eHam Forums => Mods And Repairs => Topic started by: EI5GUB on April 22, 2012, 05:44:24 AM

Title: MFJ 986
Post by: EI5GUB on April 22, 2012, 05:44:24 AM
Hi im using an MFJ-986 tuner,when I switch to coax 2 the radio goes dead,do they give trouble inside,I only run 400 watts into it and use a carolina windom and a loop antenna on it,any ideas,cheers EI5GUB

Title: RE: MFJ 986
Post by: K9YLI on April 22, 2012, 06:30:50 AM
 a 'tuner'  is  pretty  much  just  wire  in some configuration.. switches  coils  coax center pins..  or the case as  common...

simple to chase the input to output coax 2   with an  ohm meter..
its is either  open or shorted  or what ever is connected to  coax 2 is  bad.
does this model  have  feed thru..  as mine  mfj does.
does  coax  2  act the same  in  bypass as it does in  tune. ??

"expliain "" radio  goes dead"""""

Title: RE: MFJ 986
Post by: EI5GUB on April 22, 2012, 06:54:58 AM
hi by dead i mean if i use an antenna in coax1 signal will be fine but switch to coax2 and al signal is gone just like if no antenna is plugged in,i got fooled by it and have just replaced a load of RG-213 cable to a 20 mtr loop

Title: RE: MFJ 986
Post by: K8AXW on April 22, 2012, 10:36:11 AM
I posted a reply early this morning during a terrific mind fart.  Please, all who might have read it, disregard it.  (There are many configurations of antenna tuners and my mind locked in on only one)

Title: RE: MFJ 986
Post by: K8AXW on April 22, 2012, 11:07:45 AM
GUB:  OK, I'm back and this time with a printout of the 986 schematic from MFJ, which you should have done!  

The way I see it there are only about 4 things that can be wrong.  They are the switch contacts on SW2a and SW2b and the coax connector between Coax 2 and the antenna and finally the antenna/feedling that you have on Coax 2.

What happens if you swap antennas on the tuner Coax 1 and Coax 2 connectors?  This is almost always a quick check to see where a problem might be.

Yes, things can go wrong inside a tuner.  Unless the tuner is abused, tuners seldom develop problems.  One of the worst things is "hot switching" which can possibly burn off a switch contact.

If I was a betting man, I'd say you have a connector problem at Coax 2.  If not that then the antenna has crapped out.

Title: RE: MFJ 986
Post by: EI5GUB on April 23, 2012, 11:52:59 AM
hi K8AXW,Richard here,no its not an antenna prob as i have just spent 2 months finding out,when I put antenna in coax1 plug it is fine,will have to take it apart and chect it out,you could be right about the switch but i rarely go between 2 antenna so I doubt it was hot switching,thanks again,cheers Rich

Title: RE: MFJ 986
Post by: WD5GWY on April 23, 2012, 05:18:45 PM
Hope this does not seem like a dumb question, but, do you have
an antenna attached to both connectors? Ant 1, & Ant 2 ?
If you have no antenna attached to one of the connectors, then
you will receive little to no signal at all when you change the switch.
 I have a MFJ 986 here that was either "hot switched" by the previous owner
or had a lightning strike.
Either way, it did burn one of the contacts on the antenna switch completely up!
I replaced the switch ( $69 from MFJ) and also replaced all the rear coax
connectors as well. Once I rewired the switch, the tuner worked just fine.
 The 986 is a very good tuner.

Title: RE: MFJ 986
Post by: EI5GUB on April 24, 2012, 12:41:37 PM
Hi James WD5,thanks for replying,yees I do have 2 antennas connected to the tuner and it blanks any signal when I go to coax2 or coax2 direct,yes I have had to change the plugs on the rear of it a few times,I will open it and look at the switch,I rarely go between antennas and would never hot switch it but you could be right about the lightning strikes,im using a new 20 metre loop here at 70 feet and its working well for DX all round,thanks again James I will let you know what I find,cheers Richard

Title: RE: MFJ 986
Post by: K8AXW on April 24, 2012, 05:30:08 PM
Richard:  What do you mean you had to "change plugs" on the rear of it a few times?  Do you mean the So-239 female connectors on the tuner rear apron?  If this is correct, why have you had to do this?  If this is correct, perhaps you have a cold solder joint on the SO-239 connector.  If this isn't the case, the only thing that it can be is the switch.  It should be conspicuous.

Title: RE: MFJ 986
Post by: WD5GWY on April 24, 2012, 05:33:49 PM
Well, it does sound like it is time to open it up!
I would suspect the switch being bad. When I
replaced the rear coax connectors on my 986
it was because the amount of internal arcing
damage that I saw inside the case and the
insulator inside one of the connectors was
blackened. I'm pretty certain that that particular
connection was where the damage either entered
or exited the tuner. But, once all was replaced,
the tuner works just fine now.
Another thought just occurred to me. You might
have a loose solder connection on the switch causing
the problem. Check the circuit with an ohm meter and
you will find the problem. Or you will see it, if there are
major burns inside!
Good luck. Hope you get it back going.

Title: RE: MFJ 986
Post by: EI5GUB on April 26, 2012, 02:12:29 PM
hi guys I opened tuner up last night and the switch looked alright,I bent in all the small tabs that the switch hits as it passes them and changed coax 1 so259 as it was faulty,when I put it back together it was ok again,one thing I noticed a while ago is that when I was using a big multi band loop that was designed fo 80 and 40 but would work on 20 it would light up trhe light inside the tuner when I spoke into the mic,maybe this burnt something,cheers and thanks for all the help,all the best Richard