I acquired an old Eistar SJ-1 Digital pulser, however the frequency it generates is always 66% of what its supposed to produce. Turns out that the frequency coming out of the frequency stage (at Pin 11) is 66% of the 16MHz crystal or about 10.6MHz. (The 74LS38 is an open collector quad NAND gate)
I opened it up and traced out the layout. The frequency stage looks like to that I have now learned as a Pierce type NAND gate oscillator. I have drawn an abbreviated schematic of the digital pulser showing the frequency generation stage.
In the Pierce type oscillators I have researched, there is a capacitor in between the nand gates where resistor R1 is; however in this design there is a 2.2k resistor (R1).
What could be causing the wrong frequency to be generated? How I proceed to troubleshoot and correct the pulser? Should I:
- Replace the crystal with another and check?
- Replace resister R1 with a capacitor as I have seen in a Pierce type oscillator. I am guessing since the design would have worked at one point in time, R1 is not an issue here.
Any guidance would be appreciated.