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By Robert W. Meister WA1MIK 		  Print this Page


Background and Variations:

Motorola MSF5000 (and probably MSF10000) power supplies come in several varieties. This model matrix has the latest published voltage and power ratings.

First, there's the AC input voltage and frequency. In North America, 110/120VAC 60 Hz is the norm; elsewhere 220/240VAC 50 or 60 Hz may be found. Some supplies have voltage selector switches on them, but you can't use a 60 Hz supply on 50 Hz power, or vice versa. All of these supplies use a ferro-resonant transformer for voltage regulation, which is frequency-dependent.

There are also supplies that accept 14 or 28VDC input; these are just DC power filters that clean the power and provide fused distribution. I won't mention these any more.

Next we have the DC Output Power. The most common are 200-250W or 500-675W. The lower power supplies would be used in the lower power stations operating in the 10-25W range. A small power distribution board provides fused protection for several output taps to feed the RF Tray and other accessories.

Some supplies can also switch to battery operation when the AC power fails. They will also slowly recharge those batteries. These are referred to as Battery-Charging, Battery-Reverting, or Battery-Backup supplies. A regulator board controls the output voltage, which is usually lower than the non-battery supplies. This board also has the fuses on it to distribute the voltage. Battery-Charging supplies usually have a slightly higher power rating than non-Battery-Charging supplies.

Last we have the output voltage. For VHF stations of more than 25W RF power, the RF Power Amplifier operates on 28VDC while the rest of the station operates on 14VDC, so the supply has to provide both 28V and 14V. For lower power stations, everything operates on 14VDC. ALL OTHER stations (UHF, 800 MHz, 900 MHz) run entirely and exclusively on 14VDC.

General Comments:

Ferro-resonant transformers have several positive qualities: they're nearly bullet-proof, they're nearly short-circuit proof, and line voltage regulation is fantastic. One negative quality is that they're not very efficient, but if you're not paying for the electricity, don't worry about that.

The battery-charging power supplies all have an adjustable output voltage since they also charge the batteries. If you don't have batteries attached, the no-load to full-load voltage drop will seem excessive: 2-3 volts. The supplies really depend on the batteries being connected to provide much better voltage regulation. The non-battery-charging supplies hold their output voltage much better; a typical 14V supply idles around 15.3V and drops down to 14.3V when the station is transmitting full power.

The ferro-resonant transformer depends on a capacitor across a resonant winding. The value of this capacitor depends on the power supply module, but in general it runs between 5 and 30 uF at 330VAC and will be 5% to 6% tolerance. This is critical; if the value is further off than the tolerance allows, the capacitor must be replaced. It's a common "motor-run" capacitor that's used with electric motors. You can usually find these at HVAC (Heating, Ventilation, Air Conditioning) supply houses for around $5US. You want one with 1/4 inch quick-disconnect connections on each terminal. Most will have two, three, or four terminals. The battery-charging power supplies use two connections on each terminal; the other supplies only use one connection.

I took some measurements on the 500w power supply on my fully functional 110w UHF repeater. There's a barrier strip mounted to the rectifier's finned heat sink (TB602). The lower pair of terminals - the ones that go to the diodes and elsewhere - had 34.0VAC on them when the station was idle and 33.4VAC when transmitting. The upper pair of terminals - the ones going only to the transformer - had 39.0VAC when idle and 38.5VAC when transmitting. The capacitor mentioned in the preceding paragraph measured 20.3 uF and is marked 20 uF.

One fellow was having serious voltage regulation problems, such that the unregulated DC voltage dropped below 9V on transmit, even with the output power set to zero, which caused the station to reset. He bought a new 20 uF 370VAC round motor run capacitor at a local appliance parts store and wrapped about 1/8 inch of electrical tape at the bottom so it could be held by the original cap's mounting clamp. The new cap was a bit smaller than the old one. That got his DC voltage up to 15.9V when idle and 14.9V when transmitting. Problem solved.

Battery Wiring:

The Battery-Charging single-voltage supplies have two battery connections because they only need 14V from one battery. The battery connects to similar terminal blocks that feed the RF Power Amplifier along the bottom rail of the power supply. A low-voltage disconnect relay should also be installed on the chassis.

The Battery-Charging dual-voltage supplies have three battery connections for a pair of batteries. The two batteries are wired in series to provide 14V across one battery and 28V across the pair of batteries. The wiring for this configuration is a bit more complex. The 28V battery voltage goes directly to the RF Power Amplifier. The 14V battery voltage goes through a relay that disconnects it if the voltage goes too low, thereby shutting off the station. A diagram of the wiring of the dual-voltage supply can be found here. Note that all three battery connections go to terminal blocks along the bottom rail of the power supply.

Standard 75A Anderson PowerPole connectors are used with the battery cables, and in-line fuses protect against short circuits.

Power Supply Manuals and Schematics:

The following PDF files are available. They can also be found on the main Index page.

Contact Information:

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

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This article first posted 31-Oct-2013

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