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.

You can download the article by clicking here.

   Click here to download this version


Click here to download this version




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

Here are some improvements I have made.

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. Some tuners do not have a stop in

this position. If this is the case with your tuner, 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. 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.

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


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.





Created December 5, 2009   

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Last updated 03/22/2011 13:38:24 PM

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