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Pet video board 320064-02 bad

Dave sorry i ve asked if the probe settings are correct (like in picture)....
Thanks!
 
daver2 said:
The video, horizontal and vertical drive signals on J7 for a start.

Do you see any signs of activity on any of these three signals at all - or are they static high or low logic levels?

You may have to 'experiment' a bit with your logic probe on a known clock signal to see what indication the logic probe gives you.

Dave
Click to expand...


So i've tested J7 with logic probe and i have these results:

Pin 5: LOW AND HIGH
Pin 3: HIGH
Pin 1: HIGH
 
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Yes. TTL and PULSE is OK.

You have no signals going to the monitor - so we need to work on the PET mainboard a bit until we see some activity on all three of these signals.

So you need this schematic http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-6.gif and work backwards from J7 pins 3 and 5 trying to work out where the problem is. Alternatively, you can work forwards from the crystal oscillator. Your choice.

I am going out for the evening - and then I am going on a business trip until Friday - so, dave_m, are you up for a bit of fault finding :)?

Good luck...

I see you have just edited your previous post and we now have a HORIZONTAL DRIVE but no VERTICAL DRIVE?

Dave
 
daver2 said:
Yes. TTL and PULSE is OK.

You have no signals going to the monitor - so we need to work on the PET mainboard a bit until we see some activity on all three of these signals.

So you need this schematic http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-6.gif and work backwards from J7 pins 3 and 5 trying to work out where the problem is. Alternatively, you can work forwards from the crystal oscillator. Your choice.

I am going out for the evening - and then I am going on a business trip until Friday - so, dave_m, are you up for a bit of fault finding :)?

Good luck...

I see you have just edited your previous post and we now have a HORIZONTAL DRIVE but no VERTICAL DRIVE?

Dave
Click to expand...

how should the signals be? Just High or High and low?
 
All three pins should be high/low oscillating.

The H and V DRIVE signals should be a constant mark:space ratio (as they are dictated by the video geometry) whereas the video signal itself will depend upon what is being displayed on the screen at the time. In general (at start up) this should be the Commodore sign-on message, the number of bytes of free RAM and the word READY (i.e. at power up the screen content should always be the same and consist of a varying high/low logic level depending upon whether each pixel is black or white).

The oscilloscope traces attached to the schematics for the monitor indicate what signals should be on each line.

Dave
 
ilpaninaro said:
how should the signals be? Just High or High and low?
Click to expand...

ilpaninaro,
High and low lights on together mean the signal is pulsing (lights are blinking fast)

Since the Horizontal may be blinking, but the video data signal and Vertical drive are stuck, there are problems with the PET master timing control. Let's first check out the basic clock countdown chain.

See schematic sheet 6 and check for fast blinking on the following clock lines from the LS93 counter: H9-pin 12 (250 KHz), H9-pin9 (125 KHz), H9-pin8 (62.5 KHz), H9-pin11. I'm guessing these are fine as you are getting pulses on the horizontal drive.

Now check for a critical signal called 'NEXT' at H8-pin12. This signal clocks the control sequencer and gets it to the next 'state'. I'm guessing it is stuck. If so, check the source of the signal at G8-pin9 on the bottom right of sheet 7. This may be the heart of the problem. We then have to find out why we are not stepping through the four states of the sequence. Once we find the bad IC (chip), we will be fine.
-Dave
 
dave_m said:
ilpaninaro,
High and low lights on together mean the signal is pulsing (lights are blinking fast)

Since the Horizontal may be blinking, but the video data signal and Vertical drive are stuck, there are problems with the PET master timing control. Let's first check out the basic clock countdown chain.

See schematic sheet 6 and check for fast blinking on the following clock lines from the LS93 counter: H9-pin 12 (250 KHz), H9-pin9 (125 KHz), H9-pin8 (62.5 KHz), H9-pin11. I'm guessing these are fine as you are getting pulses on the horizontal drive.

Now check for a critical signal called 'NEXT' at H8-pin12. This signal clocks the control sequencer and gets it to the next 'state'. I'm guessing it is stuck. If so, check the source of the signal at G8-pin9 on the bottom right of sheet 7. This may be the heart of the problem. We then have to find out why we are not stepping through the four states of the sequence. Once we find the bad IC (chip), we will be fine.
-Dave
Click to expand...

Hi Dave,
Thanks so much my friend!
So i've checked counter Ls93 H9 and i'ive high and low signal on pin 9-8-11-12
I've checked also H8 at pin 12 and i've low signal.....same at G8 pin9 (low signal)....

Must i change h8 and g8???
Thanks!
DAvide
 
ilpaninaro said:
Hi Dave,
Thanks so much my friend!
So i've checked counter Ls93 H9 and i'ive high and low signal on pin 9-8-11-12
I've checked also H8 at pin 12 and i've low signal.....same at G8 pin9 (low signal)....

Must i change h8 and g8???
Thanks!
DAvide
Click to expand...

No! It's probably OK. One of the many signals that feed the D input of the G8 flipflop is the bad signal we will have to find.
 
dave_m said:
No! It's probably OK. One of the many signals that feed the D input of the G8 flipflop is the bad signal we will have to find.
Click to expand...

Next check on sheet 7, the AND gate G1-pins 1,2 and 3. I'm guessing input pin 2 is stuck Low which will keep the output (pin3) Low. Let us know when you get a chance. You are being a good circuit detective!
-Dave
 
dave_m said:
Next check on sheet 7, the AND gate G1-pins 1,2 and 3. I'm guessing input pin 2 is stuck Low which will keep the output (pin3) Low. Let us know when you get a chance. You are being a good circuit detective!
-Dave
Click to expand...

I ll check this tomorrow Dave!
in Italy it is now evening and it is time for dinner ;) :p
Tomorrow i'll update!!
Thanks so much ;)
 
dave_m said:
Next check on sheet 7, the AND gate G1-pins 1,2 and 3. I'm guessing input pin 2 is stuck Low which will keep the output (pin3) Low. Let us know when you get a chance. You are being a good circuit detective!
-Dave
Click to expand...

Hi!
I've checked 74ls08 (G1):

Pin 1= HIGH AND LOW
Pin 2= LOW
Pin 3= LOW

It's bad ic??
 
The signals /200 Lines and /20 Lines are critical in the video timing circuit.


The lines being counted are Horizontal scan lines with 200 lines equaling 25 character rows times 8 scan lines each. You can see on the schematic that there are a lot of things that can go wrong in this part of the circuit.
 
Can you still help me for the next step? ;(
Thanks for your patience!
 
Possibly, the entire circuit page 7 will be locked up, with the /20 & /200 lines signals not pulsing if the /RELOAD signal pin 13 of H5 is absent (because of the loop created by the /NEXT signal from G8's /Q sent to H5's pin 12).

So if the /RELOAD signal has not been checked for pulses yet, it is worth doing that initially.

Also check pin 9 & 10 of H5 to see of its both the /20 and /200 signals inactive, or just one of them.
 
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Hugo Holden said:
Possibly, the entire circuit page 7 will be locked up, with the /20 & /200 lines signals not pulsing if the /RELOAD signal pin 13 of H5 is absent (because of the loop created by the /NEXT signal from G8's /Q sent to H5's pin 12).

So if the /RELOAD signal has not been checked for pulses yet, it is worth doing that initially.
Click to expand...

Hugo, thanks for helping. I think /RELOAD is pulsing as it should be free running as NEXT is low so /NEXT is high. But it will be good to check: ilpaninaro please check H5-pin13 and pin11 for pulsing. Pin 12 should be HIGH.

You are so right that the sequencer is locked up and it makes things look static. However the counters F2 and F4 should be free running. Ilpaninaro, please check for pulsing on the following counter outputs (note we expect pulsing so if we find one stuck, we have found a problem):
G6-pin3, F4-pins 5,9,2, and 12. F2-pins 5, 9, 2 and 12. Maybe we get lucky and find a bad counter.

Also check pin 9 & 10 of H5 to see of its both the /20 and /200 signals inactive, or just one of them.
Click to expand...

Actually /20 and /200 must both be inactive HIGH as G1-pin 2 was reported LOW.
-Dave
 
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