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Commodore pet 2001-8 no video from logic board

RetroGadgetMan said:
Ok so swapped out A1 and sadly no change on screen. 🤔
Click to expand...
No worries. Scope pins 9, 8 and 11 of A1 and try and get nice traces. You could do pin 1 again too to keep @Hugo Holden happy

and order a 74LS20 for B1... (incidentally B1 is replaced on my PETunia; A1 is original; maybe I should have guessed the other way around but the A1 outputs looked wrong to me i.e. not like mine in #315)
 
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Nivag Swerdna said:
No worries. Scope pins 9, 8 and 11 of A1 and try and get nice traces. You could do pin 1 again too to keep @Hugo Holden happy
Click to expand...
New A1
Pic 1 pin 1 20us
Pic 2 pin 8 50us
Pic 3 pin 9 50us
Pic 4 pin 11 0.1 us
 

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No change then... OK... I guess we are off to B1... Replace B1

If you wanted something to do in the meantime... put your digital probe on pins 1,2,3,4,5,6, 8,9,10,11 of C3 and ensure they are all pulsing.
 
Nivag Swerdna said:
No change then... OK... I guess we are off to B1... Replace B1

If you wanted something to do in the meantime... put your digital probe on pins 1,2,3,4,5,6, 8,9,10,11 of C3 and ensure they are all pulsing.
Click to expand...
Just checked, they are.
Jumping forward. My character set, shouldn't I be seeing all the characters on the screen yet. At the moment it's just blocks and lines.
 
Just gone back to J7 for an update.

Pic 1 pin 1 video
Pic 2 pin 3 vertical
Pic 3 pin 5 horizontal

Video looks to have improved since we started
 

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Nivag Swerdna said:
pin 13 of B1 should be the same as p1 of A1? (and the same as HSYNC... i.e. 15kHz)
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This is correct. A1-1(74LS93) should be clocked every horizontal scan line (64 uS). B1-8 should pulse every eight horizontal scan lines after a complete character row is finished. This pulse is used to latch the data into E6 (74100) with the starting address of the next row of characters. This latch will hold for the needed eight scan lines to reload the counters with the starting row address until the full 40 character line is complete.

This sequencer circuit does so much. It keeps track of the complicated video addressing for refreshing the screen, generating the serial video data, timing for top blank, video on, and bottom blanking all with a minimum of 7400 type parts.

Elegant when it is working but a nightmare when something breaks. One can see why Commodore switched to the 6545 CRTC in later designs.
 
RetroGadgetMan said:
B1
Pin 8 50us
Click to expand...
looking at this on the scope recording post #350, the rate does appear to be x 4 faster than the normal about 1.96kHz it should be. We know the clock pulse to A1 is good.

This can only happen, as mentioned before, in the case that A1 is ok, is if B1 is defective with either: 1) open tracks/connections leading from the two msb's from A1 to the inputs of B1 , or alternatively B1 has has two internal open circuit inputs in the IC body which are interpreted as logic high by the remaining working IC die.

Though it would be good to have the better scope image of B1's pin 8 without the overlapping traces on post #350 just to be 100% sure.

It will be interesting to see what happens with a new B1. And if it is B1 that is defective, the credit goes to Nivag for figuring out it was likely this sub-circuit that was defective, I was still looking in other areas.
 
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dave_m said:
This is correct. A1-1(74LS93) should be clocked every horizontal scan line (64 uS). B1-8 should pulse every eight horizontal scan lines after a complete character row is finished. This pulse is used to latch the data into E6 (74100) with the starting address of the next row of characters. This latch will hold for the needed eight scan lines to reload the counters with the starting row address until the full 40 character line is complete.

This sequencer circuit does so much. It keeps track of the complicated video addressing for refreshing the screen, generating the serial video data, timing for top blank, video on, and bottom blanking all with a minimum of 7400 type parts.

Elegant when it is working but a nightmare when something breaks. One can see why Commodore switched to the 6545 CRTC in later designs.
Click to expand...

But it is a very cool circuit. I like the fact we can get inside it.

If one of many failures occurs inside a CRTC chip, it spoils fault finding fun, because you put in a new one and the fault goes away and you are none the wiser what the fault was, almost too easy and not as much fun.

I kind of prefer circuits with lots of TTL IC's rather than LSI chips for this reason. Plus, nearly all the original TTL's are still easy to get, unlike a lot of more modern parts that go obsolete ASAP. I'm glad my PET has this interesting & unusual circuit and not the CRTC chip !

Plus, the more faults that crop up in it as time goes by and we problem solve them, the more familiar we will become with the circuit. You don't get that learning experience, not to the same extent at least, plugging in new LSI chips.
 
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