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.
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
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. 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
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
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
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 its 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!
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.
Thanks John, great job. ---- Jim
Created December 5, 2009
Last updated 03/22/2011 16:09:35 PM
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