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Pet 2001N horizontal line

F4:
1 P
2 P
3 L
4 L
5 P
6 P
7 L
8 P
9 P
10 L
11 L
12 P
13 H
14 H
 
Excellent,

I am just doing another job at the moment, so will have a look when I get back home.

I am surprised about the pins marked as INACTIVE though. Perhaps you can look on the schematic for these pins? If they are actually wired up, they would look out of place.

Don't swap the devices until I have checked though...

EDIT: I had a very quick look. The INACTIVE pins are actually correct. Device F2 (74177) looks a bit suspect to me. Do you have any spare 74177 devices in your bits box?

I would still like to check a bit later before swapping the device though, as this state machine can be a little confusing...

Dave
 
Last edited:
Ok i saw for example that I1 pin 1 (inactive) it's connected with G7 pin 8 (HIGH).
to be more precise I must say that pin 1 of I1 behaves strangely .... when I touch the tip of the logic probe, it flashes a little with the green LED (low) and then disappears and remains inactive.
 
I1 pin 1 (on the schematics I am looking at) is not connected to G7 pin 8. It is in the main 16 MHz clock circuit. This gate (and its friend I1 pins 13 are 12) are operated as an analogue circuit rather than a digital circuit - so your logic probe will 'lie' to you! It is telling you that the signals are invalid TTL digital signals (which they are by design)!

I1 pin 4 is strange though - it should have a 16 MHz clock on it. This high frequency may be confusing your logic probe though, or there may be some dirt on the pin. The 16 MHz oscillator is certainly working (because we have some clocks in the machine and this is where they are all sourced from).

Dave
 
What can i do to solve this big problem on video ram please?? My scope can help me? :(
 
daver2 said:
Just wait until I have checked out your results...

I have just got home, so I can print out the schematic.

Dave
Click to expand...
Thanks so much, i hope that i can still learn something with this repair!
This is my last Pet ;)
 
Replace F2 (74177).

Your oscilloscope (in fact any test equipment) doesn't tell you what the fault is - your brain does! This is why you need to have the theory and understanding.

It still doesn't guarantee that we are correct, or that we have fixed one fault - only to find another surface...

Dave
 
daver2 said:
Replace F2 (74177).

Your oscilloscope (in fact any test equipment) doesn't tell you what the fault is - your brain does! This is why you need to have the theory and understanding.

It still doesn't guarantee that we are correct, or that we have fixed one fault - only to find another surface...

Dave
Click to expand...
Dave, i replaced F2
 
No need for the unhappy face... We have learned something new!

My guess is that F2 pin 5 is being held LOW by something that it is connected to.

I assume that you installed F2 in a socket? If so, can you remove F2 and use your logic probe on pin 5 of the socket. What do you observe?

Dave
 
Can you bend pin 5 of IC F2 carefully at 90 degrees to the body of the IC and reinsert it back into the socket (so that pin 5 is not in the socket) and then test with your logic probe on the loose pin 5 of F2.

Dave
 
daver2 said:
Can you bend pin 5 of IC F2 carefully at 90 degrees to the body of the IC and reinsert it back into the socket (so that pin 5 is not in the socket) and then test with your logic probe on the loose pin 5 of F2.

Dave
Click to expand...
High signal Dave!
 
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