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Another PET 2001 motherboard needs your help...

dave_m said:
From looking at the latest batch of photos, it is clear to me that the real culprit is the D8 NAND gate. The reset pulse output from D8-8 is activating low, well before the enabling 8 MHZ clock goes high. Basically, the NAND gate is acting like an inverter of the flip flop Q output and feeding it right back to the flip flop reset, creating a glitchy signal on C8-3 instead of a normal 43 nS pulse.
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Absolutely! Oh my, what a good night's sleep can do to your ability to think, so that I only see this today. Indeed it looks like the NAND simply inverts the one input.

André
 
MikeS said:
Andre: Did you get my email about your page on 6502.org?
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Yes I got it. It's still in my TODO box for web site updates. But I have to check it, because IIRC the ROMs were coded so that you could put them into some(!) different sockets and the thing would still work.
 
dave_m said:
On your monitor, did you have to fiddle with the horizontal hold or did it sync right away?

Is the PET 9" video signal totally NTSC compatible?
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No, the 9" PET actually has 16000 cycles per screen, i.e. a refresh rate of 62.5 Hz screen refresh rate. It's close though. But even the German 50Hz models have this number of cycles (seems logical, I haven't seen any modifications / configuration jumpers in the schematics). I found that when I tried to used the screen refresh rate to find the actual speed of the system for my 8MHz accelerator board benchmark...
 
fachat said:
Absolutely! Oh my, what a good night's sleep can do to your ability to think, so that I only see this today. Indeed it looks like the NAND simply inverts the one input.

André
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So if we are all in agreement, do you think I should change out D8?
 
Sometimes you roll the dice and you win
Sometime you roll the dice and you lose.

Today...we won!

Longer message coming soon.
 
So the summary goes: either troubleshoot with every possible device you got and replace one chip at a time, or save some time and replace every possible chip all at the same time? Then again, you probably could build your own PET from scratch if you were to replace every chip, good or bad.
 
carlsson said:
So the summary goes: either troubleshoot with every possible device you got and replace one chip at a time, or save some time and replace every possible chip all at the same time? Then again, you probably could build your own PET from scratch if you were to replace every chip, good or bad.
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Gee, I thought we did pretty well remotely diagnosing the old-fashioned classic way, without replacing every possible device; pinpointed the 'LS165 immediately as the cause of the vertical lines, Dave and Nicolas diagnosed the GAL address problem, and eagle-eyed Dave noticed the IC D8 anomaly that the rest of us all missed.

I'd say that the summary goes: take lots of scope measurements, show them to Dave, and don't listen to Mike. ;-)
 
My version of the complete fix started in July 2009, about a month after buying the PET from Yahoo Auction Japan.

It arrived DOA, The first thing I did was to replace the caps and 7805's in the power circuit, but that did not help. Fortunately Ethan Dicks from the cbm-hackers list was kind enough to offer to take a look at it if I sent it to him. In the end I sent him two PET 2001 boards. The other had been given to me and was also in some trouble. Ethan was able to get the other board fully working, and fix the power circuit in the Japanese board.

So the complete list of thanks and fixes are:

• Ethan Dicks - Replaced dead diodes in power circuit.

• Burn new character ROM (CHARGEN ROM) as the original was dead.

• Ruud - For writing a test program that could be written to an EPROM and used in troubleshooting.

• MikeS - For suggesting to replace the 74LS165 at B2 to fix the vertical line problem and for also suggesting to remove C8 (C8 was actually ok, but doing this allowed for much more detailed testing).

• dave_m - For finding the final fix for the offset characters. 74LS00 at D8.

• André - For following this fix from cbm-hackers to VCF, and for his brilliant timing diagnosis which pointed us all in the right direction for the final fix.

• Nicolas Welte - For correctly diagnosing a build issue with the RAM/ROM board I was using for testing. I had placed the resistor pack in the wrong way. The other issue with the RAM/ROM not booting BASIC4 was solved by me, as the Flash ROM which came with the board was corrupt. I also built one of Nicolas's 6550 video RAM replacement boards which was installed in the machine too.

• Anders - For sending me a keyboard from a PET 3000 series machine. I built up an adaptor to connect it to the 2001. Works great.

• Matthew D'Asaro - For making the PET composite video adaptor, which was invaluable for testing.

Hopefully I have remembered everything correctly.

Well, I'm sure there are more people to thank, but I think this list covers off on the top 8. Thanks everyone.

I learnt a lot of things during this fix, in particular I learnt how to use a scope for basic testing.

I have to say I feel a little sad that the machine is fixed. What am I going to do with my evenings now?…Oh yeah, I'm going to try out the floppy drive and I/O interface that Tezza sent for my Dick Smith System 80 :)

Thanks again everyone.

Phil
 
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You forgot to thank your wife for her infinite patience.

It's been fun, and thanks from me as well; I enjoyed the thread and learned a few things along the way, and a good result is always satisfying.
 
MikeS said:
It's been fun, and thanks from me as well; I enjoyed the thread and learned a few things along the way, and a good result is always satisfying.
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I echo that sentiment as well. I never knew the details of how the original PET handled video until this exercise.

As I put my schematics away, I can hardly wait for the next guy's PET problem. :)
-Dave
 
Well done guys!

Yes, this is the thing after such a marathon as this. After the euphoria wears off you think "what next".

The System 80 disk controller shouldn't give you as much trouble Phil. In fact, you should be able to just plug and play. "Should" does not always equate with "does" where old flaky vintage computer equipment which has travel half the globe is concerned though. (-:

Tez
 
tezza said:
The System 80 disk controller shouldn't give you as much trouble Phil. In fact, you should be able to just plug and play. "Should" does not always equate with "does" where old flaky vintage computer equipment which has travel half the globe is concerned though. (-:
Tez
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I'll see if I can't fire it up tonight. Still have the disks I was supposed to send weeks and weeks ago. I will try to get them out soon.

Speaking of which, Mike, I think I owe you some RAM too. Send me a PM with your address and I'll see what I can do.
 
Oh, in that case it took less than two years from when you got it to it being fully fixed up. Not so bad given the computer itself should be about 33-34 years old.
 
MikeS said:
It's been fun, and thanks from me as well; I enjoyed the thread and learned a few things along the way, and a good result is always satisfying.
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Yes, absolutely true for me as well. I'll post something with a link to this thread on my "PET repairs" page on my next web site update

André
 
I really love that hybrid. You have a Japanese PET motherboard with a 6502 RAM/ROM adapter as well as composite video adapter IIRC, connected to a Swedish PET keyboard and mounted in a steel briefcase. Now whenever those memory card based interfaces get common and inexpensive, you'll have room to fit one of those inside the briefcase as well. Just don't bring it to an airport. The security might get curious about the contents...
 
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