My Defender was working fine for years. Now boots up to the rug pattern. I have 3 led's on the power supply. I reset the ram and rom chips, I also rest the connector between the boards. How do I check to make sure I am getting the correct voltages?
Does it halt at the rug pattern or reboot after ?
LEDs show voltage present not correct voltage. get yourself a cheap digital multimeter, a copy of the schematics and measure the voltages at the test points on the PCBs to start with.
on the schematics "Defender_Drawing_Set_(16P-3001-101_R-T)_April_1981.pdf" pages 10 and 11 that show the pcb layout and the schematic. Measuring the voltage between Ground and Test point 1 (TP1) should read 5.1v or close to it (not over 5.25, not under 4.75v) the close to 5.1 the better.
then need to check the -5v rail (they can go bad). Measuring the voltage between Ground and Test point 5 (TP5) should read -4.9v or close to it (not over -5.25, not under -4.75v) the close to -4.9 the better.
similarly, need to test the 12v rail - test point 4 should read 12.1v or close to it.
check those then report back.
you can also check the voltages at the corners of the RAM ics . page 4 of the schematics shows the main cpu layout. ICs 1R thru 3Z ar ethe ram ICs. the black dot in the upper left corner of each ic indicates pin 1.
Ground is the top RH corner of each ic, so measure between ground (pin 16) and pin 1 (-5v), pin 8 (12v) and pin 9 (5v)
check the voltage on a few ram chips or all ram chips and report those back.