Trouble With MFJ Tuner

I bought an MFJ 941E tuner and the transmitted and reflected power gauges don't seem to be working, at least in my setup. I'm feeding 450 Ohm Window line from the balanced output. I sent it home with a coworker and he plugged it into a dummy load. It seemed to work just fine there. I've put up to 100 watts through this thing (on the appropriate setting, 30 watts on the low power setting) and the needles don't budge. But, it worked fine for my coworker. I'm thinking it must be the impedance of the ladder line. The internal Balun is a 4:1 and I suppose I should really have a 9:1. I was expecting the gauges to still move a bit... but, it's not a good match.

All this said, I was able to tune for signal strength on receive and make contact with a ham in Ontario (from Ohio). My first HF QSO! I think I must have been marginal.

So, if I get or make a 9:1 balun, will this likely solve my troubles? Or do you think there's something else wrong here?

You must have something else wrong. My meter works just fine with ladder line. Pay attention to the jumper between the wire and balanced terminals as well as the selection of wire/direct and wire/tuner.
BB

First, congratulation on your first HF contact! I hope there will be many more in the future.

Whether you have a 4:1 or 9:1 balun should make no difference. If you are putting power through the unit you should see something on the meter, good or bad.

Initially there could be two possible problems. One is that it is hooked up wrong. Double check all of your connections. But even with some odd connections you should see something on the meter.

The second one I mention is because I have a MFJ-941D. It is essentially the same tuner except that the "D" has a single meter, with a FWD/REV button, instead of the dual meter. I mostly use it with my IC-735, but it seems to works with all my rigs, 5 to 100 watts.

I bought the MFJ-941D at a ham fest for $15, because the meter didn't work. As it turned out, the original owner over powered it. This blew the diodes in the SWR bridge, the null capacitor in the SWR bridge, and melted the case that is around the 4:1 balun. Oddly enough, it didn't hurt the balun. I replaced the diodes and null capacitor and it all worked again. However, the SWR bridge seems to be overly sensitive, because I have had to replace the diodes two more times. Even though I have never over powered it.

Its a pretty simple check, once you get the case open. You can check the diodes with a analog multi-meter. You might also check to make sure none of the capacitor plates are touching. One little touch or arc could blow the SWR bridge diodes.

Note that the diodes are germanium diodes (1N34), not common silicon diodes (1N4148 or 1N914). Germanium diodes have a lower voltage drop and are commonly used in SWR bridges. However, they are not as easy to find lately. Radio Shack does not carry them. I got my replacements at Frys Electronics, but I don't recall the equivalent numbers.

Have you checked the cable between your radio and the tuner? Do you have the jumper wire on the back where the dotted line is to connect the internal balun?
prcguy

The meter will not show anything unless you switch to cw or am transmit. SSB transmissions will register only when talking. I have made this mistake once or twice...its easy to forget.

sloop said:

The meter will not show anything unless you switch to cw or am transmit. SSB transmissions will register only when talking. I have made this mistake once or twice...its easy to forget.

Click to expand...

Good call. Sometimes I forget to suggest the simplest things.

The SWR steering diodes in the MFJ are notoriously sensitive to static charges on the antenna line, always put the antenna switch on a dummy load if not using the unit.
I keep a plastic baggie full of the diodes inside the tuner so when things go whacky I can just pop the top and solder in a new pair.

Bruce

Thanks for the replies everyone!

sloop, you called it. User error this time around. Make's perfect sense in hindsight. USB doesn't let the carrier through so, if I'm not talking, nothing is there to measure. AM works fine. I don't have a key so I had no luck with CW. If I key the mic in CW my radio will go into transmit mode but nothing registers on the gauge. Must not actually let the carrier through until you key down on a paddle.

It does sound like the diodes on this unit are flaky so I'll keep that in mind.

Would I still benefit from a 9:1 balun? Or can the tuner compensate for that? I haven't actually played around enough to see how low I can get the reflected power yet.

If you had another ham check it out on a dummy load and it worked fine then I would say the diodes are fine. The diodes don't know what antenna you are connected to. They only read the power going to or coming from the antenna feedline either coax or balanced.
BB

In most cases a 1:1 balun is best and even though you might be using 450 ohm ladder line or 300 ohm TV twinlead, the impedance across the bands seen at the tuner can go from a few ohms to thousands of ohms depending on the antenna type. If you had an antenna that had a 450 ohm feedpoint impedance and used 450 ohm line all the way to the shack, then a 9:1 balun might make sense and you probably would not need a tuner in that case.

Your tuner should be able to tune a wide range of impedance and I think the main benefit from a balun is to isolate or float the unbalanced radio and tuner from the balanced antenna system. Without a balun one side of the balanced feedline would be simply grounded at the tuner, which would unbalance the antenna system causing RF currents to flow on the radio chassis, microphone and anything else attached to the tuner.
prcguy

userdnl said:

Thanks for the replies everyone!

sloop, you called it. User error this time around. Make's perfect sense in hindsight. USB doesn't let the carrier through so, if I'm not talking, nothing is there to measure. AM works fine. I don't have a key so I had no luck with CW. If I key the mic in CW my radio will go into transmit mode but nothing registers on the gauge. Must not actually let the carrier through until you key down on a paddle.

It does sound like the diodes on this unit are flaky so I'll keep that in mind.

Would I still benefit from a 9:1 balun? Or can the tuner compensate for that? I haven't actually played around enough to see how low I can get the reflected power yet.

Click to expand...

Thanks guys!

WA0CBW, I think the diodes are fine for now. I just meant I'd remember that for future reference. And I'll take the advice not to leave my antenna selected when not in use.

prcguy, that makes sense. I suppose I would have to use Kuroda's identities to find the true impedance at the tuner. But, at the end of the day, if the tuner can get a good match, I've got no reason to change anything.