MFJ-9420: A Few Questions

[W3OWL]

I noticed that most of the recent photos of the MFJ-9420 (and others in the series) now show a "Fine Tune" knob on the front panel.  Is this something new?  Does it actually work, or is it just a gimmick?  Can the 9420 be fitted with an external speaker jack (front, rear, wherever)?  Are digital readouts available for the 94 series, and how difficult are they to install?  I recently had cataract surgery and my kit building/soldering days are over for a while.  Many thanks.  73

[AD5X]

I used to have a MFJ-9420, and I'm sorry I got rid of it.  Sure worked well.  I have a modification article on my website (was publiushed in QST several years ago) where I added a jack for external headphones or speaker.  I also added a RIT and made a few other modifications.  Tuning was a little touchy, so I bet that's why they added the fine tune control.  Check out the article in the "Equipment Modifications" section of my website at www.ad5x.com.

Phil - AD5X

[AA4PB]

I don't think anyone sells a digital readout specifically for the 9420 and I'm not sure you'd have room to add it to the front panel anyway. There are some digital readouts that could probably be adapted to the 9420 - maybe in a separate case.

The new 9420 has a hole in the rear panel made for a CW key jack (when you add their CW adapter). I'll bet you could mount a headphone/external speaker jack in that hole.

[W3OWL]

Excellent gentlemen, tnx for your information.  Currently I just turn down my Icom 718 to 5 watts or so to operate QRP, but I thought a dedicated QRP SSB rig would be fun.  Headphones would be a great benefit.  TNX again.

[VE3WMB]

I have the older model 9420 and it has both an internal speaker as well as a headphone jack
(1/4 inch) already mounted on the back.

I "solved" the tuning issues on my rig by installing a Small Wonder Labs FreqMite, which
is a frequency counter that outputs the frequency in Morse and injects it into the rig's
audio stage.The only modification to the rig's case is a hole on the front panel to install a
momentary SPST switch.

 I documented this at :

http://www.qsl.net/ve3wmb/9420mod.html

The FreqMite is a pretty simple build and can be had from

www.smallwonderlabs.com

Best of luck, the 9420 is a fun little radio and quite miserly on power consumption with a
pretty good punch.

Michael VE3WMB

I noticed that most of the recent photos of the MFJ-9420 (and others in the series) now show a "Fine Tune" knob on the front panel.  Is this something new?  Does it actually work, or is it just a gimmick?  Can the 9420 be fitted with an external speaker jack (front, rear, wherever)?  Are digital readouts available for the 94 series, and how difficult are they to install?  I recently had cataract surgery and my kit building/soldering days are over for a while.  Many thanks.  73

[W3OWL]

Thanks Michael and great information on your web site.  I think I'll get one of these little QRP rigs, and it will be fun to twirl the analog VFO knob again, just like when I started years ago.  73

[WB7RBN]

I've had several of the MFJ-94XX series radios and they all are great fun.  They are a bit touchy to tune but I really never needed a fine tune control.  In fact I have a MFJ-9420X now.  Just a word of caution about adding an external speaker or headphone jack.  The MFJ-9420 and other 94xx series radios use a ungrounded speaker.  Grounding either side of the speaker leads results in smoking resister R37.  No audio after that.  If you add a external jack for phones or speaker it must be insulated from the case.  See Phil's (AD5X) article on his website about adding a head phone jack.

73's

Jim

WB7RBN

[W3OWL]

Thanks for the cautionary note Jim.  However, I received my new MFJ-9420 yesterday, and guess what?  It has a 1/4" phone jack already installed in the rear panel!  How about that.  This one must be a very new unit because this jack is not evident on the MFJ web site or in the 2010 catalog.  The jack is labeled "Headphones" and powers my Sony MDR-V150 headphones perfectly.  However, it's very underpowered when attached to various external speakers I tried today.  Does anyone have anys suggestions for a good external speaker for the MFJ-9420?  Many thanks to all.  Bill W3OWL

[AA4PB]

Radio Shack used to sell (maybe still does) a couple sizes of amplified mono speakers with an internal battery. You should get plenty of audio out of one of those.

[WB7RBN]

Bill,

Later model MFJ-94XX series radio have the phone jack as standard.  Also the very latest series are hybrids.  That is half of the componets are surface mounts.  I like the earlier version as they are easier to work on if you need to replace any componts.

I use a MFJ-281 ClearTone Speaker with my MFJ-9420 and it has plenty of audio.  It has a 1/8 inch plug so you will have to use a 1/8 to 1/4 inch adapter plug.

Don't use a powered external audio filter (common power supply source) as most all have one side grounded speaker inputs and the 9420 internal speaker will get grounded through the negative power lead on the external filter and smoke the audio resister.  I found that out the hard way on the first MFJ-94XX series radio.  The only way I know to keep this from happening is to put a isolation audio transformer between the radio external speaker/phone jack and the powered audio filter input.

Jim

[W3OWL]

Jim:

Thanks for the warning on using an externally powered speaker.  I just bought a small pair of powered (wall wart type power supply) speakers and was just about to plug them in to rear speaker jack on the new 9420.  Do I understand you correctly that doing so could fry a resistor, or worse?!  Yikes.  Could I simply power these speakers from my current Samlex 1223 PS along with my Icom 718 and MFJ-9420?  I have tried using my old Radio Shack Nova Minimus .5 bookshelf speaker (8 ohm) with the MFJ-9420 and the audio is horribly weak, requiring the MFJ volume control to be turned up almost to maximum.  The Icom 718 and the old Radio Shack scanner powers the Minimus .5 just wonderfully, with a great full rich sound.  I'd like to achieve that same level of sound quality from the 9420 if possible.  Thanks again for your help and warnings.  Bill (alpha272003@aol.com)

[WB7RBN]

Bill,

Do not power the speakers and MFJ-9420 from the same supply.  The 9420 speaker will be grounded through the common negative power supply lead.  Doing so will fry R37 on the audio op amp resulting in no audio output.  I'm not sure about using a seperate power supply (wall wart) but I would not try it.

Jim

[W3OWL]

Jim:

Thanks for the quick response, and for saving my new MFJ.  I guess I'll just try a number of unamplified external speakers and see if I get some audio improvement.  So far the 9420 works fine and I've already had a number of stateside and European QSOs with the 10 w. output.  As you mentioned, tuning is a little touchy and I'm never exactly sure where I am on the band, so I just stay away from operating close to my band edge of 14.225.  The fine tune knob helps a little.  Bill W3OWL