Teardown, Repair & Calibration of a Fluke PM6680B 225MHz High Resolution Frequency Counter

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.


  1. Peter O'Neill says:

    Nice repair video. A few years ago I bought a LeCroy LW420 Arbitary Waveform Generator, these have two switching supplies one for the positive rails and the other for the negative. The negaitive rails were all zero and the supply used the same voltage reference and PWM controller chips as the PM6680B but in my case one of the secondary side rectifier diodes shorted and blew the primary side drive transistor, PWM controller, voltage reference, current sense resistor, and a few other parts just to protect the local fuse! Once the neg. supplies were working the software diagnostics helped find a small number of damaged components and now the LW420 is operational.

  2. Jim Littlefield says:

    The default oscillator included the PM6680B is certainly not the best. A dramatic improvement can be obtained by installing a better OCXO (PM9691 or PM9692). Better yet, use an external 10MHz reference, such as an rubidium standard or a GPSDO.

    Thanks for the great videos!

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