In this episode Shahriar repairs a Fluke PM6680B 225MHz High Resolution Frequency Counter. This unit which is equipped with a soft power button, does not power on. The standby LED is dimly lit and a high-frequency noise is emitted from the unit. This points to a switching power supply failure. The power supply is a simple fly-back converter with three outputs (5V, 18V, -8V). The 5V output is regulated while the other two outputs are single diode rectified.

The fault is traced to a bad precision reference IC which is replaced. The performance of the power supply is verified and the instrument powers on. An OCXO is used to calibrate the 10MHz reference crystal of the unit after the repair is completed.

The Signal Path Blog
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