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Replacing the Potentiometers on the MSF5000 Uniboard By Robert W. Meister WA1MIK

The Problem:

Eventually, the power control pot on the front of the MSF5000 RF Tray will either break or fail to operate properly. Besides mechanically falling apart, other symptoms such as the station keying up and then immediately unkeying, or the inability to adjust the power may be the most obvious indications. The pots can't be repaired; they must be replaced.

Motorola used (at least) two styles of pots: the ones shown here with the metal rotating contact disc, and ones with larger plastic rotating pieces and a ceramic body. My boards all had the metal ones. I recently had to replace the electrolytic capacitors on a board that had the plastic/ceramic pots and all of those were fine. Here's a photo of this style. Remember, these aren't the ones that seem to fail.

The power control pot is mounted vertically on the component side of the Uniboard, the main board inside the RF Tray, so the front of the pot faces the front of the RF Tray, making it easy to adjust through the hole in the front panel. The Uniboard is installed with the solder side up, so when you remove the RF Tray's top cover, the pots are on the other side of the board. For the pots that are mounted horizontally (flush to the board), there is an access hole in the board through which you can insert an alignment tool or small screwdriver to adjust the pot from its rear. Some of these pots can even be adjusted through holes in the RF Tray's top cover. The ones on my board were Bourns 3359-series, which are adjustable from the rear but are obsolete and no longer available new. The photo below shows one pot still installed on the board, and a removed one sitting next to it, so you can see that it's adjustable from the rear of the pot.

I acquired a UHF Uniboard, TRN7231A, built in 1992, that had been exposed to a salty air environment. The ground foils on the component side of the board were badly corroded. All of the pots were frozen in place and refused to rotate. I managed to get the pots to turn but the power control pot measured open from the arm to either end. Since I had to replace at least that pot and it made sense to replace all four of them. (Some boards have five pots while others have just three pots.)

Choosing New Parts:

I took some measurements and found that they are 9mm wide. The lead spacing is 5mm from end to end. All of the pots are 10K. I looked very carefully at the data sheets for those pots and could not tell if they were adjustable from the rear. Bourns makes the 3319 and 3309-series pots that look similar and have the same form-factor but I couldn't tell if they were adjustable from the rear. The parts list in the MSF5000 service manual mentions Bourns 3319-series. Then I came across CTS 262-series pots. Their data sheet clearly shows and says that they are adjustable from either side with a 3/32-inch screwdriver slot that goes all the way through. The ratings were similar and they were the same size as the Bourns pots. The CTS pots are the ones you want to buy. Vital statistics are summarized below.

Mfgr.	Series	Orientation	Power	Material	Digikey Part #
Bourns	3309	Horizontal	0.5W	Cermet	3309P-103-ND
Bourns	3309	Vertical	0.5W	Cermet	3309W-103-ND
Bourns	3319	Horizontal	0.2W	Carbon Comp.	3319P-103-ND
Bourns	3319	Vertical	0.2W	Carbon Comp.	3319W-1-103-ND
CTS	262	Horizontal	0.15W	Cond. Plastic	262UR103B-ND
CTS	262	Vertical	0.15W	Cond. Plastic	262XR103B-ND
Bourns	3359	Either	0.5W	Cermet	Obsolete; N/A

Mouser has the CTS pots as well: 774-262UR103B and 774-262XR103B for under $0.60 each.

UPDATE: January 2021: Apparently the CTS pots are obsolete. I did some research and it seems that Piher Sensing Systems makes an equivalent pot, their PT10 series. They cost less than $0.50 each at Digikey. The horizontal mount, vertically adjusted 10k screwdriver slot pot for the power adjustment is p/n PT10LH01-103A2020-S, Digikey p/n 1993-1107-ND. The vertical mount, horizontally adjusted 10k screwdriver slot pot for the other flat-to-the-board pots is p/n PT10LV10-103A2020-PM-S, Digikey p/n 1993-1081-ND. I have NOT bought any of these parts yet but they do seem to be the same size and form factor as the CTS pots were. I'm guessing that Mouser has them, or has something functionally equivalent. The horizontal vs vertical designation is somewhat confusing; look at the photo on Digikey's web site to make sure you get the correct number of parts in each configuration.

Removing the Uniboard:

1. Disconnect the AC power.
2. Remove the large screws holding the RF Tray to the station frame.
3. Slide the locking tabs inward and pull the RF Tray out as far as it will go.
4. Unlatch the plastic Control Tray on top of the RF Tray and swing it up.
5. Remove the screws holding the RF Tray's top cover on and remove the cover. All of the stations I've seen use three to five Torx T20 screws that are retained in the cover but some use larger thumbscrews.
6. Starting at the left rear, unplug the plastic-covered RCA-female coax cable going to the Injection Amplifier and slide its ground clip off the casting. Unplug the plastic connector going to the receive VCO. Unplug the bare RCA-male coax cable going to the receive VCO. Unplug the bare RCA-male coax cable going to the IPA and slide its ground clip off the casting. Unplug the bare RCA-male coax cable going to the transmit VCO. Unplug the plastic connector going to the transmit VCO. It's pretty hard to plug these into the wrong places when you're through.
7. Gently start lifting the rear edge of the Uniboard up slightly as there are a lot of connector pins holding it in place. There are two plastic standoffs, one at each end of the Uniboard mid-way between the front and rear edges; these may come off with the board or might stay in the RF Tray. I find it easier if they stay in the RF Tray. Push these out of the board if necessary. Once the board is free, lift it out of the RF Tray using the large black plastic handles. The board is full of CMOS ICs so take the usual anti-static precautions.

After replacing the pots, follow the steps in reverse to install the Uniboard. Be very careful that the pins sticking up from the RF Tray are properly aligned with the sockets in the Uniboard. It should fall into place but you don't want to force it. Once it's lined up, gently press down on the rear edge of the board to seat the connectors. Also make sure the plastic standoffs are in place. Reconnect all the cables etc.

The Results:

I ordered enough of the CTS 262-series pots to replace all four pots on my Uniboard and one of each of the Bourns 3309 and 3319-series pots to see if they would work or not. When they arrived I took some photos of the rear of the pots. Click on any photo for a larger view. You can see the front by searching for those parts on Digikey's web site.

As you can see, neither of the Bourns parts has a slot that you can use to adjust the pot through the circuit board from the rear. This doesn't matter on the vertically mounted pot, which is only adjusted from the front, but only the CTS pots have a slot that allows adjustment from the solder side, through the Uniboard.

The pins on the vertical pot seemed to be just a bit larger than the holes in the circuit board. I had to trim them down just a bit to get them to fit. The horizontal pots just plugged right in. I'm not sure if the solder holes weren't clean enough or if the pins on that pot were just a bit wider.

When I plugged the board into my station, I was happy to see that the TX power adjust pot worked fine.

Contact Information:

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 09-Jul-2015

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.