10th Mar 2021, 1:32 pm | #841 |
Octode
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Re: Non-working Commodore PET 3016
Progress with a capital P.
I replaced UH6 with a new chip and I now have something on the screen. I could have a little weep. The screen is full of the semi-colon character, but I'm still happy. Where do we go from here? Colin. |
10th Mar 2021, 2:11 pm | #842 |
Nonode
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Re: Non-working Commodore PET 3016
Fantastic! That's real progress as the screen display will help you diagnose the remaining problem(s). I think (remembering I'm no expert) that displaying a single character repeatedly could indicate an issue with the CPU or the clock signal. Are you using the original or the replacement CPU? Is it properly seated?
Alan |
10th Mar 2021, 2:26 pm | #843 | |
Octode
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Re: Non-working Commodore PET 3016
It does the same with both CPUs, and also with no CPU inserted - I have soldered in a new 40 pin socket during this thread though.
At least it means the monitor works - I was not looking forward to fiddling with the voltages in there. Colin. Quote:
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10th Mar 2021, 2:30 pm | #844 | |
Octode
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Re: Non-working Commodore PET 3016
Quote:
I thought we'd established the CPU is running, so I'd plump for dodgy ROM or a stuck buffer in the display logic. |
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10th Mar 2021, 2:32 pm | #845 |
Nonode
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Re: Non-working Commodore PET 3016
The fact that the screen doesn't change with no CPU is a good starting point and much better than having to delve inside the monitor. Worth just checking that 5V is still reaching the CPU but I'm sure others will be along shortly to advise on further investigations.
Alan |
10th Mar 2021, 2:36 pm | #846 |
Nonode
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Re: Non-working Commodore PET 3016
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10th Mar 2021, 2:49 pm | #847 |
Octode
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Re: Non-working Commodore PET 3016
Great minds think alike. About to do it.
Colin. |
10th Mar 2021, 4:04 pm | #848 |
Dekatron
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Re: Non-working Commodore PET 3016
This is excellent news! It's certainly close to working now.
__________________
If I have seen further than others, it is because I was standing on a pile of failed experiments. |
10th Mar 2021, 4:37 pm | #849 |
Dekatron
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Re: Non-working Commodore PET 3016
Yes, a great leap forward, and what a relief that the monitor appears to have survived despite having been exposed to all that damp.
I did think Colin had already read the content of the character ROM but it's not a bad idea to read it again. (I didn't get that one to check with the others). I think it is reasonable to assume that the CPU is OK because we were able to execute NOPs, as shown by the fact that it ran smoothly through the address range when we did. |
10th Mar 2021, 4:42 pm | #850 | ||
Octode
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Re: Non-working Commodore PET 3016
Quote:
I think if it was the display ROM then you probably wouldn’t see correctly formed characters. Try checking the input and output pairs on UB3. Pin 3 compared to pin 2 etc. |
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10th Mar 2021, 5:09 pm | #851 | |||
Octode
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Re: Non-working Commodore PET 3016
UB3 is a new IC - replaced earlier on in this saga.
Happy to still check it; are you saying to check all pins 2-9, 11-18 for waveforms? Colin. Quote:
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10th Mar 2021, 5:35 pm | #852 |
Octode
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Re: Non-working Commodore PET 3016
Are you sure that was UB3 ? What was the post number that was referenced?
I think you need to compare the input pins with the output pins. One probe on pin 3, second on pin 2. Then pin 4 and pin 5 etc. |
10th Mar 2021, 5:51 pm | #853 |
Dekatron
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Re: Non-working Commodore PET 3016
UB3 and UC3, which are the address buffers, have been replaced. I wonder if Mark is maybe thinking of the data bus buffers UE9 and UE10?
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10th Mar 2021, 5:56 pm | #854 |
Octode
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Re: Non-working Commodore PET 3016
post 435
I've actually replaced UB3 and UC3 at the same time. Colin. |
10th Mar 2021, 6:00 pm | #855 |
Octode
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Re: Non-working Commodore PET 3016
Sorry, looking at the wrong schematic. Video data latch, UF9, 74ls373, if I’m looking at the right schematic now.
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10th Mar 2021, 6:06 pm | #856 |
Dekatron
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Re: Non-working Commodore PET 3016
We haven't worked in that area before, so UF9 has not been changed. It may be worth looking to see if there is an active 'video latch' signal going to UF9 pin 11. Depending on whether there is or is not, we'll proceed from there.
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10th Mar 2021, 6:14 pm | #857 |
Octode
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Re: Non-working Commodore PET 3016
OK - what am I looking for on pin 11?
Colin. |
10th Mar 2021, 6:14 pm | #858 |
Dekatron
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Re: Non-working Commodore PET 3016
A waveform, rather than the absence of one.
Also while in that area, can you have a look for activity on pin 10 of either UF7 or UF8. |
10th Mar 2021, 6:25 pm | #859 |
Octode
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Re: Non-working Commodore PET 3016
UF9/11 - consistent waveform at 1Mhz. It's a bit more rounded than square, but the timing, shape and frequency are consistent
Nothing on either UF7/10 or UF8/10. Colin. |
10th Mar 2021, 6:29 pm | #860 |
Dekatron
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Re: Non-working Commodore PET 3016
OK on the 'Video Latch' signal.
When you say you have nothing (presumably meaning no waveforms) on UF7(10) / UF8(10), what do you have instead? Steady high state or steady low state? |