1200XL Boots About Half the Time

[toddtmw]

Hi. I just acquired a 1200XL. I added a Best replacement Mylar (Awesome!!!) to get the keyboard working. I also gave it the salon treatment with a couple of hours in the sun to get the unsightly yellowing out of the case.

 

When I first got it, it would only try to "go" about half the time. (getting to the Atari Logo or booting DOS or booting the cartridge depending on the circumstance) The keyboard didn't work, so I couldn't really do much else with it. Once I got the keyboard working, I connected the keyboard to the bare board and played around with it a little. I booted 4 or 5 times and it booted fine each time.

 

Then, I put it all back together and now about half the time, it doesn't boot. Most of the times when it doesn't boot, the screen is green. (Sometimes it is just black, this seems to be more often when there is a cartridge in.)

 

I can tell when it is "going" to boot because it makes the tick sounds Atari's make when they first turn on. (not the drumroll, just the two taps like what you listen for on an 800XL to know that you can let go of the option key)

 

Once it boots, it seems to work well. I left it runninng all tests on the self test for about a half hour with no issues.

 

Has anyone seen a computer that only fails to boot sometimes? Thoughts on what could be causing the green screen?

 

Any other advice?

 

Thanks!

[+DrVenkman]

The solid color/"wants to boot" thing indicates that the ANTIC is working, at least. Could be the little 47uF (I think that's the value) capacitor in the reset circuit is starting to fail, could be the CPU is failing. Could be one or more of the IC's inside has a dodgy socket or there's corrosion/dirt on the leg of a chip and it's not always making good contact. Could be one of the two (yes, two!) 7805 voltage regulators is starting to fail. Really hard to tell, honestly. Just out of curiosity, what happens if you press RESET when you get the green screen?

 

Anyway, after my ordeal with mine, the very first thing I'd do is open it up again, look at everything closely under very bright light and magnification and just see if I see anything, especially debris around circuit traces or the legs of the two voltage regulators. Then I'd measure the output of the rectifier (the giant four-contact block mounted horizontally in the right/rear of the case) by measuring the input legs of the 7805's. It should be at or near +12VDC. Then I'd check the output legs of both 7805's, which should be very close to +5V DC.

 

If those all check out, I'd start looking at the chips needed to boot, starting with the CPU. It might be it merely needs to be removed and reinserted, or even simply raise up each side a bit in the socket and reseat it, to break any surface corrosion. Do that for all the main chips and then see if it changes anything.

[toddtmw]

So, that was an interesting test. Pressing reset on the green or black screen causes it to boot just fine.

 

Not sure what that means...

Edited August 17, 2017 by toddtmw

[+DrVenkman]

So, that was an interesting test. Pressing reset on the green or black screen causes it to boot just fine.

 

Not sure what that means...

 

Yeah, I don't either honestly but I'll tell you why I brought it up.

 

My usual "daily driver" is an 800XL with an Ultimate 1MB and UAV Revision D board inside. When I have nothing connected to the SIO port, or when I have only my chain of two 1050 drives connected, powering on is completely normal. However, if I unplug my physical drives and connect my usual chain of P:R:Connection + Lantronix box, with an SIO2USB device plugged into the P:R:Connection, I very often get a solid color screen. But if I press RESET, it boots up normally.

 

I don't know what that indicates, exactly. In the case of my 800XL, I think it's likely got something to do with the reset circuit and how it interacts with devices on the SIO chain that draw current. I might replace the reset circuit capacitor next time I have it open just to see, but for now I don't worry about it.

[+Stephen]

So, that was an interesting test. Pressing reset on the green or black screen causes it to boot just fine.

 

Not sure what that means...

Sounds like the issue is in the reset circuit upon powerup.

[Keatah]

Did you use any chemicals when you set it out in the sun?

[toddtmw]

Yes, I used the hair stuff from Sally's. but it was the empty case and I washed it and let it set for three days before putting it back together.

[Bryan]

I have a 1200XL with the same problem, so it's a thing. Haven't looked at it too much yet.

[toddtmw]

Yeah, now that I know I can press reset to get it to go, I'll probably leave it until it really breaks, and then trouble-shoot it.

 

Thanks everyone, especially DrV.

[sup8pdct]

I pulled mine out after some years of non use. 1st turn on, got normal boot blue screen for 1/2 a second then nothing. After LOTS of messing around swapping and checking chips, power rails, verifying roms etc, I finally traced it to C7, part of the reset circuit.

Only clue that I had was that after inserting Star raiders (which has a bit set to tell the os it is a diagnostic cart and boots it after minimal os setup), I could sometimes see the star field then disappears but mostly hear the sound it makes when it starts, constantly start and stop. I fixed it by feeding +5v into the + end of cap for a while. Maybe not a permanent fix, but did get it going again.

 

James

[+DrVenkman]

I pulled mine out after some years of non use. 1st turn on, got normal boot blue screen for 1/2 a second then nothing. After LOTS of messing around swapping and checking chips, power rails, verifying roms etc, I finally traced it to C7, part of the reset circuit.

Only clue that I had was that after inserting Star raiders (which has a bit set to tell the os it is a diagnostic cart and boots it after minimal os setup), I could sometimes see the star field then disappears but mostly hear the sound it makes when it starts, constantly start and stop. I fixed it by feeding +5v into the + end of cap for a while. Maybe not a permanent fix, but did get it going again.

 

James

 

Why not just replace the capacitor?

[1050]

And why not double it's value to increase RESET pulse width

while you are at it which might be the underlying issue on the

1200XL. Perhaps even go with 74HC14 for U16 to insure that

all chips getting the reformulated TC (time constant) reset pulse

have it being pulled to ground with authority.

 

Bryan put it best imo, 'so it's a thing'.

[sup8pdct]

 

Why not just replace the capacitor?

That would mean finding the soldering iron, solder and a radial cap. All far too much at this point in time.

 

P.S Complaining? Ooohhh hell yes

 

James