Keying the Harris RF-104 Amplifier


A 3-1000Z in Operation

(The distortion in the photo is caused by the glass chimney)

Although Ceramic External Anode Tubes are now the defacto choice where high power is needed, the glow from the anode of a glass enveloped tube is still something to behold.


In the 1970s and 1980s, the RF Communications Division of Harris produced a range of RF Power Amplifiers for the top-end commercial market.

These models included the RF-103, it's later variant the RF-103A (see my HZ1AB pages), and the RF-104. I also believe that the RF-102/RF-102A was another in the range but I have no other details. There may have also been other models in the range of which I am unaware.

The RF decks for all models are essentially the same, having a single 3-1000Z tube operating in grounded grid. The HT power supply is very substantial using a "swinging choke" design. The power supply is intended for full "key down" continuous operation at the rated 1kW output in any mode. The amplifier weighs over 100kg.

These amplifiers provide full coverage between 2 and 30MHz and the operating band is selected via a 10 position rotary switch on the front panel. The tuning within the selected band is via a motor driven roller inductor. The amplifier may either be operated with pre-programmed channel tuning, or via a manual tune switch - the later mode being normally used by Amateurs.

The main significant difference between the RF-103/RF-103A and the RF-104 is that the RF-103/RF-103A has a capability for 10 pre-tuned channels, whereas the RF-104 had 64 channels as standard expandable to 96.

Harris specifically designed the RF-104 to use positive line keying to interface with their line of transceivers and ATUs, whereas virtually all Amateur Transceivers are designed for amplifiers with "make to ground" keying.

The RF-104's Program and Junction Board (Control board) does have the facility for "make to ground" keying but it is necessary for the user to insert a diode in this board which is located to the rear of the front door.

The Program and Junction Board is a large and complex circuit schematic which covers a total of four pages in the manual. For clarity each page is scanned in three parts, and the zipped pages may be downloaded by clicking on the page number below:

Sheet 1 (633kb)

Sheet 2 (538kb)

Sheet 3 (584kb)

Sheet 4 (550kb)


The Program and Junction Board

Find CR32 on drawing "4b of 4" - this is a 1N4454 diode and with the text "See note 6" written just above it. On the board it is physically located just below the fuse. Any small signal diode such as a 1N4148 or 1N914 can be used in place of the 1N4454. The banded end of the diode (cathode) should be to the right.


The Program and Junction Board parts placement


Location of CR32

(Cathode (Banded End) to Right)


Now by grounding pin 25 of either J23 or J24 - these are the 37 way "D" connectors on the rear panel - the amplifier should key. (Pin 25 of J23 is internally connected to Pin 25 of J34).


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Page last updated 4 Oct 2003