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VCO Cover to Allow Tuning Access
By Robert W. Meister WA1MIK
and John Haserick W1GPO
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Modifying the VX-6000 or PM-1200 VCO Cover to Allow Tuning Access By Robert W. Meister WA1MIK and John Haserick W1GPO |
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The Vertex VX-6000 and Motorola PM-1200 are nearly identical low-band radios. Neither will work throughout the 50-54 MHz range without adjusting the two VCO coils. Unfortunately these are under a well-sealed shield on the main board. You have a few choices to access these coils for adjustment:
This short article documents this last choice.
This procedure requires the following warning: DO NOT use a twist drill to drill each hole. As the hole reaches its final diameter, the metal will grab the drill bit and pull it into the VCO, where it will hit the VCO tuning core and destroy it. Whatever you DO use, don't go further than 6 millimeters (1/4 inch) below the VCO cover. You have been warned!
Other model radios (VX-4000, VX-5500, etc) and other bands (VHF-high or UHF) have a different VCO layout and the hole locations are different for these radios. The diagram and dimensions indicated below are ONLY for the Vertex VX-6000 and Motorola PM-1200 low-band high-power mobile radios. Remember, if you can remove the shield from the board to drill the holes, that's the safest and most preferred method.
John used a 2mm round diamond burr bit in a Dremel rotary tool to carefully create the two holes. The round bit (at the bottom of the photo) is just about 1.5mm in diameter. He started with it until it got through the metal then changed to the tapered cylindrical bit with a pointed tip (middle bit). The flat tipped straight conical bit (top bit) was not needed but is shown as an alternative. Click on any photo for a larger view.
He marked their location by taking measurements from another radio that had its VCO shield removed. He held the radio almost vertical so the chips wouldn't get stuck inside the VCO circuitry. Once he made it through the cover, he enlarged it slightly and centered it to allow full and easy access to the VCO tuning core beneath. He then used a vacuum cleaner and compressed air to remove any metal pieces from under the VCO shield. The final hole diameter is just under 3mm.
Looking at the VCO from the front of the radio, the right side hole was centered 4mm from the right side of the shield, and 7.5mm from the front side of the shield. The left hole was centered 4mm from the left side and 11mm from the front side. See the photo below.
The VCO ferrite cores require a rectangular tipped tuning tool, although a small 1mm straight-blade screwdriver will work if you don't force it. Ferrite cores crack very easily. If you DO use a metallic tool, it will detune the VCO while it's inside the shield, so you'll have to monitor the VCO Test Point, adjust the core, withdraw the screwdriver, and see if the tuning voltage is where you want it. If you use a plastic or wood tool, you won't have to remove it each time and the VCO will tune up much faster. You can use some metallic aluminum tape or adhesive copper strips to cover the holes after you've tuned the VCOs.
Contact Information:
The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.
Article photos and some text provided by John Haserick W1GPO.
Article text, layout, and HTML coding by Robert W. Meister WA1MIK.
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This web page created 02-Dec-2019
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.