I know several of you have written post about this repair but I am stumped and i need a little help to hopefully get me moving in the right direction. I have been working on this board for a week and i can't get it past one beep and the LED stays on continuously. I treated the board for a small amount of alkaline damage to the usual area under the battery using a toothbrush and white vinegar and rinsing thoroughly with distilled water. I used a fiberglass pen to scrape off the residual crud and clean up the traces and transistors, resistors, and chip legs. Following the manual for trouble shooting the self-tests I have replaced the Z80 with a new one from Bob Roberts and replaced chips 7G and 8G along with all of the transistors in the reset circuit. I did have to run a couple of jumpers from 7G due to a couple of pads evaporating when I was desoldering the old chips. After all of this I was able to test the reset circuit and verify that it is working correctly using my logic probe on pin 26 of the ZPU(on start up it is low and then switches to high, when I press the reset it switches to low and then high also) . I replaced the flip flop divider 8A and transistor Q12. Still no change. I have checked continuity of the chips on the address bus(6B,7D) and the LED latch (7G,3A,8G). I am still working on continuity checks for I/O Decode circuits chips(5A,4A,6A,7E). My questions to you who have been down this road before are:
What else can I check for besides continuity before I just start replacing these chips? Does anyone have tables showing what the correct voltages should be on the pins of these chips? What is the likely hood of ROM chip(s) being bad and how can I test them?
Please any help would be greatly appreciated!
Thanks,
Jon
What else can I check for besides continuity before I just start replacing these chips? Does anyone have tables showing what the correct voltages should be on the pins of these chips? What is the likely hood of ROM chip(s) being bad and how can I test them?
Please any help would be greatly appreciated!
Thanks,
Jon