System 80 - Garbage on screen.

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Gary Hucker

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Feb 28, 2016, 12:30:52 AM2/28/16
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Hi,

I recently pulled out my System 80s and decided I would try to fix a problem with one of them displaying garbage on the screen.
I have another good unit I have been using to diagnose this problem. The broken unit has had many mods done to it, most of which I'm not sure of their function. Being new to these computers, I learning as I go.

I did some basic checks as follows...
Pressed in all of the socketed chips.
Swapped out the ROM and the three socketed chips on the video board with no change. Sorry not sure of their names.
Reseated the piggy backed RAM.
Swapped out the CPU with another of unknown state but the problem is still there.
Installed the CPU board from the other machine with no apparent change.
There appears to be some gunk close to the cap on the lower right of the video board but it is not bulging. Could the fix be as simple as replacing this cap?
The video column select switch does change the garbage on the screen.
Cannot tune the signal via RF.

If anyone has any idea about what I could try, that would be appreciated.

Thank you in advance.
Gary Hucker.

Terry Stewart

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Feb 28, 2016, 4:04:01 AM2/28/16
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A couple of other possibilities..

One, it could be a RAM chip.  4116 chips do fail.  Swapping the RAM out with known good RAM is one way to check, but if it's 48k and piggybacked, then this is not so easy. From memory I think some track cutting or jumpering was also done with the 48k mod, so it's not just a case of filling the sockets up with 8 4116 chips. 

Those plastic covered steel connecting ribbons connecting the two main boards.  These may not be seated properly or a wire could have snapped off the soldered base at one end.

Terry






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don gillies

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Feb 28, 2016, 6:22:37 AM2/28/16
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Did you try the known good video board with the CPU board from the bad computer? And while you’re swapping boards, there is a good chance you’ll break a fragile connection where the 32-way cable connects to the boards, compounding problems. Like Terry mentioned, there is a good chance this cable is the cause of the original problem, before you started adding new problems by disturbing this cable. It’s worth checking the signal on each wire in the cable, at each end, at the component the signal is supposed to be getting to/from (not just at the cable end).
 
It’s definitely worth checking the dc voltages if you don’t know the history of the computer. If you know it once worked, there’s a very good chance the problem is with the cable. If you don’t know whether it ever worked, then all those modifications are a worry, and I wouldn’t know what to suggest other than  painstakingly trying to restore it to the original circuit as per the schematics available on Terry’s website.
 
My knowledge of this is not as good as other members of this group, but in my limited experience the garbage screen was never caused by a faulty CPU or ROMs, and only once ever by a faulty logic chip (on the video board) so I tend to regard the probability of a faulty logic chip as extremely low. Also if the computer is known to have worked, the chance of a RAM suddenly failing without being interfered with in my experience is extremely low (like, never). Hopefully no-one has been pushing too hard on the RAMs in the hope of fixing connection problems.
 
Don
 
 
 
 

phil colbourn

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Feb 28, 2016, 2:38:05 PM2/28/16
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You could post a picture of this garbage.

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Gary Hucker

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Feb 28, 2016, 9:42:24 PM2/28/16
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Thanks Terry,

I checked the continuity between each of the cable strands by metering on the back side where the connectors solder into the boards and they are okay.
I'm thinking there may not be any problem with the left hand board because I swapped in one from a working system and the garbage is still there.
I will try the video board from my good machine and see what happens.

I love your videos. They are a great source of inspiration.

Gary

Gary Hucker

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Feb 28, 2016, 9:53:10 PM2/28/16
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Hi Don

Thanks for your input.

The working state of the machine is unknown to me as I bought it via trade me in as is condition. I did score 'Programming the Z80' by Rodnay Zacs with it along with a Microsoft Basic book which are useful.

I was thinking of swapping in the good video board but there will be a bit of work to temporarily disconnect some of the mods. I think it is worth doing. I'll try checking continuity of interconnect cable between components. Checked the dc voltages from the power supply and they measure okay but I didn't check them for ripple as I'm not sure how to do that exactly.

It will be good to have a good working machine for spares etc.
I'll let you know how I get on.

Gary.
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