Need a bit of discrete help!

[martin martin]

Greetings all the smart folks.  I purchased this low-cost board from Taylor Electronics in San Diego, CA.  I wanted to build up a 1970s IC style.

Here's my issue:

I am unable to get a 60Hz square wave from pin 6 of U14.  I double checked the values of R8 and C1 as that seems to be where the Run signal starts.

Notes from the designer says  The default time constant with 39K and 0.47uF is 1.8ms x 3 = 5.4mS.

I can send scope pics if needed.

Thanks in advance!

[gregebert]

What are you seeing on U14, pin 5 ? That is where the 60Hz reference signal comes in.

[J Forbes]

That's the schematic for this clock I built 25 years ago?  :)

https://selectric.org/nixie/index.html

at the bottom of the page.

I forget what it took to make it work, that was a long time ago.

[martin martin]

U14, P5

[Dekatron42]

Looks like the zener CR1 isn't working, possibly not soldered in as it should make the signal look like a half wave rectified waveform, making U14 trigger on each positive swing.

[gregebert]

I was expecting to see something similar to a sinewave with the bottom-half chopped-off, and the peaks limited to 5V. This looks more like a sinewave, so that's a suspect and could explain why you dont get anything at pin 6.

Have you tried placing a resistor, something between 10K to 100K, from pin 5 to GND ? This may help pull-down the signal to GND and give it a better shape. Lower values should give a better shape, but if they are too low, the amplitude will degrade.

[martin martin]

Thanks to all!  I have some homework to do. I will let all know soon...

On Monday, September 28, 2026 at 8:36:11 PM UTC-7 gregebert wrote:

[martin martin]

I am seeing 1.8 V at the cathode of the Zener

[gregebert]

As in DC voltage ? It sounds like an open circuit to the AC reference signal, because undriven TTL gate inputs will float to around 2 volts.