I am trying to troubleshoot an old heathkit frequency counter. The time-base circuit uses a 4mhz oscillator to create a squarewave. This go's into pin 14 of a 7490. A monostable multivibrator puts a square wave into pin 7 of the 7490. The output is from pin11 and then go's into the pin 14 of the next stage. Pin 5 has 5v going to it and Pins 10,3,2 are grounded. Pin 1 is connected to pin 12.

When I put the scope on pin 14 of the first 7490 I get a nice square wave (a little ringing but pretty nice looking). When I put the scope on pin 7 I get a nice pulse going to it from the monostable vibrator. Now when I put the scope on the output I get the signal pulse divided by 10....but when the monostable pulses the output go's to a straight line 5v....then when the pulse quits the output is the divided by 10 pulse.

Is this normal? I guess I was expecting just the input pulse divided by 10.

 

Is pin 6 connected to pin 7 or 5v?

Pins 6 and 7, when both high, will set all outputs high.
When one or the other returns low, the count will resume.

 

Pin 6 is connected to nothing. Pin 5 is connected to power.