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Power Supply Wiring
By Robert W. Meister WA1MIK
with lots of input from Bob McKinlay VE3DJ
Once upon a time, Motorola included power supply documentation in their station manuals, but then some bean counter decided they could make more money by removing that information from the individual manuals and combining it all into one thin but expensive Power Supply Service Manual, 6881094E30. When they did this, they threw out the information for the old power supplies and only kept the information for the latest revisions. Also, they did not include any information about where the batteries get connected. I think they expected us to "Use The Force" to figure this out. If you've already got a working station with the battery charging (also known as battery reverting or battery backup) power supply, you can follow the wires and see where they go, but there may be times when those wires are missing or have been disconnected and it sure would have been handy to know where they all go. Well, this article will hopefully clear up a lot of mysteries.
The Motorola MSF5000 stations are available with several power supply combinations. The input power can be DC, 120VAC, 240VAC, or 120/240VAC. The output power can be 250, 500, 575, or 675 watts, depending on application. The supply can have a single output voltage (14VDC nominal) to be used on low-power VHF and all UHF, 800, and 896 MHz stations, or dual output voltages (14VDC and 28VDC nominal) to be used on 25 watt and higher VHF stations. Finally, many stations can run on, and charge, external batteries such as NiCd or lead-acid.
What We Have To Work With:
The power supply manual has an overall wiring diagram, which they refer to as a "Hardware Kit". This shows the AC input, the transformer, the rectifiers, and the DC filter components. This PDF file has the diagram for the VHF 14V/28V power supply. Note that there is a terminal block (TB651) labeled "A+ TO PA" but you have to look carefully at the wire going to P602, that plugs into a printed circuit board, to discover that this is 28V. And of course TB651 is not labeled on the power supply chassis; you're supposed to know that the wires from the Power Amplifier (PA) go to the leftmost terminal block along the bottom of the power supply chassis. Remember that on low-power (under 25 watts) VHF stations and on all UHF and higher stations the PA runs on 14V.
The printed circuit board has several fuses and it distributes 14V power to the rest of the station. The manual calls this a "Distribution Board". It also contains the battery charging circuit. This PDF file has the diagram for the battery charging distribution board. J602 at the left receives 14V power from the bare power supply and sends it out to the rest of the station at the right via J603. But then we have this little J600 at the bottom of the diagram that says "TO/FROM BATTERY". Well, that really clears things up, doesn't it? The connector is rather small and when you look at a VHF power supply you find four thin wires going to a relay. So far we haven't seen any signs of where the battery connects to, nor have we seen a relay.
The relay is documented as a Service Manual Revision (SMR) that came with the manual I bought. This PDF file shows the documentation for this SMR (I won't bore you with the parts list on the back). The relay contacts are going to J605 on the station junction panel, which they label "BAT +", and to TB601. They call it a "low-voltage disconnect kit" and it's left to the reader to figure out just what it does. The manual already lists a relay kit with a different number which is not documented anywhere. For some reason, Motorola renumbered all the parts so they start at 650 and work their way up. So, TB601 would be equivalent to TB651. That helps somewhat as that's what the PA connects to. The relay's contacts go to a second terminal block, which is not shown on any of the diagrams. The photo of the supply calls it TB650. Here's the photo from the manual; click on it for a larger view:
VHF Station Wiring:
On the VHF station there are TWO batteries and there is that little relay, so how do these all interconnect? Bob VE3DJ has one of these stations and one day the power supply blew the main line fuse, so he took it out to investigate. In the process he had to disconnect all the wires going to the power supply and luckily documented where they all went. This PDF file shows the big picture in a pictorial diagram. First, note that there are now TWO terminal blocks along the bottom edge of the power supply. The left one still feeds the PA and the 28V battery goes here as well, but the right one handles the 14V battery through the relay contacts. The relay is energized if there is sufficient power to operate the station. When it drops out, the 14V from the battery is lost and the station shuts off. The PA is still connected to the 28V battery source but since there is no drive, the amplifier draws no current. The diagram also shows how the 14V battery power connects to the 14V power supply.
The TPN1271A VHF battery-charging power supply manual only discusses the three leads: 28V, 14V, and GND. It completely omits the important fact that the negative terminal of the 28V battery goes to the positive terminal of the 14V battery. Note that both batteries are identical but they are wired in series to provide 28V (nominal) to the station.
On the dual-voltage (VHF) supply, the 28V battery connects to the output of the 28V supply at TB601/651. The 14V battery connects to the output of the 14V supply (C655) through the undocumented relay. The battery GND goes to the GND terminal at TB601/651, which is a common ground throughout the power supply.
Single-Voltage Supply Wiring:
The single-voltage supply is similar to the low-voltage section of the dual-voltage supply. The power to the PA comes from the 14V supply, the two diodes and filter components for the 28V supply are not installed, and there is no 28V wiring anywhere. The manual for the TPN1185A power supply can be found here.
The battery-backup wiring for the 14V-only power supply is similar. There may or may not be a relay in series with the battery wire, but eventually the 14V battery power goes to the PA wires at TB601/651. If there's a relay present, the 14V battery wire probably goes to the second terminal block TB650, and the relay makes the connection from TB650 (the battery) to TB601/651 (the 14V supply).
Just like on the dual-voltage supply, when the battery voltage drops too low to support station operation, the relay drops out and opens the connection between the battery and the power supply and the station shuts off.
The MSF5000 power supplies are all inherently self-regulating through the use of the ferro-resonant transformer. This feature relies heavily on the AC capacitor that's across one of the transformer windings. The value is typically 30uF but they can be from 5 to 50uF at 330-660VAC. The tolerance is 6%, so if the cap's value is off by more than this amount, it's bad. For comparison, a typical DC electrolytic capacitor might have a tolerance of 20%. If this capacitor changes value, the output voltage or regulation may change. It's easy enough to check, and most appliance or air conditioner supply houses will have replacement caps for $5 to $10. Bring the old one with you so you get one that's the same physical size, has the same microFarad value and the same or higher voltage rating, and has the same terminal configuration.
The battery charging power supplies have a regulator board that switches an inductor across the resonance capacitor to change the transformer's output voltage slightly. An SCR drives a Triac to perform this switching based on the 14V power supply's output voltage. The inductor should only be in the circuit for a fraction of the AC power cycle. If something goes wrong on the regulator card, it can leave the inductor in the circuit 100% of the time, causing the supply to draw a lot of power, make a growling sound, and possibly blow an input fuse. While the regulator could probably be repaired, it's faster, easier, and safer to just replace the board.
Credits and Acknowledgements:
Bob McKinlay VE3DJ provided hands-on examination of his VHF battery-charging power supply and drew the pictorial diagram.
Schematics and photos came from the Motorola Power Supply Service Manual.
MSF5000 and a whole bunch of other terms are trademarks of Motorola, Inc.
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.
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This article first posted Sep 16, 2013.
Article text and hand-coded HTML Copyright © 2013 by Robert W. Meister WA1MIK.
Pictorial diagram copyright © 2013 by Robert McKinlay VE3DJ.
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.