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  #1  
Old 10-18-2007, 10:18 AM
Architect Architect is offline
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Default Alive but not working

I finished the kit and it seems to work. However, the CLEAR button doesn't clear the data LEDs. The INPUT LED lights up though. Some other key combinations result in most of the LEDs to go out, so it is not the K-register. Any data key pressed leads to all data LEDs to light up.

I understand that the keys pressed are mapped onto 377 and the LEDs are mapped on 200. So it could be the memory. But that is just a first impression. I will look into it in more detail tonight.
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  #2  
Old 10-18-2007, 01:12 PM
Geoff Harrison Geoff Harrison is offline
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Quote:
Originally Posted by Architect View Post
I finished the kit and it seems to work. However, the CLEAR button doesn't clear the data LEDs. The INPUT LED lights up though. Some other key combinations result in most of the LEDs to go out, so it is not the K-register. Any data key pressed leads to all data LEDs to light up.
That's similar to the problem had at first, especially having all the data lights light up for any data key pressed.

It sounds like the K register is shifting when it shouldn't. Try checking the ICs and signals driving pin 6 on IC 104 & 105 (i.e. ICs 107, 116, 120, 121). Make sure pin 6 is not stuck low. In my case I had a bad 7420 @120.
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  #3  
Old 10-18-2007, 07:42 PM
Grant Stockly Grant Stockly is offline
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Do you have a way to test the TTL devices?

I hope you find the problem easily!
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  #4  
Old 10-19-2007, 10:52 PM
Architect Architect is offline
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Default Found It !

Also in my case the culprit was a 7420. It was IC52 (page 24). I found it by using signal IN (pin 5 of IC117, page 04) to trigger my oscilloscope (also vintage) to see what happens the moment 377 is read/written while I push CLEAR or one of the data buttons. It is not that IC52 was functionally faulty, but it negatively influenced IC40 and IC41 to push up the signals WD and WT to 1.6 V which makes them 1 in stead of 0, causing a delay-line full of 1s.

Of course I cannot run programs now because I used IC75 in its place. I'll solve that tomorrow.
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  #5  
Old 10-20-2007, 01:54 AM
Grant Stockly Grant Stockly is offline
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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
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  #6  
Old 10-20-2007, 02:37 AM
Geoff Harrison Geoff Harrison is offline
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Quote:
Originally Posted by Grant Stockly View Post
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.
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  #7  
Old 10-20-2007, 12:02 PM
Reactive Reactive is offline
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Quote:
Originally Posted by Grant Stockly View Post
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
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  #8  
Old 10-20-2007, 05:29 PM
Architect Architect is offline
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Default 74ls20

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.
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  #9  
Old 10-20-2007, 11:00 PM
Grant Stockly Grant Stockly is offline
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Quote:
Originally Posted by Architect View Post
But I was good: I didn't end up with 50,000 LEDs.
Too bad. Better luck next time!

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  #10  
Old 10-31-2007, 09:46 PM
Architect Architect is offline
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Default Everything works

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.
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