Antenna Tuner Questions

[kb2mob]

I have a FT-901DM and a MFJ-949D tuner that a friend gave me. The thing is, when I switch to direct on the tuner, the incoming noise and signal level is pretty loud. I can hear great. When I switch tothe tuner, the nosie level drops, signals seem to drop slightly, likefor example, right now I'm listening to K1KBW on 3885Khz AM and direct he comes in like 20+ with the tuner, he comes in around just shy of 10+

How can I trouble shoot this? I got into the hobby to rag chew, but now I'm reallying wanting to get intot he whole technical aspect of the hobby. Building and troubleshooting etc.

Any tips or links to sites with info to help me would be great!

[K1JJ]

Hi Rich,

Make sure the T/R relays or coax switches are really working, the connections thru the tuner are good, etc. Use a VOM to check and see near zero ohms connections. [1-2 ohms is OK on some relays that get pitted from high power sometimes]

Next, see what the swr is thru the tuner to the antenna. Hook the swr meter right at the tuner input and see. If it is near 1:1, then that tells you a lot. That means the whole system is working FB without blown baluns, shorted cables, etc.

A 10db drop is way too much, as you know. I think the connection check and swr check will identify the culprit. If not, then report back here and we'll try soemthing else, OM.

73,
Tom, K1JJ

[w3jn]

MFJ tooners have a propensity for cold solder joints, as well has having parts that are barely inadequate for the rated power.  Open the bad boy up and give it a good inspection, see if there's any bad solder joints or scorched components.

An antenna tooner would be an outstanding first homebrew project.  You can build something that is much better than anything commercially made for much, much less.

73 John

[Paul, K2ORC]

What's been said already.  I wonder if the balun may have been cooked by a previous owner, or as has been suggested, there may be a cold solder joint or a broken connection.   No matter.  Chalk it up to experience and roll your own. 

There are some articles here.   http://amfone.net/ECSound/

Also, check the following which features links to a ton of antenna related websites.    http://www.ac6v.com/antprojects.htm

Good luck, and welcome.
Paul K2ORC

[kb2mob]

Ok this is what I'm seeing here. Lemme know if I have it wrong... a variable cap in the center with 2 diodes on either side. The diode to the right seems to have a chip or something in the glass/placstic on the cathode end if I'm reading this right, and the variable cap looks like it's been beaten to death. It looks like there is solder on the screw and when I went to check it with my multi meter, some of the top seemed to flake off!

If I'm reading these schematics correctly, that variable cap is 3-10 pF, And those diodes are 1N34A's.

Wish I had a real digicam instead of a web cam to get better details...

The actual traces are on the bottom of the board so I couldn't see them at all to inspect them , but I did check from the base of the componets to where the trace goes and just about everyone I saw was 0 Ohms. Some were 1 or 2.

When I switch to direct on the tuner the SWR shoots to 5:1. With the tuner I get 1:3. But there is that pesky signal loss.

This sucker was free so I couldn't beat the price on it. Plus he paid shipping to get it to me when I lived in South Dakota.

BTW: If anyone would like to see the schematics I have a flatbed scanner and can upload them as a jpg. 

[K1JJ]

Hi Rich -

Those diodes and associated parts are most likely the swr directional circuitry and/or relative RF output metering stuff. Will not affect the power transfer of the tuner. If they were affecting it, they would be toast by now..    But to be safe, do you have a working external SWR bridge to try into the tuner's input for a verification of your readings?

Ques: Why is the antenna 5:1 without the tuner? What kind of antenna and what kind of feedline?

And why can't you get the swr down below 3:1 WITH the tuner in line?

If it were me, I'd eBay the tuner - sell it as is or fix it first.

Then build up the JJ super tuner. Simple and match any load at any power level. 

READ THE FOLLOWING THREAD FOR DETAILS:

http://amfone.net/Amforum/index.php?topic=5931.0

Here's Fred/MopMans's Version. Notice how simple. Just one tuning cap and a big coil.. and a 5 turn link inside you cannot see. Vac variable is nice, but breadslicer is OK too. Everything floats so single ended cap is all that's needed. Notice the color coded taps for quick band changing with alligator clips. You can match anything - any band with the feedline clips and capacitor clip taps. [four taps plus variable capacitor adjustment]



73,
T

[kb2mob]

My antenna is a Cushcraft AV5. I could never seem to get it tuned right. 20 mtrs seems to be the oly one I can get it to work decent on. I'm feeding it with 50 Ohm RG8. I can get it below 3:1. I must made a typo in my previous post. I can get it to 1.3:1 SWR with the tuner inline. And I wish I had a seperate SWR bridge to use. Money is tight and I seem to get freebies from folks that have upgraded and have their old stuff still lying around.

I'll check out the thread you gave me and give it a shot building a tuner.

[K1JJ]

OK, Rich -

You mean 1.3 : 1 SWR, I think.  And is it 1.5 : 1 w/o the tuner?

That's pretty good, if so.  But, still doesn't tell us why you have a 10db RX drop with the tuner in line.... Try another receiver just to eliminate something funny going on in the input matching. 

You see, if you can put power thru the tuner and get 1.3:1 swr, then the receiver should see the same thing, unless there is a cable in the T/R switching to the receiver that is defective and used only for the receiver.

Keep checking things and I'll think about it more.  Hope ya build up the new tuner. It will solve your problems and let you move on to higher power later on.

T

[kb2mob]

No the reflected needle shoots up to 5:1.

Yeah.. I'm good at typos

[Ed/KB1HYS]

I've got a MFJ 949E tuner here. 
The Inductance switch on mine Zorched out. gave me very similar problems to what you are seeing.  It was just a thin FRG wafer switch. Like they say, a lot of components in there are overtaxed (CHEAP)   

I shoehorned a small sized ceramic switch in there, works as good as it ever did.  I also reflowed all the joints, just because it was opened up.

Mighty Fine Junk... but easy to fix. 

[Paul, K2ORC]

My antenna is a Cushcraft AV5. I could never seem to get it tuned right. 20 mtrs seems to be the oly one I can get it to work decent on. I'm feeding it with 50 Ohm RG8. I can get it below 3:1. I must made a typo in my previous post. I can get it to 1.3:1 SWR with the tuner inline. And I wish I had a seperate SWR bridge to use. Money is tight and I seem to get freebies from folks that have upgraded and have their old stuff still lying around.

I'll check out the thread you gave me and give it a shot building a tuner.

Rich, where you live do you have the room and a couple of supports that would allow you to build a dipole? 

[kb2mob]

Yeah, I can put up a dipole. Infact I built one and put it up, but the driven came out of the SO-239. I thought I had it in there good. The only thing now it, the snow has fallen and I don't wanna climb up on the roof to take it down to fix it then put it back up. My AV5 is ground mounted, so I simply switched to that one when I discovered my dipole was kaput for the time being.

[K1JJ]

Yeah, I can put up a dipole. Infact I built one and put it up, but the driven came out of the SO-239. I thought I had it in there good. The only thing now it, the snow has fallen and I don't wanna climb up on the roof to take it down to fix it then put it back up. My AV5 is ground mounted, so I simply switched to that one when I discovered my dipole was kaput for the time being.

Paul, you axed the right question...

Yes, Rich, the dipole is the antenna of choice, esp for 160-40M where all the local AM action is. It will be as much as 20-30db!  louder than the vertical for the locals.

As for the coax falling out of the connector...  Make sure to use the silver plated Teflon PL-259's. Solder coat the braid of the coax before insertion and drip some solder into the little holes. I use a propane torch for the job. Fast heat is the way... slow long heat will melt the coax insulation. Tape the junction between the connector and the coax real well for mechanical strength after soldering.

SUPPORT the coax at the center insulator with a rope pipe hitch knot. Know how that's tied? Wrap about ten turns of the rope around the coax and then tie a square knot. It acts like a Chinese finger grip and distributes the stress over a few inches on the coax hanging.  Tie the rope to the center insulator. Have the coax looking down at the ground in a loop to keep water from getting in and wicking down the coax run, along with a good taping job at the on all connections.  etc.

73,
T

[wa2zdy]

And exceptions not withstanding, most ham radio gear made in Mississippi is garbage. 

Isn't MFJ in Mississippi?