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8032 50 Bytes Free when switched on.

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waltermixxx

Experienced Member
Joined
Apr 3, 2015
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132
Location
Canada
Hi folks I was wondering if anyone has encountered this issue before.
(First let me start by saying i had a good read through the "CBM8032 What does the start up chirp really mean" thread,
lots of very good stuff in there. ) Okie doke back to my issue.

I am fixing an 8032 Universal board for a friend. When powered up, the computer chirps and reports Commodore Basic 4.0 message along with
the amount of free ram. Here is where the issue is. It says only 50 bytes free. I have had luck in the past fixing ram issues by piggy backing
ram onto existing ram, to locate the bad ram. That trick did not work this time, even with 16 good 4116 rams chips piggy backed. it got me thinking, basic is working, some ram is working, the CRTC is working. What could the issue be? So I loaded up an eprom with the DiagV4 diagnostic rom image (PETTESTE2K – Version04.), turned on the computer and got the image below.

The diagnostics stops on the page 0 and page 1 test it looks like.... it got me thinking:

Could it be the CAS0 or CAS1 signal? I'm thinking: So without proper RAS and CAS signals I think the dynamic ram on the pet bottoms out to 128 bytes of ram. So I am going to scope the ras and cas0 and cas1 lines and see what is there... I'm kinda thinking it's not a ram issue, could be a support chip, either one of the 74LS244's or perhaps the 74LS00 (UD5) I will check what's going into those chips, and what's coming out. The data bus and the address lines in general appear to be good because Rom is being accessed for basic and the video ram is being accessed properly at 8000 (nothing strange on the screen) I'm very new to fixing pets, and sometimes a little bit of knowledge is a dangerous thing. :) I'm hoping I'm on the right track. the only thing that does not support my theory is the b's in the Pet tester diagnostic result.... hmmmmmmm

ramtest.jpg
 
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waltermixxx said:
It says only 50 bytes free.
Click to expand...

That means the quick memory check on power-on found an error at around memory location 1075 (1025+ 50). Since piggy-backing did not work, the fault may be a a bit stuck at zero.

So try a poke 1075, 255 and read back the location with print peek(1075). If it reads back something else, that will tell you what bit is bad when you convert the data to hex.
If that test passes, they a poke 1075,0 and read that location again with a print peek(1075).
 
Thank you, I will give that a try :) and report back. :)

Ok, poke1075,255 then print peek(1075) yields 239, which is the 5th bit stuck off?
1,2,4,8,16 so bit 5 appears to be stuck off, on the 1075th memory location? So I am back to a ram
issue? but this kinda limits it to 1 ram chip, D4 stuck at zero (UA11/lowerram) :) so I can pull that one, and see.
(or perhaps try a piggy back ram on that chip alone... )... Am I on the right path? :)

or follow the bit back, before the ram, and see if it's stuck there... :)

ok so D0-D7 with no additional letter means these go right to the processor,
so chances are it's not anything (most likely) before the ram, as those signals are not buffered.

so back to ram. :) sorry for the stream of consciousness, but it allows folks to see my train of thought,
and they can advise if I'm barking up the wrong tree.

So I will report back after i have either successfully piggy backed a ram, or removed the ram,
and socketed one, with success or no luck. :)
 
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As dave_m has previously stated, piggy-backing only works in some circumstances.

If a RAM bit is being driven LOW (by a faulty RAM device say) - then piggy-backing it with a healthy RAM device will not help (the bit will always get dragged low).

Dave
 
Okie doke. It's time to fire up the PACE de-soldering tool and pull out UA11. :)

I appreciate you guys stepping in to assist. I will follow up once i get a chance to pull that chip,
and install a socket, and replace it.
 
Thank you Gentlemen, I now have have 31743 bytes free. :)
seems to be working. I will put the PETTESTERv4.0 in and just let it do it's thing.
and see if anything else is up. :)

Thank you again. :)

Not sure how to change the title of this thread to indicate SOLVED.
 
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Excellent.

Yep, let my PETTESTER have a crack at the memory now to make sure everything is now clean.

Dave
 
Yup I let it run for at least 45 minutes or so, and it had done quite a few successfull passes of the final ram test.
i just let it keep going. :) It did not fail :) thank you again for the tools and assistance with this repair. :)

I put the board back in the SuperPet, plugged in the "super pet" board along with it's MMU sub board,
and all is working... The only thing left is the keyboard quite a few of the keys do not work, I am no stranger
to all the screws required to get to the board to give it a good proper cleaning. along with all the little plungers.
I've done a few already :) But I think all the hard stuff is done. :) Thank you again :)
 
Just catching up on all of the posts - nice work tracking down the faulty IC. Quick question, the MMU board, is that an original TPUG or one from Retro innovations ?

What OS/9 programs do you run ?






waltermixxx said:
Yup I let it run for at least 45 minutes or so, and it had done quite a few successfull passes of the final ram test.
i just let it keep going. :) It did not fail :) thank you again for the tools and assistance with this repair. :)

I put the board back in the SuperPet, plugged in the "super pet" board along with it's MMU sub board,
and all is working... The only thing left is the keyboard quite a few of the keys do not work, I am no stranger
to all the screws required to get to the board to give it a good proper cleaning. along with all the little plungers.
I've done a few already :) But I think all the hard stuff is done. :) Thank you again :)
Click to expand...
 
I wish it was mine, but it's not. I was just fixing it for a friend (Steve Gray). When I'm trying to fix a board, I pull it out of the original machine, and set it along the left side my working 4032 (with a universal board in it), so the power, keyboard and monitor cables of my working pet can reach it. So I have unobstructed access to it. Because it was a universal board, I guess I wanted to focus on that, as it indicated 50 bytes free with or without the Super Stuff. :) The MMU board is a Retro-Innovations' board. :) I have no experience with the SP9000 but I should have a chance to play with it a bit before he comes over to pick it up. :)

I once again would like to thank you guys for your help with this. I can use what you taught me here, with any other boards that come my way. Along with that incredibly helpful Diagnostic Rom :)

Cheers. :)
 
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You're welcome.

The diagnostic ROM has been worth the effort! I need to find some time to produce V05 at some point and fill the 2K up a bit more...

Dave
 
waltermixxx said:
I was just fixing it for a friend (Steve Gray).
Click to expand...

Walter,
I'm glad you are doing a good job for Steve Gray. Say hello to him for me. I got good use out of his Editor ROM to allow use of a C64 keyboard in a PET, and also his CBM Transfer (CBMXer) program which simplified the transfer of PET programs from the PC to a Commodore 4040 floppy via a gadget called Zoomfloppy.

He creates good software and hardware to support the PET.
-Dave
 
Aha.. Thank for the reply. Steve’s a good guy ... enjoyed working with him on the Supersoft stuff
 
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