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The Dentron MT-3000A Antenna Tuner and How to Find Some Good and Readable Documentation

 Key search words:  Dentron, MT-3K, MT-3000A, built like a battleship, W-2 Wattmeter, MT-3000A Operation Manual

At the peak of the years of the Dentron Radio Company in  Ohio, (1970s) this company built some really heavy duty manual antenna tuners.   Built like the proverbial battle ship, hundreds, if not thousands of these tuners are still in use today.  Someone searching for one on eBay will be surprised that they have maintained their value even today.  If you’ve got one of these treasures I suspect that you'll be more apt to sell one of your relatives (not a spouse or a child but, perhaps a your bum of a brother) rather than getting rid of your Dentron MT-3000A.  This tuner is well built, will easily handle the maximum legal power limit and operates over a wide range of impedances, even up in the 160 meter band.

During the 1970's I owned and operated a ham radio store called "Derrick Electronics" in Broken Arrow, Oklahoma.  I've written about this in several other places on my website so I'll not repeat that information but I was a dealer for the Dentron Company.  In addition to that, I was an active proponent of their equipment and owned several of their pieces, including tuners and amplifiers.   I still own most of these pieces and use them often.  I actually own two of the MT-3000A tuners, one that I bought back when I had the store and another I bought off eBay.  The seller of this second one has originally sold it to someone else and when he shipped it to the buyer, he shipped it via UPS.  UPS, in their own special shipping style known to many of my readers, dropped the well-packed parcel and severely broke several pieces inside.  Both of the variable capacitors were sheared from the Plexiglas bar insulators which Dentron used to insulate the capacitors above ground.  The variables, now freed, shifted inside and broke components in the wattmeter circuit.  It also broke the heavy-duty ceramic 18 position Inductance Selector switch.  The meters and several other pieces, however, were still good and reusable.  The seller, having collected on the destruction insurance from UPS, advertised the unit as a basket case of parts.   Knowing full well that I might be buying a pig-in-a-poke, but hoping that I could salvage a few parts as replacements for my original tuner, just in case, I bit on and won the "tuner kit."

When it arrived (via UPS without insurance) and I was encouraged with what I saw.  After studying the sad heap of pieces, I took a positive inventory of what I had.  The meters were good, the knobs were OK, the other Antenna Selector rotary switch and the push button switches were OK and part of wattmeter circuit was still intact, although it had taken quite a licking and was no longer ticking.  I tried to repair the broken 18 position switch but that didn't work at all -- J.B. Weld just couldn't do the job on the three pieces of the switch.  I was surprised, however, to find an exact duplicate of this switch wafer in my junkbox.  It didn't have the shaft assembly but the one in the new "junk pile tuner" had just what I needed there.  Merging the old with the newly-found, I had an exact replacement of the 18 position Inductance Selector switch.

I was able to find Plexiglas stock and duplicate the variable capacitor insulator mounting strips, along with some nylon screws and hardware I already had.   The variables has received enough shock to bend a few plates but remarkably few were bent and no plates had been knocked out of their mounts.  I was able to get them back in alignment without much difficulty.

The wattmeter took me a while to rebuild, mostly because of the difficulty in finding the correct replacement parts.  In time, however, I did find them and was able to get the wattmeter working correctly again.  I was well on my way to refurbishing my heap of parts and converting them back into what it had begun its life to be, a Dentron MT-3000A antenna tuner.  This tuner now spends most of its time hooked to my 160 meter antenna, although I have on occasion, used it in other configurations.  It works exactly like my original MT-3000A and shares a spot atop its twin on my operating desk.

Documentation

One of the major complaints of owners is the manual which came with it from Dentron.  The document is just a few pages and the printing, particularly the schematic, is quite light.  Although there are several copies of the original scanned manual out on the Internet, it looks like all are copies I have found have been light copies, copies of copies, and copies several generations away from the original.  Most of these scanned sheets are difficult to read and the schematic is particularly obscured.  For that reason, and also because there wasn't anything particularly good on TV one evening, I set about to redraw the original schematic and also to scan the other scanned pages and enhance them with some graphics software I had.   I've placed my enhanced MT-3000A scanned pages of the operating manual in my "download" section and you can download the zipped file by clicking on download MT-3000A manual.

I must admit at the outset, that I am not an artist, never was, certainly never will be.  When I try to draw a picture, the best I can do for drawing people is to use stick figures.  When I taught Oklahoma History many years ago, I tried to explain to my 9th graders how the great cattle drives, from Texas to Kansas, began after the Civil War.  I could only draw stick longhorn cattle and my students kept asking me why anyone would want to move or buy such a skinny cow.  Sometimes now, after 40+ years when I meet an ex student, they remind me of my artwork with those cows.  Ah, but back to the antenna tuner and the need for good documentation.

This task of improving the original schematic, however, did not require any original artistic creation ability, only a redrawing over what someone else had done quite a few years ago.  I use the old Paint program that has been included with copies of all the Microsoft Windows software for many years.  Perhaps it was part of the original operating system, I didn't try to go back and see when it first appeared on the scene.   I do know that it's been around for many years and many versions.  It's simple, cheap, fairly intuitive, and produces a pretty good copy.

This first copy, reproduced below, is the re-do of the schematic page I found in my original manual.  I did notice that the C5 disc capacitor was left off the schematic.  I caught this when looking at the schematic of Dentron's W-2 wattmeter which was basically the same circuit used in the MT-3000A.

MT-3000A Schematic.JPG (226435 bytes)

           <--- Click on thumbnail for expanded view

 

Click here to download this version

The second file here is the same thing but additionally, I have identified the parts values given by Dentron on the actual schematic.  Hopefully this will provide additional help to the user.   Let me offer one other caution, the original MT-3000A showed the values of R1 as 10 ohms and the R2 value at 43 ohms, both 1/2 watt resistors.  Typically, in most circuit diagrams used, these two resistors are of the same values and are usually around the value of 50 ohms each.  The Dentron W-2 wattmeter used duplicate values for R1 and R2 and they were 43 ohms.  I looked at my second (rebuilt) MT-3000A, which happened to be conveniently open on the bench and both resistors had the value but it was 47 ohms.  This could lead to some real confusion to someone checking their unit for the first time, but, suffice it to say, you might find some deviation in these two resistors in your own tuner's wattmeter.  One would assume that the exact value may not be critical, as long as they are equal values, however, going back to the parts list in my original MT-3000A manual, I find those two different values - 10 and 43 ohms.  Go figure.....

MT-3000A Schematic with values.JPG (273964 bytes)          <--- Click on thumbnail for expanded view

Click here to download this version

 If you are fortunate enough to own one of these fine old Dentron tuners (I hate to call it an antique but a 40+ year old piece of ham gear does have some mileage on it), consider yourself to be quite fortunate.  Many newcomers shun the use of an outboard antenna tuner because they say, "my new transceiver has a built-in tuner."  Those built-in tuners, at best, can only help reduce an SWR of less than 3:1 whereas tuners like the MT-3000A can deal with much higher SWR conditions and bring them in line with what the transceiver expects to see.

I hope this article and the attachments included are helpful to you, whether you be newcomer or old timer.

There's some additional information on adding some modifications to the MT-3000A tuner from W1AN.  Click here to see that.

Created October 5, 2009   

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