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Author Topic: Any danger in opening/cleaning an MFJ tuner?  (Read 4816 times)
KK4RXN
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Posts: 123




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« on: August 05, 2013, 11:31:11 AM »

I would like to open and inspect my MFJ 949 tuner after using it last night and it seeming not to want to tune. I was thinking about gently cleaning with DeOxit, and is there any danger of voltage left in it even disconnected?
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
AA4PB
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Posts: 13032




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« Reply #1 on: August 05, 2013, 11:49:09 AM »

Nothing in there to hold voltage when it's disconnected. Be sure to disconnect the 12VDC meter bulb supply.
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KK4RXN
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Posts: 123




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« Reply #2 on: August 05, 2013, 12:09:24 PM »

That's what I figured. I thought it'd be better to ask a dumb question than to wish I had. Thanks!
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
WB6BYU
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Posts: 13578




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« Reply #3 on: August 05, 2013, 01:39:26 PM »

Certainly no danger in opening it - I don't think the lid has been secured to mine in years.

There isn't a lot that can go wrong inside the tuner, however.  Assuming that you can still
read reflected power, the power detection circuit works.  Beyond that there are two variable
capacitors (which can arc if they get a lot of crud on them, or if a plate gets bent), a coil
(the plastic bars that hold the turns apart can melt and deform if it gets overheated:  losses
in the coil can be high with some load impedances), and the switchs that select coil taps and
antennas (possibly poor or arced contacts).  With both switches, make sure that the knob is
installed on the shaft properly so the pointer lines up with the proper marking on the front panel.

Also check for poor contacts in the coax connectors:  a common problem is when the center pin
gets spread out by a plug with some excess solder on it - it can then make poor contact with
the cables plugged into it (especially those with crimped connectors.)
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KE3WD
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Posts: 5689




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« Reply #4 on: August 05, 2013, 06:27:29 PM »

Sure, clean and lube switches, contacts of roller inductor if it has one, and very tiny wee bit of deoxit at only the brush contact points of variable caps. 


And since it is MFJ made, always a good idea to visually inspect for cold solder joints, touching up any that look suspicious - and especially to look for any joints that should have been soldered but never were! 


73
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KK4RXN
Member

Posts: 123




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« Reply #5 on: August 06, 2013, 05:19:14 AM »

I cleaned it up a little last night and it seems after opening it up that there are no major problems at all. There are brown (oxidation?) spots on just about every solder joint, so maybe I could freshen them up. I've never soldered before, but it's time to try it out.

When I buttoned it back up and tried it out last night, it does have distinct changes when rotating the inductor knob, so that's good. It could be my 135' delta loop that is causing a little of the trouble, but when trying to tune it I can't get it tuned like I'd like to. The SWR is actually better sometimes when bypassing the tuner than trying to tune it.
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
K4JJL
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Posts: 503




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« Reply #6 on: August 06, 2013, 08:17:32 AM »

Brown schmutz is leftover rosin from soldering it together.  Won't hurt anything.
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WB6BYU
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Posts: 13578




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« Reply #7 on: August 06, 2013, 09:41:17 AM »

Quote from: KK4RXN

...It could be my 135' delta loop that is causing a little of the trouble, but when trying to tune it I can't get it tuned like I'd like to. The SWR is actually better sometimes when bypassing the tuner than trying to tune it.



Generally when that happens one of the variable capacitors is at one extreme - either fully meshed
or fully open.  If you can't get it to match using other inductor settings and by varying the other
capacitor as well, then the load impedance is out of the range that your tuner can match.  (Sometimes
you have to go back and forth between the two capacitors several times to get a low SWR.)

How are you feeding your delta loop?  Depending on the feed method and feedline length you
can get a very high SWR on some bands, which the tuner might not be able to match. Sometimes
adding some extra feedline will shift the impedance enough that you can match it, though often
in such cases the coax losses will be high due to the high SWR.
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KK4RXN
Member

Posts: 123




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« Reply #8 on: August 06, 2013, 10:00:28 AM »

It's fed with about 30' of 300 ohm lead line.
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
SWMAN
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Posts: 600




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« Reply #9 on: August 06, 2013, 10:18:52 AM »

 The last time I opened an MFJ product to look inside, a couple of screws fell out and I had no idea where they were suppose to go or where they came from Huh? Scarry
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KK4RXN
Member

Posts: 123




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« Reply #10 on: August 06, 2013, 11:31:22 AM »

Quote from:  link=topic=91412.msg691109#msg691109 date=1375809532
The last time I opened an MFJ product to look inside, a couple of screws fell out and I had no idea where they were suppose to go or where they came from Huh? Scarry

From all the reports I've read about these tuners, I was expecting the same thing! I was extremely surprised to not have any surprises.
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
WB6BYU
Member

Posts: 13578




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« Reply #11 on: August 06, 2013, 11:49:08 AM »

Quote from: KK4RXN
It's fed with about 30' of 300 ohm lead line.



That should be a pretty benign load impedance on 20m, and not too bad on 40m.
Pretty radical on 80m however.  What band are you having problems with?


You have the twinlead connected to the balanced terminals, right?  Do you have the
required jumper wire in place from one of the balanced terminals to the Wire terminal?
And the switch in the Single Wire position?  If so, how can you measure the SWR without
the tuner in the line?

(To answer the last question, instead of the required jumper between one of the Balanced
terminals and the Wire terminal, I used a longer wire with a banana plug on the end and
plugged it into the center conductor of the COAX 1 output jack.  That way I could switch
between the COAX 1 and COAX 1 DIRECT switch positions to feed my loop through the
tuner or direct.  But that isn't the ordinary mode of operation, and certainly isn't described
in the manual.)
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KK4RXN
Member

Posts: 123




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« Reply #12 on: August 06, 2013, 12:07:52 PM »

I know running through the tuner that 80m was working better on the bypass side than through the tuner. 40m may have worked better in bypass as well.

Yes, the twinlead is connected to the 2 balanced line terminals; the jumper wire is in place from one balanced line terminal to the Wire terminal, and I had the switch in the Balanced Line Wire position.

When running it in the bypass position the SWR meter was showing SWR as lower than it was when going through the tuner.
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Barry
KK4RXN
Jeremiah 29:11-13 / John 3:16
WB6BYU
Member

Posts: 13578




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« Reply #13 on: August 06, 2013, 01:49:26 PM »

Quote from: KK4RXN

When running it in the bypass position the SWR meter was showing SWR as lower than it was when going through the tuner.



But there is no bypass position for the single wire / balanced output.  The switch
positions should be:

COAX 2 BYPASS
COAX 1 BYPASS
COAX 2 (tuned)
COAX 1 (tuned)
SINGLE WIRE / BALANCED
DUMMY LOAD

If you're measuring in the bypass position, you are connected to COAX 1 or COAX 2, not to
the balanced line.  (Or perhaps you are connected to the dummy load, in which case the
SWR certainly should be low!)

(This assumes that the schematic in the copy of the manual on the MFJ website is correct.
There certainly is room for doubt in that regard.)



I'd have to model the antenna to see what the impedance would be on 80m, but at a rough
estimate the tuner will need to match 10 - j100 ohms on 80m.  It might not have enough
capacitance to get there.  If it can, it likely will require some careful adjustment.
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N4ATS
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Posts: 851




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« Reply #14 on: August 11, 2013, 12:01:50 PM »

If it is MFJ , scrap it , get a Palstar or any other product
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