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Author Topic: MFJ 941D  (Read 3803 times)
K3MOV
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« on: July 28, 2010, 01:01:16 AM »

I apparently made a mistake when purchasing a used MFJ 941D which worked "perfectly".  Shoulda known better Embarrassed. When I use the tuner to receive, everything seems to function okay.  I can peak the incoming station by using all three controls just as one would expect with this type of tuner.  However, when I try to tune my transmitter into an antenna or dummy load, the SWR simply doesn't vary at all - the meter shows infinity regardless of how I try tune it.  I feel kind of sorry for the tuner as it appears to have lived a pretty rough life before I gave it a home.  But I would like it to do what it is supposed to do (get the SWR down) before it finds another home in my junk pile.  Any suggestions for resuscitating this thing would be greatly appreciated.

Tnx es 73, Tom K3MOV
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VK2GWK
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« Reply #1 on: July 28, 2010, 01:12:29 AM »

If this tuner has been used with too much power - or ill matched antenneas - it is most likely that the two weakest components have burnt out: the balun and the switch. On receive this might still work because of capacitive coupling. But on transmit the power does not get through....
Replace the switch.... preferably with a heavier type.
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W8JI
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« Reply #2 on: July 28, 2010, 05:01:33 AM »

I apparently made a mistake when purchasing a used MFJ 941D which worked "perfectly".  Shoulda known better Embarrassed. When I use the tuner to receive, everything seems to function okay.  I can peak the incoming station by using all three controls just as one would expect with this type of tuner.  However, when I try to tune my transmitter into an antenna or dummy load, the SWR simply doesn't vary at all - the meter shows infinity regardless of how I try tune it.  I feel kind of sorry for the tuner as it appears to have lived a pretty rough life before I gave it a home.  But I would like it to do what it is supposed to do (get the SWR down) before it finds another home in my junk pile.  Any suggestions for resuscitating this thing would be greatly appreciated.

Tnx es 73, Tom K3MOV

Tom,

If it peaks the signal, it MUST be changing SWR. Before I started throwing expensive of difficult to change parts at the tuner, I would find the bad part and replace it. It also might be cockpit trouble with the pilot. :-)

Does it vary SWR on the meter in your radio when you adjust it??? Did you read the tuner manual and follow instructions??


73 Tom



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WA3SKN
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« Reply #3 on: July 28, 2010, 05:59:41 AM »

OK, you have a "sick" MFJ941d.
Luckily, this is a simple unit that should not be hard to repair!
Break out a VOM meter and open up the case.  Using the ohmmeter, start at the transmitter jack and check resistance to the switch.  The capacitors should not be a problem unless the plates have become shorted.  This is a simple "T network" device.
You should have low resistance from the transmitter jack to the switch, continuity through the swith, open or very high resistance accross the capacitors, and low resistance accross the inductor and the balun coil.
I imagine that either the coil or balun has been damaged by high levels of current/voltage.
AND, more importantly, MFJ has parts!
Good luck with the project.
73s.

-Mike.
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W3LK
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« Reply #4 on: July 28, 2010, 08:29:11 AM »

uuuuhhh ...

You are using a constant-carrier mode to adjust the tuner, aren't you?
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K3MOV
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« Reply #5 on: July 28, 2010, 09:05:51 AM »

Thanks for the replies.

Before posting I did read the manual thouroughly.  I had one of these years ago and it worked fine so I am familiar with the way it is supposed to function. I also got inside the unit and looked for anything obvious (broken contacts, cold solder joints, etc.) and cleaned the contacts.  I tested it with both my MFJ 259B and with my IC 7000.  After peaking the received signal on several bands, I actually paid more attention to the SWR meter on the 7000 than I did on the tuner as I know my auto tuner brings the SWR on the rig down immediately, as it should.  I used the RTTY constant carrier mode on the 7000 otherwise I would not have had an infinity reading on the meter.

Well, time to start digging.  Thanks again es 73, Tom K3MOV

Tom, by the way I also took a breathalyzer test before posting just to make sure it wasn't pilot error ;-)
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W8JI
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« Reply #6 on: July 29, 2010, 03:31:40 AM »

Before posting I did read the manual thouroughly.  I had one of these years ago and it worked fine so I am familiar with the way it is supposed to function. I also got inside the unit and looked for anything obvious (broken contacts, cold solder joints, etc.) and cleaned the contacts.  I tested it with both my MFJ 259B and with my IC 7000.  After peaking the received signal on several bands, I actually paid more attention to the SWR meter on the 7000 than I did on the tuner as I know my auto tuner brings the SWR on the rig down immediately, as it should.  I used the RTTY constant carrier mode on the 7000 otherwise I would not have had an infinity reading on the meter.

Well, time to start digging.  Thanks again es 73, Tom K3MOV

Tom, by the way I also took a breathalyzer test before posting just to make sure it wasn't pilot error ;-)

Hi Tom,

Lacking details of what you read on the meter, it is difficult to guess the bad component. If you have high equal readings of forward and reflected power, it can be an open or short in the directional coupler. It is a bridge circuit so several components or connections could make the bridge stay unbalanced.

A bad diode (wich is the second most common failure other than operator error) would normally just make one side, forward or reflected, read low.

You will have to inspect and dig. Anyway, now that you looked at the meter on the radio you know the problem is not the bandswitch or anything you are doing. :-)

73 tom
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K3MOV
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« Reply #7 on: July 29, 2010, 02:51:42 PM »

I haven't had time to get into this thing yet but I want something to use for the weekend.  I paid $40.00 for this used brick.  I ordered a new cute little MFJ 904 from R&L for $114.00.  Incredibly, I was able to connect and operate this complicated piece of electronic equipment without breaking out the manual and it works great.  This is only about the tenth MFJ tuner I have had since 1960 so I feel pretty confident in setting them up without the manual or fear of "pilot error".

I really do appreciate your comments and I must say the fact that it functioned on receive but not transmit did have me scratching my head a bit.

Tnx agn es 73, Tom K3MOV
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WB6BYU
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« Reply #8 on: July 29, 2010, 03:33:45 PM »

A couple other thoughts.

First, an open ground connection in the coax could let it peak a signal on receive but not drop the SWR on
transmit.  So could a bad switch antenna switch connection, if there is enough capacitance or leakage
resistance through the terminals to let some signal leak through.  Same with the output connector.

Could also be a semi-short circuit somewhere that breaks down on transmit.

If it is like my MFJ and has preset resistors to calibrate the voltage ranges, they could be way out of
adjustment (so that the reflected power sensitivity is too high), or if the ground lead is disconnected
on the reflected power pot you could get a similar result.  And if the directional coupler is not properly
nulled the SWR won't drop very low even when properly tuned.

So I'd start by putting it in line with another SWR meter and a dummy load.  Does the other meter also
show a high SWR?  If not, that points to a problem in the SWR meter circuitry.  Is the SWR low on the
external meter in bypass mode to the dummy load?  If so, check the switches and grounds - there isn't
much else in line between input and output to go wrong.

If you have narrowed it down to an actual fault of the tuner circuitry (the switch, coil, two capacitors,
balun, and connections) then check the coil for damage and shorts (or crud) between the capacitor
plates.
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K3MOV
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« Reply #9 on: July 30, 2010, 11:03:59 AM »

Dale, now that I have another MFJ tuner that I know functions properly, yours sounds like a reasonable first approach to track down the problem.

Agn, tnx to you and all others for their input.

Tom K3MOV
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