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Commodore pet 2001-8 no video from logic board

Hugo Holden said:
You are not making a recording of the vertical drive signal (if it is there ) yet, because in the three photos the scope timebase looks like its set on 20 uS/division.

To see the vertical pulses the timebase needs to be slowed right down because the spacing between the pulses is about 16.67 mS, so set the timebase for 5mS/division (or similar) when checking for the vertical pulses.

Make sure to have the scope's probe input on DC coupling (looks like it is), and set say for 2V/large division on the attenuator with a x1 probe, or if its a x10 probe set it for 0.2v/large division, which gives 2V/ large division with the x10 probe (a large division is usually a centimeter on the screen of most scopes). To be sure you have got that right, connect the probe onto the 5V supply and the trace should jump up by 2.5 large divisions = 5V.
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With the settings you suggest I get this for vertical.
 

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Founds these in the schematics. Can someone confirm if these are the settings "on the right page" used on the scope to read the outputs for video vertical and horizontal
 

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Yep,

The settings are Volts/div (Y) and time/division (H) to give the image referenced.

You appear to have no vertical drive signal (if I am reading your photograph correctly).

The vertical drive signal should be 16.67 ms between pulses.

At 5 ms/div (as per Hugo) this would give 3 divisions between pulses.

At 2 ms/div (as per Commodore) this would give 8 divisions between pulses (as indicated in photograph # 2).

Dave
 
RetroGadgetMan said:
Well I have been back tracking from the vertical signal and pin 3 of C7 is not outputting anything on the scope. Probe is lo on pin 3. It is getting a clock signal on pin 12 pulsing so I suspect it is that it is C7 74ls107 possibly causing the vertical loss.
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RetroGadgetMan said:
For a few minutes I had a full screen of lines and mess now nothing again. No output from horizontal either. So I guess something else has failed.
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Unfortunately, this is something that can occur with old electronics that have not been powered up after a long, long time in hibernation...

I am afraid you just have to backup with the diagnostic process again and start from the beginning to find the next pesky device that has failed...

Dave
 
IMHO Forget the scope! Get that logic probe going again!

So do you have pulses on J7 pin 5 now? If so the horizontal might be better.

For the vertical...

How about J7 pin 3?

How about pins 2,3 and 5,6 of B6?

You do need to be very methodical here otherwise it will go pear shaped.
 
Nivag Swerdna said:
IMHO Forget the scope! Get that logic probe going again!

So do you have pulses on J7 pin 5 now? If so the horizontal might be better.

For the vertical...

How about J7 pin 3?

How about pins 2,3 and 5,6 of B6?

You do need to be very methodical here otherwise it will go pear shaped.
Click to expand...
B6
Pin 2 lo
Pin 3 hi
Pin 5 lo
Pin 6 hi
J7
Pin 3 Hi
Pin 5 lo
No pulse on any of these .
I was getting a good square wave on the horizontal after changing C9 with a white line on the screen and for a short while a full screen which came and went.
 
RetroGadgetMan said:
B6
Pin 2 lo
Pin 3 hi
Pin 5 lo
Pin 6 hi
J7
Pin 3 Hi
Pin 5 lo
No pulse on any of these .
I was getting a good square wave on the horizontal after changing C9 with a white line on the screen and for a short while a full screen which came and went.
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OK... Again pin 2 and 3 being opposites and pin 5 and pin 6 being opposites is a good sign for the health of B6... but they should be changing

So I assume pin 9 and 12 of B6 are also not changing?

What is happening on pin 12 of C7 and also pin 12 of C8?

Also what about pin 2 and 3 of C7? and... pin 2 and 3 of C8?
 
B6
Pin 9 lo
Pin 12 lo
No pulse
C7
Pin 12 all lights pulsing
Pin 2 pulse only
Pin 3 all lights pulsing

C8
pin 12 all lights pulsing
Pin 2 pulse only
Pin 3 all lights pulsing
C9 pin 11 all lights pulsing
Did I miss any 😆
 
Sorry I mixed c7 up with c8
C7
pin 2 hi no pulse
Pin 3 lo
Pin 12 all light pulsing

B6
Pin 1 hi
Pin 2 hi
Pin 3 lo
Pin 4 lo
Pin 12 lo
Pin 13 hi
No pulsing at all on b6
 
It is not C7...

C7 is not toggling because pin 1 is permanently LOW.

Move backwards to C6 pins 11, 12 and 13 next.

Dave
 
Nivag Swerdna said:
Replace 74LS107 at C7

Play with your scope whilst you wait for your parts to arrive :)

A trace of pin 12 of C7 would be nice and keep Hugo happy
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Thankyou for confirming my suspicions on C7.
I ordered one earlier today.
I will try and get a picture of the scope reading of pin 12 before I extract it and post.
 
Pin 1 (K) and pin 14 (/RESET) are both pulled HIGH. This means that pin 1 (the J input) is required to go HIGH for the J/K flip-flop to toggle. This is not happening.

C7 pin 1 comes from the AND gate at C6 pin 11. We need to see what is going into the AND gate on C6 pins 12 and 13 before suspecting C7.

Dave
 
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