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
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.
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
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.
Created December 5, 2009
Page visited 102 times
Last updated 03/22/2011 13:38:24 PM
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