Making Motorola MSF5000 VHF VCOs Operate Below 146 MHz

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Back to Home Making Motorola MSF5000 VHF
VCOs Operate Below 146 MHz
By Scott Hilton NØOBA and
Paul Thompson WØOD

We recently acquired several VHF MSF5000s in the Denver area and found they would not work on our repeater frequencies below 146 MHz. We actually looked on repeater-builder to see if someone had done this and was surprised there was no info there already, so we thought it would be nice to share our modification with everyone else.

Although this modification was quite simple, we of course make no claims that you will be as successful, so please do this at your own risk.

Background:

Our repeaters were stock range-2 units that covered the 146-174 MHz band. The VCOs would not lock at our frequencies of 144.590 RX and 145.190 TX since they were out of their normal operating range. VHF VCOs are not tunable or adjustable, unlike all the other models. The range-2 VCO assemblies we had were RX VCO TRD1842 and TX VCO TTD1732. In order to work on our repeater frequencies, we needed to convert the high split VCOs to the lower split (range-1, 136-162 MHz). The range-1 VCOs are TRD1841 and TTD1731 and the C367 option provides these. Here is how we modified our range-2 VCOs to be used on the range-1 frequencies, at least low enough to cover the bottom end of the two-meter band.

In the MSF5000, the transmit VCO operates at the desired transmit frequency, but the receive VCO operates 10.7 MHz lower than the desired receive frequency (this is due to the intermediate frequency of the receiver).

Getting It To Work On The Bench:

Paul did the initial investigation and testing. Here's how he went about working on the receive VCO.

I set the VCO up on the bench. I provided 9VDC to the Vcc Supply line and the Band Shift line, and I grounded the Steering line. I measured the VCO output frequency then I took off the top cover. I thought the frequency would increase, but it decreased 500 kHz. I starting by cutting the inductance trim jumpers. This lowered the operating frequency by about 800 kHz. Then I bridged the capacitance tuning stub and got closer. I needed a bit more capacitance, so I just ran the wire closer to the ground strip. That did the trick. The frequency increased by the same 500 kHz when I put the cover back on.

Step-by-step Procedure to Extract the VCOs:

Do each VCO separately or make sure you mark them so you can put them back in the correct location.

Unplug the power cord.
Remove the four screws holding the RF Tray's front panel to the cabinet, release the latches, and slide the RF Tray all the way out.
Unlatch the plastic Control tray and rotate it all the way up into the service position.
Remove the five Torx screws holding the aluminum cover to the RF Tray and remove the cover. The photo below shows the progress at this stage.
Unplug the right-angle RCA plug and the DC signal connector from the VCO.
Extract the VCO by lifting it straight up.
Remove the four Torx screws at the four corners of the top cover of the VCO.
Do the modifications detailed below.

Here's the view after the RF Tray's cover has been removed, identifying the VCOs inside. Click on any photo for a larger view.

Reverse the above steps to restore normal operation.

The Physical Modifications:

The two VCOs are very similar. The transmit VCO has additional components to allow audio to modulate the carrier frequency. The RF sections are essentially identical. Here's a photo of the unmodified receive VCO with the various modification areas noted.

And here's a photo of the unmodified transmit VCO. Compare these to the modified versions below.

There are two foil areas that are trimmed at the factory to get the VCOs to operate at the correct frequencies. At the ground end of each VCO inductor line (the copper trace along the bottom edge, just left of center), are four small foil traces. We're calling them "inductance trim paths." All four need to be cut to give us more inductance to lower the operating frequency. The receive VCO photo below shows this modification.

The upper tuning stub (the copper trace that runs across the top edge just below the board numbers) acts as a multi-section capacitor. It was necessary to increase capacitance to lower the operating frequency. We did this by soldering a piece of thick bare wire to the foil at each end. On the TX VCO we didn't need to add too much capacitance, so just restoring the entire tuning stub by bridging it was all that we had to do. For the RX VCO, we needed to place the added wire nearer to the top of board (do not let it touch the numbers as they are grounded) to get added capacitance. [Editor's note: You can also use insulated solid wire or put sleeving over the bare wire.] The wire should be as close to the circuit board as possible. The photo below shows the transmit VCO modifications. Note that the four inductance trim paths were NOT cut on this VCO, although they should have been. It is recommended that they be cut. An X-Acto knife works fine.

After the modification, the RX VCOs worked down to roughly 132.700 MHz (allowing a receive frequency of 144 MHz) and the TX VCOs worked easily down to 145.190 MHz. The transmit VCO meter reading was 13uA; the receive VCO meter reading was 15uA. If the inductance trim paths were cut on the transmit VCO the meter reading would have been a bit higher. As long as the VCOs lock they're good to go. Your results might differ slightly but after modifying eight of these and having them all work perfectly, we're confident you'll have your repeaters up and running in record time!

Acknowledgements and Credits:

Credits for the original idea and the fix go to Paul WØOD.

Bob WA1MIK provided the schematics for the four VCOs, which came from a Motorola Detailed Service Manual. He also turned the original notes into a repeater-builder article.

Photos were taken and supplied by Scott NØOBA.

Motorola and MSF5000 are trademarks of Motorola, Inc.

Contact Information:

Scott can be contacted at: nØoba1 [ at ] comcast [ dot ] net.
Paul can be contacted at: snowshoe9 [ at ] comcast [ dot ] net.

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This page originally posted on Monday 07-Mar-2011

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.