Of course we can !
You appear to have horizontal and vertical synchronisation signals (i.e. A full screen raster), but no video signal.
The brightness looks as though it is turned right up possibly (or this may be a failure mode of the fault).
The first thing (as usual) is to check if you have any video signal at the connector on the PET main board that connects to the monitor. I can't remember if you have an oscilloscope or not? That would be the best piece of test equipment to use for this.
No video signal would indicate a fault on the main logic board. The presence of the video signal would indicate a fault on the monitor.
Divide and conquer!
Dave
fine thanks, just extremely busy at work...
Can you measure the dc voltage on j7 pin 1 relative to 0v/gnd with your multimeter please. It should be somewhere between 0v and 5v and post the result.
Dave
A voltage reading of 4.8 Volts seems too high to me. The implication being that the video output from the logic board is permanently high.
Can you detect any pulses (with your logic probe) on J7 pin 1 as a further cross-check?
The worrying thing is that a video signal level being high should be full white on the monitor (or green for a green screen monitor)...
I don't think you have two simultaneous faults, but it is possible...
Let me have a think for other tests that you can easily perform. dave_m might join in as well...
Dave
Definitely no video from the main logic board then.
Not sure (with the signal level being high) why you haven't got a white/green raster...
Dave
Today i try to read all roms with eprom programmer, so i can compare with the original files....
Not needed yet.
You need to see why no video data. See p.8 of schematic http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/320349-8.gif
Check why G11-pin 6 is stuck high. Check inputs with logic probe: pin 1,2, 4, and 5 of G11 7420 4-input NAND gate. One of those may be stuck low.
OK thanks.
So G11 pin 4 is bad.
Can you check the following pins for pulsing with your logic probe please:
G10 pin 6 (74LS00).
G10 pin 8 (74LS00).
G9 pins 8 and 9 (74LS74).
E11 pins 7 and 9 (74LS165).
Basically, I am working backwards on the schematic diagram until we find where things go from GOOD to BAD...
Dave
OK thanks.
So G11 pin 4 is bad.
Can you check the following pins for pulsing with your logic probe please:
G10 pin 6 (74LS00).
G10 pin 8 (74LS00).
G9 pins 8 and 9 (74LS74).
E11 pins 7 and 9 (74LS165).
Basically, I am working backwards on the schematic diagram until we find where things go from GOOD to BAD...
Dave
My bet is with the E11 side of things but without data back, that is just a unfounded guess.
OK thanks.
So G11 pin 4 is bad.
Can you check the following pins for pulsing with your logic probe please:
G10 pin 6 (74LS00).
G10 pin 8 (74LS00).
G9 pins 8 and 9 (74LS74).
E11 pins 7 and 9 (74LS165).
Basically, I am working backwards on the schematic diagram until we find where things go from GOOD to BAD...
Dave