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  Making an MSF5000
Run Without a
Power Amplifier

By Robert W. Meister WA1MIK
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I know several people have asked if this is possible, either for testing or to drive a non-Motorola power amplifier. I had a reason to do this myself, and necessity IS the mother of invention, so I looked at the schematics and did some experimenting. I thought I'd share my results.

I performed this on a C74CXB7106BT station, 110w digital-capable UHF repeater that put out 60 watts at the antenna jack. The same procedure should work on an analog station or any base station.

Note that this method should NOT be used as a permanent solution. The power control circuit runs with very high gain and is not stable without a true feedback signal that follows the RF output of the station.


I disconnected the N-male connector going from the RF tray to the input of the 3-section pre-filter on the internal filter/duplexer. I ran this cable into a Telewave 44A wattmeter, which was terminated with a 50-ohm load.

Bob McKinlay VE3DJ drew a very nice pictorial diagram of the 6-pin Molex connectors, which can be found here as a 114 kB PDF file.

I disconnected the 6-pin Molex connector going from the RF tray to the PA. I connected a 2.4k 1/4w resistor from pin 4 (the green wire - forward voltage) of the Molex connector coming from the RF tray (the one with the exposed male pins) to TP5 on the SSCB, a source of +9.6VDC. This was in the front corner next to the Set/Select momentary switch on my board. I did measure +9.3VDC there by itself.

I connected a Fluke 189 digital multi-meter to the resistor lead on pin 4 of the Molex connector so I could monitor that voltage. In receive mode, it measured +2.79VDC.

I pressed the front panel Xmit switch. The wattmeter moved quickly up to 12 watts. I measured +2.83VDC on the resistor lead. All of the normal front panel LEDs lit up as if the station was feeding its PA and making 60 watts.

I tried adjusting the front panel Power Output control. The slightest movement caused the RF output to drop to zero. The internal power control circuitry has very high gain and depends on the forward voltage signal coming out of the PA to carefully regulate the output power. This is why the station is able to hold its output power so well.

At this point, the power control circuit is running open-loop, as it has no feedback from the PA to control things. The normal rated power for the IPA is 3-9 watts. The maximum amount of output power the IPA is capable of is 15 watts (slightly less on 896 MHz), but it's not rated to run that high continuously. In the 6w stations, the IPA feeds the circulator directly, and in all stations the forward voltage is sampled just prior to the circulator. A resistor value is changed to produce about the same +2.8VDC at rated power for each power amplifier.

So it is possible to do it, although you can't realistically control the output power. The proper way to do so would be to fashion a forward voltage sampling circuit to the output of the RF tray and hook it to the right pins of a 6-pin Molex connector, so it would provide some feedback based on output power.

A More Permanent / Alternative Solution:

The following information was posted on a news group and has been transcribed here. The author makes no guarantees as to the usefulness, completeness, understandability, or accuracy of this information.

MSF5000 PA Bypass:

Required parts: 1.5K 1/4 watt resistor and two ferrite over wire.

  1. Remove RF Tray top cover.
  2. Make a pathway for a wire exiting the IPA area by either:
    1. Drilling a small hole at the TX VCO end nearest the Uniboard, or
    2. Lifting the TX synthesizer coax and grinding the slot deeper.
  3. Connect a 1.5K 1/4 watt to Q451-C (middle lead). This is on the IPA control board and is mounted to the chassis with a screw. Two IPA layouts - one using an RF brick (900 MHz) and one using discrete parts (UHF) - can be found in the MSF5000 library.
  4. Slide one insulated lead of the ferrite through the opening made in step 2.
  5. Connect the other ferrite lead to the free end of the 1.5K resistor.
  6. Connect the protruding ferrite lead to P402-13 (Final Forward Voltage). This is the right-most connector along the back edge of the Uniboard. A color scan of the Uniboard X-ray view can be found in the MSF5000 library.
  7. Move JU1 from the 'normal' to 'service' position (removes PA Fail beeps when not in TX). Note that this jumper only applies to ANALOG stations, those with a Station Control Module (SCM). The analog-plus and digital-capable stations require a different method to put them into the 'service' mode; details are buried in the instruction and operation manual.
  8. Replace the RF Tray top cover (be sure your wire routing will not be pinched by the cover).

This makes a connection between the voltage that controls the IPA and the Forward Voltage signal, that would come from the PA if it were present, into the TX Power Control circuit, providing feedback that tricks the station into thinking there is a PA attached. Power should be adjustable from near zero to about 12.5 watts.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This article first posted 27-May-2008

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.