Alive but not working

[Grant Stockly]

Wow...I must have a marginal batch of 7420s. Geoff had a problem with one of his 7420s too. Geoff, was yours functionally good but having out of spec logic outputs?

You can use any 20, like a HC20, LS20, L20, etc. You might be able to find one in another piece of hardware.

If you can't locate another one immediately you might be able to use a pull down resistor to bring the low voltage into spec?

Grant

[Geoff Harrison]

Originally Posted by Grant Stockly

Geoff, was yours functionally good but having out of spec logic outputs?

I haven't tested it yet, I had to return my IC tester. I'll check the chip when I get it back. Or perhaps I'll jut put it back in the Kenbak and scope it.

[Reactive]

Originally Posted by Grant Stockly

If you can't locate another one immediately you might be able to use a pull down resistor to bring the low voltage into spec?

Grant

I'd use a RAMWall, personally.


Henry

[Architect]

The days you could run to the local store and pick up a 7420 are definitely over. How sad. No Kenbak programming this weekend. I ordered several 74**s online and they will probably arrive on Wednesday.

The LS versions are 0.52 a piece and so I simply had to order another $ 20 of stuff I do not really need to prevent the $8 shipping. But I was good: I didn't end up with 50,000 LEDs.

[Grant Stockly]

Originally Posted by Architect

But I was good: I didn't end up with 50,000 LEDs.

Too bad. Better luck next time!

[Architect]

To make a long story short: see the attached picture of IC25. The problem had nothing to do with a faulty 7420 as I mistakenly claimed before. It really is this simple. Only some people (Geoff) find this faster than others.

[Geoff Harrison]

Hmmm, that looks familiar

Congratulations, I'm glad you got it running. What are we up to now, three running Kenbak-1 Series-2 machines? Four, including Grant. That makes us a more exclusive group than owners of original Kenbak's

[Grant Stockly]

Originally Posted by Architect

To make a long story short: see the attached picture of IC25. The problem had nothing to do with a faulty 7420 as I mistakenly claimed before. It really is this simple. Only some people (Geoff) find this faster than others.

I bet you felt like yelling, laughing, and crying all at the same time when you found that. Were you studying the trace layout of the board to debug the circuit and see the lack of a solder joint by accident?

So, have you run any of the examples from the manual yet?

Grant

[Architect]

I suspected the IC52 and I replaced it with the one in IC77. Then the problem of all the LEDs being lit constantly was gone, and I wrongly concluded I found the answer. But it just accidentally worked because I took out IC77. When I put in a new 7420 at IC77 it was back.

Since I swapped IC25 and IC41 before (with the 7400 from IC99 remember), the only thing I could think of is that IC25 was upside down which can easily be detected by checking the voltage at 7 and 14. Only then I saw 1.5 V at pin 14 of IC25 and started disassembling the machine.

Warned by Geoff, I carefully checked the board after soldering and back then I found two similar incidents. But apparently overlooking a third.

[Architect]

I suspected the IC52 and I replaced it with the one in IC77. Then the problem of all the LEDs being lit constantly was gone, and I wrongly concluded I found the answer. But it just accidentally worked because I took out IC77. When I put in a new 7420 at IC77 it was back.

Since I swapped IC25 and IC41 before (with the 7400 from IC99 remember), the only thing I could think of is that IC25 was upside down which can easily be detected by checking the voltage at 7 and 14. Only then I saw 1.5 V at pin 14 of IC25 and started disassembling the machine.

Warned by Geoff, I carefully checked the board after soldering and back then I found two similar incidents. But apparently overlooking a third.