I spent some time reconciling the components found on the PCB and in the schematics. Once I had my spreadsheet in good shape I was able to quickly locate parts for beeping out connections.
I discovered quite a few that were on the schematic, in the range of R82 to R88 along with associated capacitors from C64 to C67 that were not on my board. Where these parts were shown on the schematic, straight connections existed.
Two resistors, R80 and R81, were added onto my board as a rework in an unprofessional style that led me to believe that the prior owner added these on their own. Another component R77 on the PCB was found to be in line with a connection that is a straight wire on the schematic, thus it was deleted in later boards.
|R80 added under U1B|
|R81 added with tubing and loose resistor|
TRUDGING ALONG BEEPING EVERY PIN TO ITS OTHER END(S)
After another two hours I had the rest of schematic page 2 verified. Perhaps I am speeding up a bit, based on a new method where I first extract all the tests from the schematic as a list of connections; a made-up example would be U4A 4 - U2A 2 - U5E 10- R22. When I had a full page of connections listed, I could swing through with the continuity tester and check off each as it passed. Thus I projected that my average speed for the remaining four would be 3 hours per page.
After only another hour I had page 1 complete - quite a speedup. Page 3 went a bit slower but by the time I reached the end of that page, without having found any broken traces, my confidence in the basic integrity of the board had grown substantially. I could still have bad ICs, since I have already found seven of them, but the board continuity is sound. I had a dinner appointment thus work on pages 4 and 5 started the next afternoon.
I wrapped up work on page 5, finding no missing connections to components. It was time to hook this back up and begin checking out the various circuits as I introduced inputs.
ODDBALL MEASUREMENT OF OPEN COLLECTOR INVERTER OUTPUT
One part of the logic that controls the BACKWRAP output signal is an open collector inverter, pulled up to +5V by a 10K resistor, whose output is feeding two AND gate inputs. When I measure the resistance on the inverter output pin U6E 10, I see 5K to up to +5V but the pullup resistor R79 is 10K.
|Portion of the BACKWRAP signal circuit|
THAT 'DOH' MOMENT
Look carefully at the schematic excerpt above, at gate U2J. I read that as 74LS00 and installed one in the board, but in fact the gate is a dual input AND, while the 74LS00 is a dual input NAND. The correct part number is 74LS08N.
After I checked all the other chips I replaced with74LS00, looking at the schematic carefully, I knew that the five such chips I declared bad based on the boolean logic behavior were in fact supposed to be AND gates.
I will remove all five (again) and populate them with the correct chip. To be safe, after the soldering I have to beep out all connections from the 70 pins, but that will be pretty quick compared to the comprehensive scan I just finished.