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Proper drive levels for a Harris SX serries transmitters

I was working on a Harris SX5 the other day and snapped a picture of the scope while measure RF drive levels.  There are still quite a few of these units out the in the field, judging from my search engine results.  I thought it would be helpful to post something about it.  The RF power amp boards for the Harris Gates solid state AM series transmitters are the same design, I believe.

In order to fully drive the RF MOSFETs in this particular series transmitters (Harris SX1, 2.5 and 5 including A models) at least 26.5 volts peak to peak is required.  Less than that and the device will turn full on and internally short.  To measure RF drive, the transmitter must be in local with control voltage on, with the rear door interlock defeated (this can be safely done if the transmitter is wired with separate AC feeds for control and RF power supply). Make sure the RF power supply is defeated and will not turn on.  Measure across the input of the each of the toriods that feed the gates of the RF devices.

Harris SX series transmitter drive level test

Harris SX series transmitter drive level test, 27.45 volts, 1,110,000 Hz

It should measure between 26.5 and 29.5 volts. This one measures 27.45 volts peak to peak. Each input toroid on every PA board should be measured as the toroids themselves have strange failure modes and may pass resistance and continuity tests, yet still not provide proper drive voltage to the attached devices. This has to do with core permeability.  Each toroid feeds two RF MOSFETs, replace part is IRF-350.

As always when dealing with a SX transmitter, good luck.

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2 comments to Proper drive levels for a Harris SX serries transmitters

  • Elliot Gehringer

    We have a SX-1 as a backup..the unit will not make more than
    25 watts..the raise lower controls have not effect..the control circuits function properly…
    The four outputs of the PDM generator appear to be normal wave
    forms at the proper levels…the input to the IPA board is 15 volts peak to peak..however the input to the PA is only 20 volts. The voltage to the PA board is around 250 volts…there is nothing in the troubleshooting section of the manual…that
    deals with this problem.. the PA volts on the digital meter
    reads 57…with current around .5..one PDM amp output has a
    shorted Mosfet..transmitter was only doing 750 watts before the current problem…any tips would be greatly appreciated..

  • Paul Thurst

    Sounds like insufficient drive to the PA board. Likely, all the devices on the PA board, except one or possible two, are shorted and will need to be replaced after the low drive level problem is isolated and fixed. Good luck.

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