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Cbm pet 8296-d black screen

Dave_C78

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Good morning,
today i started pettester and
at a certain point, the screen froze in this mode:
 

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Dave_C78

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I turned the computer back on and now i have this screen....what's happened?? :(
 

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Dave_C78

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Update...i tryied piggyback on all 2114 rams and i saw that Uc7 video ram was bad!
I changed and now seems to be fine...
 

daver2

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Yes, it looks like the video sub-system is not fully stable yet. I suspect the “cause and effect” of removing the 6520 and the video sub-system “fixing itself” may have just been down to good luck!

To remove the 6520 you powered off the PET and “fiddled”.

Yes, from the displays the problem was associated with the ODD half of the video memory - and UC7 fits the pattern. Also, I could determine that is was data bit 4 that was stuck high, and this is also on UC7.

I suspect you are still guessing rather than identifying the fault from the observed results and the schematics. Yes? Perhaps I need to do a little “teach in” for you based on this specific fault. Would that help?

Remind me, didn’t you change UC7 a while ago?

What you need is to get the PET into a stable state, so that every time you turn it on it works!

However, I can’t see how a video fault like this would cause the system to “freeze”. There may be some instability still lurking yet... The only way to guarantee the system has frozen is to look at the CPU SYNC pin. If pulsing it is still doing something... It might not be doing the ‘right’ thing though! But the CPU is doing something.

Dave
 

Dave_C78

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Perhaps I need to do a little “teach in” for you based on this specific fault. Would that help?
Yes, I am a bit ignorant on this issue unfortunately :(
When you have time, it will be an honor and a pleasure to learn something!

Now pettester still working again...i bought from Netherland two 6520....
Pettester still running but i see than letters flickering...
 

Dave_C78

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Some chars are stable and others moves :(
as soon as I turn on everything is stable, after a few seconds some characters start to move...

 

Dave_C78

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after a few minutes of operation, the chars return to being stable...
 

daver2

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This looks either like an IC socket issue or a temperature dependent issue.

1. Have you ordered a can of freezer spray?

2. Have you cleaned up the flux from around all the pins of your soldering?

3. Have you got some contact cleaner to spray on the pins of the ICs in the sockets and then just remove/insert the ICs a few times? When you check with your multimeter - everything may be OK. However, after a period of time, you start to get problems... Try the socket associated with the character generator.

Dave
 

Dave_C78

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No,i haven't bought yet freezer spray...tomorrow!
I have cleaned flux with isop. alcool....Maybe UA3 white socket it's no longer good, if i touch rom can i see little noise on the screen...i used contact cleaner but nothing changes :(
 

daver2

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Yes, if you touch UA3 and the noise pattern changes on the screen - you may have found the culprit.

Equally well, it could also be the ROM... But this is the last scenario you should consider. Assume it is a contact problem first.

As an aside, if you are going to be repairing vintage computer equipment as part of a hobby - then can I suggest writing out a list of tools and test equipment you will need. As time progresses, you could consider purchasing some more useful tools. The top of the list would be a better oscilloscope - along with learning how to use it. Finding noise issues like this is relatively easy with an oscilloscope - but you do need to know what 'good' and 'bad' looks like. Unfortunately, this can only really be determined by looking and experience.

Dave
 

Dave_C78

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Yes, if you touch UA3 and the noise pattern changes on the screen - you may have found the culprit.

Equally well, it could also be the ROM... But this is the last scenario you should consider. Assume it is a contact problem first.

As an aside, if you are going to be repairing vintage computer equipment as part of a hobby - then can I suggest writing out a list of tools and test equipment you will need. As time progresses, you could consider purchasing some more useful tools. The top of the list would be a better oscilloscope - along with learning how to use it. Finding noise issues like this is relatively easy with an oscilloscope - but you do need to know what 'good' and 'bad' looks like. Unfortunately, this can only really be determined by looking and experience.

Dave
Hi Dave,
yes i would buy a new scope, a ram tester and logic tester...
When I have bought a new oscilloscope, can you teach me something please?
 

daver2

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>>> When I have bought a new oscilloscope, can you teach me something please?

Of course. However, before you buy - let's make sure you are buying the correct thing (in terms of specification). There are some excellent (downloadable = free) books on measurement techniques using oscilloscopes.

The one you have (I have a similar thing incidentally - however I also have a much more capable unit) is OK to about 128 kHz - but you need something that can resolve to 16 MHz (if you want to look at the actual clock signal of a PET). In order to resolve 16 MHz, you will require a bandwidth etc. higher than that.

It also depends if you want to buy new (and get support and a Guarantee/Warranty) or second hand.

Dave
 

Dave_C78

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Ok i think buying used oscilloscope....

I ordered some sockets.....i'll change ua3 socket and the two 6520 as soon as it arrives!
We update soon! Good evening sir.
 

Dave_C78

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Good evening Sir Dave, i changed Ua3 socket but i have same troubles... When i touch ua3 rom, chars they moves! So i burned new char eprom and now seems to be fine! Now i still waiting for the 2 6520...we Can waiting for the Jigsaw solution now???
 

daver2

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This is the PET machine that just keeps on giving!

The character generator must have been failing for it to be sensitive to the electromagnetic field you were inducing into it from your body.

Let's see what happens when you get the replacement 6520s...

We finally finished our jigsaw. We are just debating breaking it up and doing the other side (it is two for the price of one). My wife is not too keen about this - but my son is up for it...

Dave
 

Dave_C78

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This is the PET machine that just keeps on giving!

The character generator must have been failing for it to be sensitive to the electromagnetic field you were inducing into it from your body.

Let's see what happens when you get the replacement 6520s...

We finally finished our jigsaw. We are just debating breaking it up and doing the other side (it is two for the price of one). My wife is not too keen about this - but my son is up for it...

Dave
Eheheh Dave...when i finished the pet ... i have a dual floppy to connect and test if it works :(
 

tradde

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Ever do a two sided jigsaw puzzle? Same picture on both sides but rotated 90 degrees on one side. Hard, but fun.
 

Dave_C78

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Good evening!
Update:
Today i receveid the two new 6520, i put them in the sockets and finally i have correct pettester screen (i hope)!
I tryied to insert edit rom but i only got the correct basic screen once and didn't hear any sound...
sometimes I turn on and get a black or blank screen or just a few words .... :(
 

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daver2

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The 00 for the keyboard look OK now.

When you press keys on the keyboard do you see one of the numbers change from a 0 to a 1, 2, 4 or 8 in response?

To get this far means that a lot of the PET circuitry is working. I have my suspicions about what is next...

What we need to work out next is "is your PET consistent and stable whilst running the PETTESTER"? For example, if you turn it on tomorrow does it work the same as it does today? What about trying it in the morning before going to work and when you get home from work?

If it turns on and works consistently, over a number of days, then we can move on to look at why it is not running BASIC. However, if it is not consistent, there is no point until we work out where the problem is...

EDIT:

>>> sometimes I turn on and get a black or blank screen or just a few words ....

Is that with the PETTESTER or the proper EDIT ROM in place?

Dave
 
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