The screen will actually make a sizzle noise and the letters will start getting blurred and fill the whole screen with a snowish look. Even parts of the screen that normally stay black will fill with the snow!

I quicky turn it off when this stuff happens.

I cant even load a program!

AHHHHH!

5 or 10 minutes pass.
I figure that maybe my baby is feeling better.

I go and turn it on and it works fine. I hold down BREAK and hit reset and pull up BASIC with no problem; everything works like new.

WTF is going on here?

 

[chuckcmagee]

Sounds like a power spike to me.

 

[Computer Collector]

Well, tonight I was adding up my checkbook on it. I only had it on for a minute and then all of the sudden...



BLIP!


The screen immediately goes dark and makes a little sizzle noise.

I quicky shut it off. I havent tried to turn it on again.


So, whats this power problem you speak of? I need to replace the power supply or what?

 

[chuckcmagee]

Oh, I had thought it might have been just a surge in the local power company supply. That was a good guess, one of many, but obviously incorrect. The high voltage part of your CRT circuits is going or has gone bad, the now infamous "flyback transformer". I say infamous because they are famous for this kind of thing.

If you are good at this sort of thing, you will likely have to find a working part in another model 4 and replace the one you have. Once they start doing what it is doing, it burns through the insulation in a progressive manor, it will only get worse.

If you are going to attempt it, BE CAREFUL! Some designs are poor and leave a very high voltage on the CRT tube, EVEN WHEN OFF! Better designs have a resistor that slowly bleeds the charge down after being turned off. I have no idea which kind is in the mod 4.

 

[kb2syd]

I speak from experience. The bad kind. And it hurts!

 

[Druid6900]

Well, also speaking from experience, I'd suggest desoldering and re-soldering all the legs of the flyback.

Just like in the early Mac one piece units, the flyback is mounted perpendicular to the ground and, from years of heating up and cooling down, they tend to develop cracks in the solder and it needs to be removed and replaced as this results in high-resistance connections.

To discharge the CRT, use a long thin flat-bladed screwdriver with a fairly heavy jumper cable (the ones with little alligator clips on each end, but not the cheap thin wired ones) connected to it and to the metal chassis of the computer with the unit plugged into the wall but TURNED OFF. Slip the blade under the rubber suction cup on the second anode which is attached to the CRT. You'll know when you hit the metal anode or the aquadag around the CRT "button". Try not to touch anything else.

After the discharge, unplug the computer from the wall, remove the CRT driver board from the side of the case and do the re-soldering to the flyback.

Chances are about 90% that this will solve the problem. Chances are only 22% that you will kill yourself in the process

 

[Terry Yager]

Experience, etc...

Experience, etc...

Chances are about 90% that this will solve the problem. Chances are only 22% that you will kill yourself in the process

So, you're saying that the odds are in favor of a successful repair? What about Murphy's Law? In my experience, Murphy always takes the side of the hidden flaw in one's logic...

--T