I was always interested in the J.W. Miller AT-2500 auto tuner. A big hulking monster box of parts which can handle 2.5 KW of power and do all sorts of wonderful things like lock out your amp, set alarm thresholds etc.
I was lucky enough to find one for sale with an outboard coupler. But that's where the problem lies with these. There are tons of these things out there without couplers. Lost in a move, misplaced, silent key estate items etc. Recently, I saved a fellow ham at the Sussex Hamfest from buying one without the coupler, sifting through the boxes the coupler turned up and he walked away with a complete setup.
These tuners should have been made with the coupler internal to the unit so it could never get lost!
Now me, I'm as dopey as they come, but I did tear into my coupler to look at what was inside, and I also took a load of pics for some dude without one. I don't know how he made out, but after figuring out the magic, I now have a solution!
Whilst clicking around on eBay, I found a fellow ham, Victor RK3R in Russia sells these neat 3KW SWR modules which are normally used for his amplifier projects. Look him up on YouTube as well and subscribe and like his vids as they are quite informative.
Anyway, I took a shot and ordered one of these boards. His eBay handle is V_RK3R and his store is easy to find and he'll answer your questions in a timely fashion. The order was placed and it arrived rather quickly, in under two weeks I'd say.
I was certainly impressed with the quality of the board and its heavy duty components. I then mounted the board on standoffs, then inside a broken MFJ dummy load, I added two standard HF end connectors, a coiled and shielded mic cord, a 3 pin din connector and voila! A heavy duty 3kw SWR bridge for the AT-2500!
It's rather simple to do, there are 3 wires from the AT- 2500 din jack, center which is din #2 is ground, number one on the din is forward, I used yellow for that. Number 3 on the din is reflected, I used black for that, and red for the center connection on the Board which is ground as mentioned, that's the center pin #2 on the din as well. Just remember to note upon completion, if the reflected (Right) Meter doesn't work (stays around 8:1 or so With RF present), then change the coax connectors around and mark them "transmitter" and "tuner." Don't ask how I know. ?~?
Anyway, I compared this home brew coupler to my original AT-2500 Coupler and it's very close, you can make minor adjustments inside the tuner to tweak things on, or even modify the coupler itself for internal adjustments. Victor has it set for his projects that he offers, but it works fine business for us as well!
I bought the following items to complete this project. Two chassis mount PL-259's, one mic cord (use the shield and two wires, one for forward and one for reflected power.) I salvaged a 3-pin muffin fan connector (which is why the colors are black red and yellow BTW) I ordered 3 pin din connectors, then order a project box that will accommodate the SWR bridge and some two chassis mount standoff connectors and remember to leave room around the board as you'll be running some power thru the board and the edges are hot with RF. screws, small wire ties.
This whole thing took me a few hours to plan out and make. The board is silk screened by Victor for your connections. I made mine fast to test the idea, it works and it was under 50 bucks to build. I'm now going to build it right as I have another JW Miller without a coupler that needs some other minor TLC as well.
In conclusion, I hope this works for you, you can probably use this for an SWR meter missing an outboard coupler as well.
Anyway, please feel free to post your ideas below to make it better or more accurate. I found in a few hours of testing, it worked nearly as well as the original coupler. Even with a totally non resonant antenna system it worked equally as well, it did take a bit more careful selection of the band selector with both, but the tests between the two couplers equaled out.
Hopefully this helps you with your AT-2500 or other project. I hope to hear your suggestions or ideas. This is a great way to get that tuner back in auto mode!