K5LAD's  Assorted Ham Stuff

 

Another article on the Dentron MT-3000A Antenna Tuner and Some Good Modifications by W1AN

 

After I published my article on my Dentron MT-3000A including an easier to read schematic, I received the following information from John - W1AN.  John has made some neat modifications to his and several other MT-3000A boxes and we thought it would be more convenient for those searching for information on this tuner to have his article published in the same place.  I had the additional webspace so I offered it to him.

The following article is fully credited to John - W1AN.   John used my redrawn schematic to add his modifications so you can download his schematic by clicking here.

MT3000A-schematic-W1AN-mods.gif (145272 bytes) Click on thumbnail for larger version

 

You can download the article in .pdf format by clicking here.  NOTE:  The schematic is a separate file and not included in the .pdf download.

  

 

 

DENTRON MT-3000A TUNER MODS BY W1AN

The Dentron MT-3000A is an excellent tuner that has been around for many years.

Here are some improvements I have made.

 

W1AN MT3Kfront.jpg (85154 bytes)

Add an additional straight through connector

The antenna selector switch will need to be modified by removing the rotation stop just clockwise of the COAX 1 DIRECT position. First make sure there is an actual spare contact in the next clockwise position. If you have an empty space you will not be able to perform this modification. If your tuner has the contact and no stop, consider yourself lucky and you can skip the most difficult part of the modification. Remove the tuner top and bottom covers. To remove the stop you will need to remove the shaft

coupler and the mounting screws for the plate securing the switch and the 3/8" nut holding the switch. Once loose, remove the two 6-32 nuts and screws that hold the ceramic wafer to the detent part of the switch. You will then notice there is a very small screw and nut which acts as a stop just past the COAX 1 DIRECT position. Carefully remove. You can leave it out or relocate it one position clockwise if you like. Note there is also a stop on the counter clockwise position just past the Balanced Line position. Leave it in.

W1AN rear apron.jpg (107132 bytes)

Please note that variations may exist. Reinstall the switch knob and hardware. You should now be able to turn the antenna switch at least one position clockwise from the COAX 1 DIRECT position. This will be the new COAX 4 DIRECT position. You can label if you like.

Drill a mounting hole for the COAX 4 DIRECT connector next to and left of the existing COAX 1 DIRECT connector on the rear of the chassis. Possibly but unlikely you can find an identical SO239 type connector. Punch or file the hole carefully for whatever connector you plan on using. Some connectors may have a flat or key on one side. Be careful to not damage the internal wiring and dummy load resistors when you do this! Mount your connector in the new hole and then using a piece of #14 solid copper wire run the center pin to the ceramic wafer switch position corresponding to the new COAX 4 DIRECT position. From the rear, this point is the third terminal clockwise from the top switch screw. Dress the wire away from the others and solder.

 

Replace the meter switch and get a bonus low power reflected power position

The existing meter switch buttons often becomes loose, pop out and generally annoying so I've simply replaced it with two rocker switches. The best fit switch I found was C&K part number DM61-J1-2-S2-05-Q available from Mouser, Digikey and Newark Electronics for about $4.00 each. To remove the old switch cut the wires close to the existing switch terminals. Remove the tuner knobs and loosen the front panel to access the mounting screws. Remove the switch assembly and reinstall the panel and knobs. With a square file carefully file out the holes to match the mounting for the replacement switches. Try to keep the holes centered with the original panel markings. Check the fit often. You do not want oversized holes. Once the holes are in and deburred, mount the switches. Some hot melt adhesive or a touch of 5 minute epoxy at the edges inside should hold them if they do not want to snap in place because of the panel thickness.

W1AN rear meters & switch.jpg (119643 bytes)

Next wire the switches. The switches I selected are DPDT.   Electrically only SPDT are required. A small loop of bare wire soldered between each terminal twin provides better reliability for the low signal levels and a more secure and easy place to solder the wires and hang the trim pot. 

There is a three conductor cable from the power meter circuit board that comes up through a hole just below the meters.

The color code varies tuner to tuner.

Disconnect the wire that goes to the bottom meter and relocate to the common (center terminal) of the 200W switch. Install a new 6 inch small gage wire from the meter point where you just removed the wire and connect to the top terminal of the 200W switch. Install and solder a new 100K ohm pot from top terminal of the 200W switch to the bottom terminal. The new 100K trim pot becomes the high power reflected power meter adjustment.

The remaining two wires from the three conductor cable can now be connected.  Connect and solder the one originating near the top potentiometer on the power meter board to the top terminal on the 2000W switch. Connect and solder the one originating near the middle potentiometer on the power meter board to the bottom terminal on the same switch. Connect and solder the loose wire originating from the top meter to the center common terminal on the same switch.

Check your work. You should now have a high/low forward power switch labeled 2000w and a high/low reflected power switch labeled 200W. Since the meter and mating switch pairs are marked the same on the panel you should have no trouble remembering which is which. The existing reflected power adjustment potentiometer on the power board is now the low power reflected power adjustment. Apply power and a proper "mismatched" load and adjust this first. Watch out for RF burns! Now adjust the new 100K trim pot for your high power reflected power. I won't go into detail here. Just remember that these readings are somewhat relative and it’s up to you how accurate you want to make them.

I've successfully modified three MT3000A tuners as indicated above. They have been very reliable under heavy use. With the new rocker switches they even look better! Enjoy your new enhanced Dentron MT-3000A!

John W1AN

11/22/2009

Other Modification Ideas

Install a fuse holder on the back panel for easy dummy load fuse replacement. Install a coax connector for external high power dummy load and a selector switch on the rear panel "Dummy Load Internal/External".

Switch to add additional loading capacitance for bottom end of160M.

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Thanks John, great job.       ---- Jim - K5LAD

Created December 5, 2009   

Last updated 03/22/2011 16:09:35 PM

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