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Replacing the MSF5000
By Robert W. Meister WA1MIK
The Intermediate Power Amplifier (IPA) and 9.6V Regulator (REG) reside on one module in the middle of the rear of the station's RF Tray casting. The combined assembly is called the IPA/REG board.
Sooner or later one of the small tantalum capacitors on the IPA/REG board is going to short out. It usually catches fire and chars the circuit board. Once it shorts, a foil on the Uniboard that carries A+ voltage to the IPA/REG board will burn open and need to be repaired, along with the damage done to the IPA/REG board.
IPA / REG Board Styles:
There are two styles of IPA/REG board. The older design has a ceramic substrate containing the IPA RF circuitry that takes up most of the space, and a carrier board around the perimeter that contains the 9.6V Regulator and the input circuit for the IPA. There is a 10 or 15uF 20V leaded capacitor (C451) along the front edge of the fiberglass carrier circuit board and it looks like it's a radial lead teardrop tantalum cap. Here's a photo of an older UHF IPA/REG board with the capacitor outlined in yellow. One manual says p/n 2384538G23 for the 15uF 20V part. Schematics are available in the "RF Tray section from The MSF5000 Instruction Manual for the analog station" on this web site.
The newer design uses an RF brick module for the IPA that takes up less than half of the space available for it. The 9.6V Regulator and the input circuit for the IPA are on the main circuit board, which is where the affected capacitors are located. The original caps are 22uF, 20V, 10% tantalum, rectangular surface-mount, and are an orange-yellow color. They are marked "226" (which indicates a value of 22uF). There are three of them on the IPA/REG board. Their location and identification numbers may change slightly depending on the operating band. The three capacitors have been outlined in yellow. Here's a photo of a UHF IPA/REG board.
For comparison, here's a photo of a VHF IPA/REG board, which is quite similar to the UHF board.
Here's the schematic of the VHF IPA/REG board with the capacitors indicated. Note that C461 is directly across the incoming A+ power line, and it's the one that usually blows up first. Click on the image for a larger view.
Finally, here's a photo of a 900 MHz IPA/REG board. This board is significantly different from the other two and the caps are in different physical locations, but they serve the same purpose.
Replacement Surface-Mount Capacitors:
It looks like the originals were Kemet T491 series, which are available from all the major parts suppliers. The Motorola service manuals just say p/n 2311049A21, 22uF, 10%, 20V, so the first value below matches the original one. The following D-size caps are all acceptable:
|Value||Kemet P/N||Digikey P/N||Price|
Choose one value and buy three of them per station. Digikey will mail parts out via USPS first-class mail for around $3, making them a great choice for small orders. Mouser also has these parts for about the same price; prefix the Kemet P/N with "80-" and remove the "AT" at the end for the Mouser P/N. The beveled or banded end is the positive end; there should be a polarity indicator on the circuit board. You don't need to remove the IPA/REG board to replace these caps. Heat the positive end and lift that end up once the solder bond releases. Then raise the cap so it's vertical and heat the other end underneath the cap and lift the cap off the board. Clean the excess solder off the pads. I added a tiny amount of fresh solder to the negative end and soldered that end first. The ground foil sucks up a lot of heat. Press the cap down as you do this. Then solder the positive end by adding a bit of solder where the lead and solder pad are exposed. Press the cap down and make sure it's firmly bonded at each end. It took me 20 minutes to open the station, replace all three caps, close the station, and test it.
I'm not very good at working with surface-mount components, although these are fairly large and are easy to install. If you want to install ordinary tantalum teardrop capacitors with wire leads, you can purchase those instead for a bit more money. Cut the leads to about 1/8 inch long, bend them to fit flat on the pads, and solder them upright or at an angle onto the existing pads where the surface-mount caps would go. There's plenty of room and placement is not critical. Just be sure you've got the polarity correct; sometimes the plus sign is very hard to see. Often one lead is longer than the other but once you cut them shorter you can't tell which is which.
You can buy either surface-mount or leaded tantalum capacitors on your favorite auction site in larger quantities for considerable savings. Just make sure the new parts have the same ratings and sizes.
Replacement Leaded Capacitors:
Digikey has several 10 and 15uF, 20V, 10%, radial, through-hole, tantalum capacitors you can choose from, made by AVX or Kemet, for between $1 and $2.
It seems that the proper way to replace C451 is to remove the IPA, unsolder and remove the old cap, install the new one, and replace the board. The ceramic IPA is quite fragile and the output coaxial cable is soldered directly to it, with little slack. So it might be easier to do all the work from the top of the board. You'll have to pull the old cap out of its holes, clean the holes completely, cut the leads of the new cap just long enough to fit through the holes but not so far that they short out to the RF Tray beneath it, and solder the new cap in place. An alternative is to cut the leads of the old cap and tack-solder the new cap to the remnants of the old leads. Sloppy but it will work fine. I have not done this procedure so your experience may be different.
Repairing Uniboard Damage:
Incoming power (A+) for the IPA/REG board starts at the power supply. It travels down a cable to the Interconnect Board under the RF Tray and passes up through the plug-in filter assemblies mounted to the RF Tray's casting under the Uniboard. The Uniboard's P402 connector, pins 2&3, picks up the A+ and routes it over to another plug-in filter assembly and P454A pin 3 that connects to the four-wire cable that plugs into the IPA/REG board. The foil that burns out runs between P402 pins 2&3 and P454A pin 3. If you see that it has burnt and/or opened, it's easy to just add a small, insulated jumper wire between the two pins, bridging the bad trace. Just make sure you've replaced the shorted tantalum cap(s) on the IPA/REG board and repaired any damage due to caps that caught fire (yes, tantalum capacitors do that). See this file for a color X-ray view of the Uniboard trace.
Credits and Acknowledgements:
The parts values came from various older Motorola service manuals.
John AD7SV provided confirmation of the capacitor brand and series.
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.
Article text, photographs, and hand-coded HTML © Copyright 2015 By Robert W. Meister WA1MIK.
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This article was created 03-Aug-2015
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