Hi all,
Yes very old post but i thought i would add my findings to hopefully be helpful to others..
My screen also has gone into the dreaded displaying garbage symptoms. After spending a lot of frustrating time playing around i think i can put my finger on the problem (mine anyway), digital is certainly not one of my strong points but here goes..
It seems the SA's display pages are changed by pin 7 of U260 (PLS159), this chip is supposed to send a negative pulse to a binary counter (U326). The binary counter counts until the correct page/s is displayed then is suppose to stop.
The binary counter also sends signals to other chips including U308 which controls blanking and "dotenable"! (one of the classic things we all see with the fault)...
What seems to be happening (in mine anyway) is that U260's pin 7 is caught in a near mid state, around 4V in mine- any noise in the circuit is +- on the 4v and sending the page select counter into near constant counting- this seems to be why we see all of these unusual displays....
Now to prove the point, i pulled the pin 7 output up to 5v with a high value resistor- the SA now always boots to a spectrum display which i assume must be default with the initial (000?) binary output from the counter.. no other screen data is displayed but at least i can see a working spectrum..
I experimented with mosfet switching on pin7 to lighten its load but had no success.. I dont think it pulls down at all...
Now all i have to do is find a PLS159, the code and try to program it! GEEZ!! (id very very thankful if one of you guys can help)
I dont think the PLS is doing very much looking at its clock, but for me the code would be very difficult to invent..
Hope i have been of help.
cheers, Graham VK3XDK (grumss@yahoo.com.au)