MFJ-986 SWR/POWER meter problems help for Desperation Engineering, Inc. ;-) Hi Hi

[K6JBG]

I modified an MFJ-986 CLC T-network antenna tuner, into a serial LC circuit to "Tune" my second story RF ground. I decided to use the SWR/POWER cross needle meter as an adjunct to the one on my Kenwood TS-570DG. The MFJ meter has a finer resolution and it acts as a check. I use a modified E. F. Johnson 250-23 "Matchbox" link coupled "Balanced" antenna tuner for my 135 foot, multiband, dipole antenna, fed with 450 ohm ladder line. Two days ago the "Reflected/SWR" needle on the MFJ meter started to NOT work the way it has been for the last year and a half. The MFJ meter "Reflected/SWR" needle readings started to indicate the "Reverse" of the built in Kenwood TS-570DG SWR meter. I took the rear meter PC board out of the MFJ-986. I left the umbilical cord running up to the front panel "Display" portion in tact. Using a schematic, kindly provided by MFJ on the internet, I inspected the board with a "Parallelogram" arm, wall mounted, florescent lamp; an intensely bright "LED" flashlight, and a large "Sherlock Holmes" style magnifying lens. All of the solder joints looked real good and solid. I lifted the cathode ends of the two 1N34 Germanium diodes off of the board using a Radio Shack suction de-soldering tool [I am 68 and this de-soldering tool is the best I have found so far]. Their forward and backward readings, with my Tektronix TX3 DMM, were in speck. I carefully re-mounted the diodes and put the rear MFJ meter board back in place [not fun...6 screws with washers...4 with nuts I used forceps to hold (tight space)...one washer and nut to hold the lamp wallwart socket...]. I put the unit back into service and got the same results ;-( I tried tuning up on 80 meters and, while messing with the pot, to limit the current going to the "Ground Tuning" current meter, I noticed the MFJ SWR needle jumping around. I went to the garage and built an aluminum shield plate. I mounted it between the MFJ meter and my added ammeter and circuit. That stopped the interaction, but the MFJ SWR needle still worked in opposition [backwards] to the built in Kenwood meter...I am stumped...any suggestions are gratefully accepted...K6JBG

[KE3WD]

...and, while messing with the pot, to limit the current going to the "Ground Tuning" current meter, I noticed the MFJ SWR needle jumping around. ...

A shield? Why?

It worked all that time without a shield...

First thing to try here, a wee squirt of DEOXIT or other control cleaner into that pot, which has probably become dried out and having contact problems at wiper or brush.

If that doesn't clear it up, suspect a bad pot, use the ohmmeter to ascertain that the pots outside two pins are still at the correct resistance for the pot, then check from each outside to center pin while turning full travel and watching the ohmmeter for smooth resistance change, checking here for a broken carbon or the likes.


73

[K6JBG]

The shield was grabbing at straws from frustration...

I put some DEOXIT D100L on the pot wiper, just like you suggested...and nothing changed... The pot is behaving fine...it it just that darn SWR or Reflected Power meter needle that is reading the opposite of the radio's SWR meter...How it could suddenly "Flip Out", or "Wig out", and "Go 'round the bend", so to speak, is beyond me at this point.

Perhaps it is an idiosyncrasy of the MFJ twin needle meter design?

73

[KE3WD]

Look for a bad connection, cold solder joint or the likes at every ground point in the circuit next, if one of those has high resistance or disconnect, meter *might* behave backwards...

Make sure the unit itself isn't somehow hooked up backwards...

Do you have the MFJ schematic for the thing?

I don't think that the twin needle meter deswign would have anything to do with the problem, after all, it is really only two separate D'arsonval meter movements, they are just housed in the same case (and one is designed to move from right to left).

My thoughts are that it must be something like the bad ground point connection or perhaps could still be those diodes, might not be a bad idea to replace both to see what happens if other strategies don't pan out.

Good Luck with it, keep at it, you'll find it.


73

[K6JBG]

As I stated in my first post, I pulled the rear meter board completely loose from the case [only gluttons for punishment should do this!], left the umbilical chord in tact, and went over that board with a 4X(?) magnifying lens and even hit a couple of

the solder joints that looked "possibly" marginal with my well tinned Weller SP23 iron.

Yes, I have the schematic, and yes there is nothing magic about the D'arsonval movements, or their opposed arrangements.

Point well taken about the ground connection(s), will have a look there next.

I learned years ago about tube testers and/or VTVM readings not representing the "Actual in-circuit loads" on a component. If all else fails, will replace both of the 1N34 Gallium diodes [it is not something I am itching to do!]

73

[KE3WD]

Quit complainig, it won't repair the thing. Often is the case where we have to do whatever it takes, revisit where we were before, etc.

The bright side is that the second time around, pulling the board should be easier to do.


So do it.


73

[K6JBG]

Okay. I just pulled the board and replaced the two 1N34A Gallium Diodes. Nothing changed. So where do we go from here? Perhaps it needs a readjustment of the trim pot(s) to get it put right?