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faulty model 4


daafies
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hi all, i recently bought faulty model 4 from ebay. its in really good condition, other then i get a screen full of characters when i start it up. also the disk drive spins a bit and stops.

 

its a non gate array, with 64K

 

here's what i tried so far:

 

1) i reseated all the ICs,

2) checked the motherboard for cracks and lose connections/soldering. eveything there looks ok.

3) checked voltages from power supply. i get like +4.9, 11.9 and -11.9

4) rearranged the memory ICs

5) i tried break+reset, enter 2x and typed "out 244,1" + enter but no drive activity

 

just now the while typing the power suppy filter caps went up in smoke lol. very exciting. could one use a more modern power supply like an ATX? i have an old dell one here with +5, +12, -12 ?

 

 

any ideas what i should try next? new CPU?

 

thanks very much in advance!

Edited by daafies
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Does the screen come up the same every time or does it change every time you turn the machine on or reset it?
If it's the same, did changing the order of the RAM change the screen.

If the screen stays the same and changing the order of the RAM chips changes the screen, I'd say it's bad RAM.
If the screen changes every reset, then maybe the CPU.
There are many other things that could be wrong though.

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Does the screen come up the same every time or does it change every time you turn the machine on or reset it?

If it's the same, did changing the order of the RAM change the screen.

 

If the screen stays the same and changing the order of the RAM chips changes the screen, I'd say it's bad RAM.

If the screen changes every reset, then maybe the CPU.

There are many other things that could be wrong though.

 

 

screen is pretty much same with every reset. i dont recall if it changed with the RAM chip reorder. i'll have to check again.

i can also change the screen with holding the break key at power up. what RAM chips should i get? old stock? or is there new equivalent i can use?

 

and on the power supply caps. can i use mylar film? i got NTE caps here laying around that meet the .01uF and .22uF. MLR103k630 and MLR224K630

 

thanks!

david

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screen stays the same. also after rearranging the RAM chips. maybe RAM is ok then? possibly :)

Maybe, maybe not. A bad ROM *might* be consistent, bad RAM *might* be consistent in spite of the chip order... it's probably not the CPU.

Turning it off and on should alter the display if the RAM is ok and the CPU isn't writing junk to it due to a faulty ROM.

I rarely see ROMs go bad.

 

Are any of the RAM chips hot to the touch?

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i added some pictures of the screed. the smaller fonts are when you turn on the computer normally


i get the larger fonts when i hold any of the funtion keys and power on the machine. (i also here a click. relay maybe?)


holding break and powering on doesnt result in anything different.


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Really, it doesn't tell me much except I think the CPU is running since it changed to 32 column mode.
Every other character is displayed in that mode.

I can also tell that all of the bits are set for some of the graphics characters to it's not necessarily a problem related to a single bit in video RAM somewhere.

However, I think that A (whatever accent character that is) is supposed to be a blank from clearing the screen.
That could still be video RAM or some data logic between the CPU and video RAM.
*edit*
Keep in mind, it could also be the character generator circuit but I think something would show up on screen like it was asking the normal startup questions on the Model III.
Cass?
Memory Size?

If you hit enter a couple times does the display change?

Edited by JamesD
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There are multiple ways... the first, a direct link to your image in a gallery like this...

gallery_35324_1027_787582.jpg

 

Or like how you did it...

 

 

Or by clicking on the "More Reply Options" button and uploading it from your PC.

 

aha! more reply options. cool :) thanks for that!

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Really, it doesn't tell me much except I think the CPU is running since it changed to 32 column mode.

Every other character is displayed in that mode.

 

I can also tell that all of the bits are set for some of the graphics characters to it's not necessarily a problem related to a single bit in video RAM somewhere.

However, I think that A (whatever accent character that is) is supposed to be a blank from clearing the screen.

That could still be video RAM or some data logic between the CPU and video RAM.

*edit*

Keep in mind, it could also be the character generator circuit but I think something would show up on screen like it was asking the normal startup questions on the Model III.

Cass?

Memory Size?

 

If you hit enter a couple times does the display change?

 

:/ no hitting enter a number of times has no effect. but who knows enter key could be bad too :) i tried the enter key on number key pad also. assuming it does the same thing

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The block diagram in the tech manual shows data buss and address buss buffers. If you can find those, reseat those as well.
That's looking very much like a ROM issue. Did you check the drive controller to see if it's chips are seated ok?
*edit*

That's definitely not video RAM.

Edited by JamesD
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