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Edit 1/1/17: I was going to take a picture of my NI Elvis breadboard as the first fee suggestions didnt work. I removed several ICs (counters) that I was going to use for the other digits and some spare wires, and now the circuit works as it is supposed to with or without the 1 nF cap. I dont know exactly what the culprit was, but as I continue to build the rest of the circuit and put the other ICs back in, the problem may present itself again, if it does I will put in another edit.


Original question:

I'm not home so can't post a schematic at the moment, but hopefully I can describe what's going on and always add one in later.

I have a 1 Hz clock signal going into CLK A, 74LS90 set up in divide by 10 mode, with both reset and set pin pairs grounded. QA goes into CLK B. QA QD QB and QC all are connected directly to QA QD QB QC inputs on the 7441 and the 0-9 outputs to a nixie tube. Both ICs have 0.1 uF bypass caps to ground.

When turning it it sometimes counts 0-9 for only one time, then it only counts 1-8. I tried inverting the 1 Hz signal, and tried buffering but only 1-8. Next I got out my scope and the QA QB QC QD signals looked ok, and I realized when my scope was connected to QA, the nixie displayed correctly, 0-9. Then, Not sure why, but I thought adding a bypass cap between QA and ground would fix the problem. I tried I think 0.1 uF first and the display was way off it only cycled a few digits. Then I tried a 10 pF and it counted 1-8. Finally I tried a 1 nF cap and now the circuit works, I get a 1 second count of 0-9.

Can someone explain what is going on? Similar designs I have seen do not mention a required cap between the BCD counter and the nixie driver. Thanks!

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    \$\begingroup\$ Do you have bypass capacitors between Vcc and Ground pins on both ICs? A 22 uF or larger electrolytic close by may also help. \$\endgroup\$ – Peter Bennett Dec 31 '16 at 23:50
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    \$\begingroup\$ How long are the wires between the two chips? \$\endgroup\$ – Spehro Pefhany Jan 1 '17 at 0:00
  • \$\begingroup\$ @PeterBennett both ICs have MLCCs going from VCC to ground. They are 0.1 uF. \$\endgroup\$ – Efram Goldberg Jan 1 '17 at 14:00
  • \$\begingroup\$ @SpehroPefhany the wires were sort of long, 22 cm jumpers. I just replaced them with 5 cm long jumpers and removed the 1 nF cap on QA and it still only counts 1-8. I then tried to put the 1 nF cap back in and it now counts properly again. \$\endgroup\$ – Efram Goldberg Jan 1 '17 at 14:04
  • \$\begingroup\$ I removed a few unused ICs from my breadboard to take a photograph for the post and after removing them, the circuit now works even without the 1 nF cap. I updated the question. As I add in the remaining digits, the problem may come up again at which point Ill put another update and photograph. Thanks for the help so far. \$\endgroup\$ – Efram Goldberg Jan 1 '17 at 14:24

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