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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, perhapas 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 way known to many of my readers, dropped the well-packed parcel and severly broke several pieces inside.  Both of the variable capacitors were sheared from the plexiglass bar insulators which Dentron used to insulate the capacitors above ground.  The variables, now freed, shifted inside and broke pieces in the wattmeter circuit.  It also broke the heavy-duty ceramic 18 position Inductance Selector switch.  The meters and several other pieces 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 won the bid and 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 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.  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.

I was able to find plexiglass 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 it was remarkably few 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 referbishing my heap of parts and converting them back into what it had begun its life to be.  This tuner spends most of its time hooked to my 160 meter antenna, although I have on occassion, used it in other configurations..

Documentation

One of the major complaints of owners is the manual which came with it.  The documents 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, 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 particularlly 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 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.  This task, however, did not require any original artistic creation, 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 wee when it first appeared on the scene.   I do know that it's been around for many years and many versions.

This first copy 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 it when looking at the schematic of Dentron's W-2 wattemter which was basically the same on used in the MT-3000A.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Click here to download this version

The second file here is the same thing but I have identified the values given by Dentron on the actual schematic.  Hopefully this will provide additional help to the user.   Let me make one other caution, the original MT-3000A showed the values of R1 as 10 ohms and the R2 value at 43 ohms, both a 1/2 watt piece.  Typically, these two resistors are of the same values are are around the value of 50 ohms.  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 conviently open on bench and both resistors had the value - 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 devices in your own tuner's wattmeter.

 

Click here to download this version

 

 

 

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 My tuner now does not look much like the original Dentron Super Tuner but the added features have made it much more useful and practical for my use.  It will match the proverbial bedspring and, in the process, show you your SWR and power output.

Created January 13, 2009   

 

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Last updated 10/06/2009 01:45:13 AM

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