I got this frequency counter and crystal tester mainly for the frequency counter. Today I decided to test a few crystals, and was successful with ones above 10 MHz but not at 4 MHz. The two capacitors labeled C2 and C3 are the feedback to the crystal, and they were originally 22 pF. So I changed them both to 39 pF. Now the 4.0 MHz resonator from an old mouse works okay. But it won’t oscillate with a 455 kHz resonator. It’s good enough to test most xtals.
