daver2
10k Member
Is there a question there?
Sorry, I think something is getting lost in translation somewhere.
Dave
Sorry, I think something is getting lost in translation somewhere.
Dave
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
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
how should the signals be? Just High or High and low?
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
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
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.
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.
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
in Italy it is now evening and it is time for dinner
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
It's bad ic??
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.
Actually /20 and /200 must both be inactive HIGH as G1-pin 2 was reported LOW.
-Dave