Running an MSF5000 without the PA attached

[kcbooboo]

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 ran a test this morning. I thought I'd share my results.

This was on a C74CXB7106BT station, 110w UHF repeater that put out 60 watts at the antenna.

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.

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 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 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 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 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, 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. It would probably be quite easy to fashion a forward voltage sampling circuit and hook it to a 6-pin Molex connector so it would provide some feedback based on output power.

Bob M.

[kb0nly]

How about changing the resistor value or putting a trimpot inline with pin 4 and the 9.6v source instead? Could that be used to keep it within the 3-9 watts without having to monkey with the front panel power output control?

[kcbooboo]

I'm sure the resistor value isn't critical, i.e. you could substitute a 2.2k or 2.7k, or run it from a different voltage and use a completely different value. As it is, your mileage may vary, and your station may put out no power at all unless you slightly tweak the power control pot.

The big problem is that with the high gain of the power control loop, a very small change makes a big difference in the IPA's output power. By making it an open-loop, the front panel power control really just becomes an on/off pot. I did try to carefully set the power to something else, but it was either 12 watts or zero. I could get it to be a couple of watts, but it was changing as I observed the meter.

The pot on the Uniboard is an inexpensive, low-precision, carbon composition, 240 degree rotation pot. Maybe if it was a 25-turn pot, it would become more useful for setting the output power when in this open-loop condition.

Without going into a lot more circuitry on the Uniboard, this was the best I could do. At least the station thinks it's running just fine and doesn't generate any alarms. Great for testing, but I can't recommend running it that way.

Bob M.

[w9sl]

I am intrigued by this thread as it got me past an impass with my UHFMSF5000 (C74CXB7106BT) the same one you are working on. I just acquired this repeater and while tweaking the Tx prefilter, I lost the ability to key the transmitter. I figured I had messed up the tuning so I bypassed the prefilter and still could not get the system to tx even playing with the front panel power output control. I happened across your clever way of fooling the power control circuitry and when I drove the green wire ( forward power line) with a resistor to 9.6v and adjusted the power out pot, I was able to get output from thr IPA,....... but only 2-3W max and not the 12W you obtained. Unfortunately the main PA doesn't respond to anything less than ~3W so I do not have enough drive! I can't imagine what's wrong and I hope I haven't damaged the IPA output transistor by previously mistuning the prefilter. Any ideas as to what to try would be greatly appreciated.
Thanks in advance,
Don

[Doug]

Hey Don,
Have you had any further luck? Have you tried by-passing your pre and post filter at the same time? Unless your IPA was getting weak I doubt that you've done any damage to it as they seem to be pretty robust.
Doug

[KF4LZA]

Can anyone shed more light on this topic and wether it is possible to fashion a power control circuit to trick the IPA that the PA is installed? I am trying to use a RF tray on a TPL 100 watt amp however I can only manage to get the IPA to transmit a full 20 watts out which will burn it up in no time and overdrive the TPL amp as well. I have tried the resistor inline but that only allows the IPA to think the PA is there and key up at 100% power level. Has anyone tried an inline pot to see if the voltage can be managed?

[GlennD]

Assuming this is UHF, if you want to pay the freight or pick it up in Long Beach, I have a Passive PA assy in my junk box. It came from a LPI repeater years ago.

Since it is built on the mid power chassis it is pretty heavy. All it has in it is the power control circuitry and a circulator.

PM me if interested

[Big BOB]

A couple of my MSF's were giving me trouble as stated. The only power control I had from the from panel was either full power or no power. No matter what I did that was all I could get.

So unplug the six pin molex going to the PA. Turn the front panel power control CCW then bring it up until you see power (Mine was full power). Turn the MSF off and open up to get to the IPA. There is a 4 pin connector on the IPA. Place a small pot in line with the first pin (brown wire). This is the voltage control line to adjust the power out of the IPA. Turn the MSF back on and adjust the pot to the power out you want. I have been using this fix on 2 of my MSF's for several years with no problem.

Big BOB

[WB6DGN]

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 ran a test this morning. I thought I'd share my results.

I always heard it was "necessity is a mother...". but that's neither here nor there. In the Low Power 800 and 900 stations, Motorola uses what they call a "Low Power Control Head" to enable running the IPA directly into the antenna for 5 or 6 watt operation. It's basically just a box with "N" connectors in and out and a single wire that goes to the interconnect board. Inside is a directional power sense circuit (pretty simple) that provides the feedback that the control needs to keep the power output normal and stable. The output of the IPA goes through the circulator, then through the low power control head, then through the low pass filter to the transmit antenna jack. A single wire, labeled "forward", p/o W203, goes to J596A, pin 2. There is no source of DC other than the rectified RF from this box. If someone can suggest a location, I can scan and post that section of the manual for reference. There are only five pages. Looks like it would be pretty easy to build one.
Tom