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Cbm 2001 Pet strange boot

Desperado said:
Hi, i repaired 74ls00 s' leg and i inserted on socket but i have always same bad signal on UD2 pin 9!
Time to wait for new 74ls00! I update this thread when it arrives!
Thanks so much!
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Well that is good.

It confirms that the fault is inside the original A4 74LS00, and was not something like a short external to it.

But what Daver2 and I are both interested in is how that IC failed in the way it has, with interacting inputs.

As you know, one "theory" is that the die inside the chip has lost its +5V power. If that was true A4's outputs would be very defective or absent too.

But it is possible that it is just the gate connected to pins 1,2 and 3 that is defective and somehow the inputs have developed leakage between each other, but this is not common either.

If you can, please leave the scope Ch1 connected to D2 pin 9 showing the bad signal, and scope around on the other pins of this defective 74LS00, for example put the scope CH2 on output pin 3 of A4 so we can see what the output of gate A4 is doing.
 
Desperado said:
Pin 1 and 2 signals are the same of UD2 pin 9 :(
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Yes, because there is leakage and an interaction happening between those two pins 1 &2 inside the IC causing the bad signal. What is pin 3 doing, can we see a scope recording ?

If you look back at this from post #1346, looking at pins 1 &2 with the scope, to see if the signals matched, was part of the process of finding this fault:

You are looking to have the scope CH1 on pin 9 to get the most stable lock you can and using ch2 as your probe. The two places to probe around are the jumper block, especially its right hand side where the pin 9 connection enters at P and around the IC's pin 1 and pin 2 A4 and pins 10,11 & 12 A3, to see if there is an interfering signal that matches what you see on Ch1(pin 9,D2).
 
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Hi guys, today i received new 74ls00 and now i have good signal on UD2 pin 9....i am desperate because if i remove Nop Generator and
i turn on the computer, i can see always scrambled chars screen!!!! :(
 
We haven't finished with the NOP generator yet my friend!

We found a problem - and then (suddenly) we tried to see if that fixed the PET...

Now you need to go back to the NOP generator. Double-check that the output signals from UD2 work and check that the /CHIP SELECT pins of the ROMs are operating now.

Dave
 
daver2 said:
We haven't finished with the NOP generator yet my friend!

We found a problem - and then (suddenly) we tried to see if that fixed the PET...

Now you need to go back to the NOP generator. Double-check that the output signals from UD2 work and check that the /CHIP SELECT pins of the ROMs are operating now.

Dave
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Hi Dave, yes i have good signals on UD2!
daver2 said:
Double-check the signals on UD2 work and check that the /CHIP SELECT pins of the ROMs are operating now.

Dave
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Pin20??
 
Pin 20 of each ROM yes.

They should have the same waveforms as on UD2 of course. We are checking the PCB tracks and the IC sockets here.

Dave
 
It doesn't really matter. Can you get to pin 20 of each socket?

Just out of interest, which version of the PETTESTER are you using and in which ROM socket?

Dave
 
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