800XL Boots intermittently

[hueyjones70]

Computer sometimes boots sometimes shows dark green screen. The good boot or no boot does not fit any pattern. It might fail to boot on 10 consecutive attempts or it may boot on 10 consecutive attempts. When it doesn't boot, I hear a click like it is going to boot but then nothing. I have tested all the 40 pin ICs in another computer and they all work.  SYSCHECK says the memory is OK. SYSCHECK always comes up so whatever is wrong must be something that SYSCHECK over rides when it is checking memory.

[xrbrevin]

5 hours ago, hueyjones70 said:

must be something that SYSCHECK over rides

Maybe OS ROM or it's co-conspirator the MMU..

can you test them in another machine?

[Mclaneinc]

The click sound is interesting, bad pokey?

 

But then again, if it's an IC it's usually bad or not so it would rarely be intermittent, which make me wonder if a component that can charge up ie caps etc are more likely targets..

 

Then again, what do I know... Best of luck OP, there's top people on here.. The syscheck always booting could indeed be a huge clue?

Edited May 31, 2022 by Mclaneinc

[Classic Pac]

You could have a bad cap

[Rybags]

A click sound is normal on powerup/reset and usually a sign the OS is at least starting up.

Press Reset and see if it keeps occurring.

 

But in any case, more info needed as in does the text screen appear?  And do you have disk drives or anything else hooked up?

[Mrshoujo]

Check solder joints and dirty IC legs and sockets. Basic things first.

[gilsaluki]

5 hours ago, Classic Pac said:

You could have a bad cap

I wish things could be cured by a bad cap.  In all my years, I have never seen one.  

[xrbrevin]

yeah its a rare instance when they do, maybe it says something about the OEM power supplies? (except the obvious 120v ingots, of course)

[hueyjones70]

When it boots, it boots to the ready prompt. I have used two different power supplies. When it doesn't boot it still has 5.0xx on several points on the MB. I didn't check the MMU yet but I will today. I haven't tried it with a disk drive but I will. When it is on a non-boot streak, Star Raiders also won't boot. 

[Mclaneinc]

1 hour ago, xrbrevin said:

yeah its a rare instance when they do, maybe it says something about the OEM power supplies? (except the obvious 120v ingots, of course)

Tis true, if it's a cap issue it's usually on the power side, and so very easy to visually spot.. I wonder if flaky ram could cause an issue?

 

All guesses on my part...

[TGB1718]

35 minutes ago, hueyjones70 said:

Star Raiders also won't boot.

Could be wrong, but doesn't that usually point to dodgy RAM as @Mclaneinc indicates

[hueyjones70]

I was mistaken when I said that SYSCHECK always comes on, and I only get the click on a nonboot if SYSCHECK is present.

[hueyjones70]

I checked the MMU and it isn't the problem. Disk drive has no activity on a nonboot

[1050]

I would have long ago changed out U18  for a 74HCT08 AND gate and since I bought a stick of them and the other one I like to change, U30 for a 74HC51 chip as well. Can't get the latter one anymore so it looks like someone will have to make something functional out of two smd AND gates and a smd NOR gate perhaps? U18 is often enough alone to give it a solid system clock, U30 just added to the RAM stability since it's creating /CAS and /RAS signals.

[hueyjones70]

On 6/2/2022 at 6:05 AM, 1050 said:

I would have long ago changed out U18  for a 74HCT08 AND gate and since I bought a stick of them and the other one I like to change, U30 for a 74HC51 chip as well. Can't get the latter one anymore so it looks like someone will have to make something functional out of two smd AND gates and a smd NOR gate perhaps? U18 is often enough alone to give it a solid system clock, U30 just added to the RAM stability since it's creating /CAS and /RAS signals.

I installed a 74HCT08 and the computer has booted every time since then. I also tried a 74HC51 and the computer would not boot with the HC51 installed.

[1050]

Thanks for the feedback, well I was half right...

 

That's a new one for me on the HC51 NOT doing as well as the HCT08. Going into my files too. I have to suspect something is special about this one, but since I'm shooting blind anyway, I have no way to know where to look first. Have you tried the selftest memory section and does it show any problems?

 

Any better diagnostic tools at your disposal? I got one of Jurgen's Sys-Check devices somewhere, but I've yet to actually use it. Don't know if it would help anyway either.

http://www.van-radecke.de/STUFF/tfhh_HW_info.pdf

 

One of the benefits of having a modern PC run eprom burner is that they often can ID AND check these kinds of logic chips for proper function. Maybe that HC51 is what they call dodgy?

[hueyjones70]

I never did run the on board Memory Check.  The SysCheck does a better job and it works when almost nothing else does.

 

[hueyjones70]

My belief that the problem was solved was premature. The computer is back to the sometimes boot/sometimes not. I recapped it just in case, no joy.  

[hueyjones70]

I think I have confirmed that the problem is a computer chip. I was swapping chips from BAD XL to GOOD XL and eventually BAD started working and GOOD kept working. I would work on other tasks then go back and power on both computers and after several repeats the GOOD stopped working and BAD continued to work. Now I have to backtrack and see if I can determine which chip is the culprit.

[hueyjones70]

This may be unrelated to the booting issue bit when the BAD computer does boot, the colors are off and dark.

[hueyjones70]

Yesterday I repaired an 800XL by replacing the BASIC socket. This one had different symptoms and I thought it might be the MMU because when it booted with SYSCHECK all of the memory checked out. I took out the MMU socket (stock single wipe) cleaned up the socket area and installed a new double wipe. That eliminated the problem. That is two computers in as many days that I have repaired by replacing sockets.