Author Topic: Help Troubleshooting COSMAC 1802 Membership Card?  (Read 1092 times)

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Offline tcv

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Help Troubleshooting COSMAC 1802 Membership Card?
« on: January 24, 2014, 12:39:03 am »

I recently put together a COSMAC 1802 Membership Card and, well, it's not quite working. It's probably more accurate to say that it's broken. :D

If possible, I'd like help troubleshooting the problems. I have been in touch with the kit's author, Lee Hart, and he's been very gracious and has pointed out a lot of helpful information. Regrettably, he's not been available to help for a few weeks and I'd like to get going on it a bit more.

I'm very new so I'll be slow!

First, here are some relevant links:

1. Schematics:
2. The Kit page:
3. Troubleshooting page:

The main symptom right now is: I can input data, but I have not been able to read anything back.

I am going over the board looking for bad solder joints. I've been through the bottom board and I think things are OKAY there, but this is my first larger project and I can't swear to everything. I have yet to go over the top board. Going through the bottom board made things a little more stable, it seems. The lights generally respond as they should.

Here's my first question:

On the troubleshooting page, Lee says, "Check to see if the oscillator is running. U4D pin 12 should be high, and U1 pin 39 should be an AC voltage (as indicated on your multimeter)."

I know I have AC voltage on pin 39 (I'll have to go back and check what the reading is). My logic probe on a BK Precision 2707b Multimeter which says, "Connect the red test lead to the test point and the black lead to the common buss of the logic circuit."

What is the "common buss" in this case?

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