My brain has melted too. It was a good test.Given we have disconnected pin 2 the counter should be free running... the counters will only be HI HI HI for a single period i.e. 64us so the output of B1 pin 8 should only be low for a period of 64us... B1 must be knackered?
View attachment 1249148
I think we are all in agreement to replace B1. Though, just in case I would test the connections between A1 and B1 with the meter.My brain has melted too. It was a good test.
I shall revert the A1 chip and await the B1 replacement if we are all in agreement.
I have already ordered a 2nd probe that has a X1 and X10 setting.I think we are all in agreement to replace B1. Though, just in case I would test the connections between A1 and B1 with the meter.
One thing you could do, that would probably work to get a good recording of B1s output on pin 8; is to put the scope on A1 pin one as you have done that shows a nice stable clock pulse. Then use another probe on channel 2 of the scope to look at the output of B1, you would also need to slow the timebase down a few steps to see a few B1 output pulses, and we could measure their frequency from that, but I think they will be 4x faster than they should be.
This might be a good time for me to mention that I have made a Verilog simulation of this circuit, basically all of page 320008-3. I originally did it to just learn what the exact timings and relationships of the video signals. It was a quick hack but it seems to be correct for all the pins I have probed in my machine. I have returned to it once in a while to try to learn how it works. Recently, I fixed some data path problems while trying to learn exactly how "snow" happens on old PETs when you write the video RAM.I was thinking, since Commodore did not really provide an adequate description of this creative circuit design, when we figure it out, we should write up a detailed description of it. Maybe they wanted to keep it "proprietary"
Hmmmm.... There might be something rotten in the state of Denmark as the old saying goes, or perhaps a red herring.Ok now have a visual on pet screen again.
Trigger set to norm
Pic 1 Pin 8 of B1 50us
A1
Pic 2 pin 9 50us
Pic 3 pin 8 50us
Pic 4 pin 11 0.1 time div
Wow, this looks like excellent work.This might be a good time for me to mention that I have made a Verilog simulation of this circuit, basically all of page 320008-3.
Ok I will do this later. Shall I remove the bridge wire and re seat pin 2 of A1 aswell.Pic 2 (aka A1 pin 9) is squarish... Pic 3 (A1 pin 8) and Pic 4 (A1 Pin 11) have never really been explained, that's why I previously blamed A1.
To B1, or not to B1: that is the question.
@RetroGadgetMan you could remove B1 and replace with an empty socket... we could then see if the outputs (pins 9, 8 and 11 of A1) recovered; temporarily removing the character ROM again perhaps.
Yeap. Tidy up now ready for the arrival of your 74LS20.Ok I will do this later. Shall I remove the bridge wire and re seat pin 2 of A1 aswell.
Having never gone much past using PALASM and ABEL in Hardware Description Languages, I do not know how to use this file in a simulation to see waveforms, etc. I am impressed that even old parts like the Commodore 6550 RAM and 6540 ROM have been modeled and are in a library.
I suppose one would have to use a powerful LINUX type machine and load up tons of special apps. Would all the required software be free or low cost?
Thomas,I run it on Vivado but I one idea I have is to dump the waveforms in a vcd file and then you could use gtkwave to browse it. I should make a top post about this and I can include a few screen shots.
Some years ago a local Electronics Supply shop in my area closed down. They had this massive stack of 74LSxxx TTL's many labelled drawers in a large cabinet. I wandered in there and said to the owner, what is up ? He said he was retiring and closing the the shop. I offered him $100 cash for all the IC's and the cabinets they were in, he agreed on the spot! What a lucky dog I was that day. There was also 4000 series cmos. They have come in so handy over the years for repairs and projects.You can never have enough 74LS244 chips.... if you don't already
I had actually already ordered some back in October in anticipation of needing them. They never showed up either. It's an omen.You can never have enough 74LS244 chips.... if you don't already
What a cool repair. I had actually thought about using different chips to use the gates I need but hoping it won't come to that yet. But going forward this may have to be the way if any of the logic becomes unavailable. At least one of the ics on the board is unavailable at the moment through eBay at least.Some years ago a local Electronics Supply shop in my area closed down. They had this massive stack of 74LSxxx TTL's many labelled drawers in a large cabinet. I wandered in there and said to the owner, what is up ? He said he was retiring and closing the the shop. I offered him $100 cash for all the IC's and the cabinets they were in, he agreed on the spot! What a lucky dog I was that day. There was also 4000 series cmos. They have come in so handy over the years for repairs and projects.
I also had a large stock of plain 74 series TTL's because back in the 1970's I got obsessed with Atari's arcade games of Pong & Tank and have to have all the spares for those. So usually, if my vintage computers break I have a spare. Not always Though, recently I had to buy spare IEEE-488 interface buffer IC's for my PET.
Also I got caught out on an IBM serial card without a spare IC, but I managed to temporarily repair it using part of the remaining IC that was still working, I called it The Lost in Space repair:
If it takes too long for that 74LS20 to turn up one option to test the theory it was faulty could be to cobble some other gates together to act as a 4 input NAND.
Yes, the HCT cmos range had their inputs conform to TTL voltage thresholds. They are really good if you want to make low power (battery operated gear)Don't forget that HCT series is a modern equivalent (and manufacturered).... so 74HCT20 would do you well. Also many devices have equivalents with differing part numbers.
This should stir a debate.... but generally a HCT should replace an LS.... and use less power, not get so hot etc... they are not identical... but close enough that in most cases you can use them interchangeably.
https://www.onsemi.cn/pub/collateral/an-368cn.pdf