The AM Forum

Here's a blurry photo (sorry about that) of a Vectronics (i.e. MFJ) "2 kw" low pass filter pc board underside.  You can see the two burnt toroids.  This was from operating with a ~275 w. carrier going through it for around 1.4 kw pep.    I found it when the vswr on the feedline went up to around 2:1 everywhere above 10 MHz.  Here's what they say on the Vectronics website about this model, the LP-30:  "Handles up to 1500 Watts Key down. Measures 8.75x3x2.75 inches. Has SO-239 connectors."  Hmmm.
Glad I don't have to fool around with LP filters now, one good thing about DTV.

73

Rob
K5UJ

I hope you didn't spend much on that toy. I would rate it at about 100 watts

Can't remember what I spent but I bought it new.  Got it 8 or 9 years ago when I was just getting back into the hobby after being QRT for 18 years and didn't know anything because I had forgotten just about everything. 

I had a similar event with my MFJ-962D 1.5 Kw PEP roller inductor antenna tuner.  Figuring I was only running max 125 Watts of Carrier for maybe 500 watts p.e.p., I thought all was fine.  It was for about two months of AM'ing at that level.  This tuner includes a torroidal 4:1 balun for balanced line (I was running coax out to a 1:1 W2DU choke balun to the last 35 feet which was ladderline, G5RV antenna) cross-hair needle meter for power and swr, and antenna switching options for two antenna's out, standby and the balanced line... .the antenna switching was done through a ceramic wafer.  That wafer burned up.  

After gutting the tuner and removing everything but the SO 239 in and out, the aircaps and the roller inductor (and a few internal wires of course).... the tuner now handles the 100-125 watts carrier.

The point?  Equipment designed for SSB P.E.P ratings are obviously not going to handle high duty cycle modes well.  

I'm keeping my eyes open for a Johnson Matchbox or some heavy duty components to build a huge balanced tuner.... I sure love seeing those huge breadboared balanced tuners that some of us here have built!  Where does one FIND those huge Aircaps and Coils!??

That's nothing.  You should have seen my "2 KW" Dow-Key relay after I fired up the Gates BC1-T on 160 for the first time.  It turned into a solid block of charcoal.

My cure was to replace it with a monster open-frame contactor relay with dime-sized contacts, specially designed for HV antenna switching, and to build a T/R relay sequencing circuit to eliminate the hot-switching problem.

That's why I don't pay any attention to that p.e.p. bullshit.  It first showed up in ham radio in QST and CQ ads in the 50's to allow the manufacturers to inflate the power ratings of their slopbucket appliance leenyars.  Some of the so-called "2 KW" linears were actually good for about 150 watts.

I gave up on the MFJ stuff.  I melted a tuner coil on their "KW" after blowing out the SWR meter on it.

HAM fests have good parts

When the specs say "up to XXX" you have to take serious what that means - somewhere below the number, anywhere below.   1500 Watts best case only - 160 meters, perfect 50 Ohm match at 0 C ambient?

Rob, 

Let me translate "marketing speak" for you.  "Handles up to" is similar to the claims "lasts up to three times longer than regular..." and "increases your fuel economy up to XX%". These are reverse guarantees  ;)   Translated respectively, we guarantee it will require no more than 1500 watts (and probably much less) to generate spectacular smoke and pyrotechnics; this product may blow up instantly but under no circumstances will it last more than 3 times as long as the standard product; and you will probably see no fuel economy gains and perhaps a loss but we guarantee that under no circumstances will it increase your fuel economy more than XX percent.

Seriously, under perfect circumstances it may withstand 1,500 watts key down if you have very short dashes and a low duty cycle.  Sort of like the term "instantaneous peak power" used for audio gear in the 70's.  Test procedure was hook up storage scope, hook up record player, turn volume up to full, drop needle on record, blip on scope screen was the IPP as it blew up-multiply this value by 1.25 and send it to the ad department.

The Coils are "found" when you go to home depot and buy some tubing, then wrap it around a chunk of PVC   ;D

The caps are ebay/flea finds, or buy them new.
Or use vac-variables....

Yep all true.  I was a little surprised about the Dow Key though.  Amp mfrs will give you the max pep for the best circumstances and leave the rest out.  I've never tried a MFJ tuner because they didn't feel right to me but I figured way back when that it would be hard to go wrong with a LP filter.  Wrong!  but I wasn't thinking of AM back then either.  yeah, those big RF switches are great.  I've seen a few that came out of AM broadcast tuning units that were made to be controlled remotely to change patterns at dusk and dawn.  If you get any of those junked tower tuning units those switches in them are gold. 

Go to hamfests; take a look at parts on eBay.  At the last local fest I bought a 12KV 1000 pf vacuum variable for $50.  That was a steal.  Unfortunately, (but someone else's misfortune is your opportunity) in markets where there are a lot of little AMs, these properties are not doing as well as they once were.  Some licensees are doing things like selling the license to any buyer who will be able to move it to another location, then they're selling the property to a developer.   Or, they may be consolidating outlets if they own a few in one market, by letting N of them go dark to keep one on the air.  These folks are often willing to let their RF gear go for next to nothing or even free if you can haul it away.  The trick is finding out about these actions in time to rescue the gear.  Station owners are often business owners who don't give a flip about the equipment, which is often old stuff they consider junk.  But in that "junk" are parts like what's in the attached photo.  Well, the photo is too big to upload, sorry about that.  All that is on their mind is to unload the property and stop the bleeding which they want to do asap so the last thing they think of is telling a ham that the bulldozers are coming tomorrow.   Sometimes you can find out what's going on by looking at FCC filings on their website--look for applications to move a city of license somewhere (COL) or a sale or deal to take one station dark to increase power on another one locally.  These all signal potential changes or dismantling of a tx site.  For us hams, the relatively low power stations yield the biggest bonanza in my opinion.

73

Rob K5UJ

I've been using an old Dow Key on my BC1T for years now and it handles the output without any trouble....I'm glad for that...I have other Dow Keys that have been run on full scrote transmitters as well...Maybe Don had one that was damaged or otherwise not in good condx.  I have never purchsed any MFJ stuff since I saw a 1 KW tuner a local buddy  fried to a crisp with 150 watts

It's getting to the point that you have to make your own tuners or pay $$$.  Here's four roller inductors that are going to be used to make a couple of balanced tuners.  In the back are some chokes and a nice 60 H 500 ma modulation reactor for my dream maul  :D

I also had a similar experience with an MFJ peak-reading wattmeter.  Fried it good running only 300 watts or so AM...and it was supposedly rated at 2 KW.   Obviously, much of this equipment is not really capable of handling "real power".   Might work OK on SSB with a low duty cycle, but not on AM or brick-on-the-key mode.  Unfortunately, also my experience with a 275 watt Johnson Matchbox had a similar outcome.  When running "high-power" on AM, one must consider that things need to be HEAVY DUTY to insure longevity.   :o ;D

73,  Jack, W9GT

I don't think anyone could afford a 2009 replica of the Johnson Matchbox.

Ditto for some other BA gear.  OTOH look at the guys buying restored vintage Collins rigs. 

The J in MFJ must stand for Junior or Jock(strap)  Meant For Jock strap.  Remember all those low power antennas with Jr. in their model numbers like the TA33 Jr.?  Handled something like 300 w.   I used to have the MFJ balanced RF amp meter in the feedline.  Now this thing is supposed to be able to measure up to 3 A, which can equal a lot of power for a ham.  So I'm ragchewing away running my 200 to 300 w. and the carrier showing a few hundred ma on the meter, and I notice the vswr has changed.  What's this?  I go retune the tuner back to 1:1 and continue.  After a minute vswr high again.  Okay something is heating up.  I put my hand on the balanced amp meter.  Warm as a toaster.  Pull it out of the feedline and everything is okay again.   I guess the rule with their RF stuff is J stands for Junior.

Rob

KX5JT asked: "Where does one FIND those huge Aircaps and Coils!??"

The best source is old broadcast transmitters that got parted out. Some AM stations do not know that hams are recyclers of these rigs, and they begin to part them out for metal value. If you can find one, the output network of Gates, RCA, Collins, and many others will yield some first class tuner components for high power, since they were running 1 kW carrier + modulation. If you can find a Raytheon RA1000, you'll even get giant air variables and balanced inductors with link output.

I blew up an MFJ 1.5KW LPF with a 200W carrier. With the filter in line the VSWR went from 1:1 to 1.2:1 What a piece of crap. It is now intermittent. I suppose I will have to remove it and open it. If it is repairable, it could be used on the 100W plastic radio.

Somewhere, there should be a site mfjsucks and post images of the cheap burned up crap.

I do have a 300W automatic tuner on the 100W plastic radio. By looking at the insides of it, it should be OK for that.

I'm going to find the good old filters at hamfests from now on. The ones with real coils and filter sections inside will echo or ring when tapped on due to the coils being supported at the ends.

(I can see MFJ reading this now and adding a tuned metal tang inside so the filter will go 'bong' when tapped on. What does a "tang tuner" make in China?)

The Dow-key would probably have held up to the BC1-T if everything had been sequenced properly.  But it didn't like being hot switched.  The problem was that the Gates continued to put out power for a split second after the relays dropped out, until the capacitors discharged and evidently the VFO didn't disengage instantaneously with the antenna. Nothing could have been wrong with the Dow-key itself to make it turn to toast the way it did.

The open frame relay I now use took care of the problem, but I built a sequencing circuit anyway.  Not even the slightest spark at changeover.  The only problem was that it is very noisy.  It uses an A.C. coil that buzzes and it makes a loud "clank" when it engages.  I suspended the relay on springs, very much in the fashion of the old double-button carbon microphone mounted in a ring stand, and enclosed the whole thing in a wooden cabinet mounted on the wall, and now it is barely audible with the cabinet door closed.

But with just about everyone receiving TV off cable or satellite nowadays, and most over-the-air TV stations moved to the upper VHF or UHF channels since the digital conversion, why the need to even bother with a LPF?

I seem to remember that one of MFJ's 2KW tuners cooked on the bench at the ARRgghhL's laboratory with only 800 watts applied. Of course, they never included that with their review in QST.  :P

I remember another one of the tests where quite a few of the high power rated tuners had issues on 160 matching low impedance loads which is usually about the toughest task for a tuner.  Most of them were lossy under these conditions and that loss is what generates the heat.  I recall the burning MFJ story also and I believe they changed the coil support material after this incident.  Probably most of the people in the MFJ market segment are not running legal limit high level AM nor using Alpha or Henry type amps and for the person running SSB or CW with a pair of 572B's or a quad of 811A's the tuners probably do just fine.

I use very simple homebrew reversible L tuners here, one in the house and another in the barn.  The inductance is a large Gates rotary inductor and the capacitance is from a 1500 uuf vacuum variable with an additional 500 pf  available via a large fixed padder.  I use BC-375 tap switches to reverse the L and provide antenna selection.  The barn tuner at the contest setup has additional small selectable fixed caps in parallel with the vacuum variable for faster QSY (less cranking of the vacuum cap).  So far I have not managed to damage either in about 10 years of use including high level AM from my Desk KW, Viking 500, and Gates BC-250GY or running CW, RTTY, and SSB at the legal limit in contest mode. Total parts cost per tuner was under $200.

if you gonna op AM, you have to use AM stuff.  8)

you can gripe about a KW flashbox being 350 bucks, but the reality is thats a bargin - because it costs as much or much less than something new that wont handle 25% of the power.

the hamfest today was interesting - there were at least 7 KW matchboxes there, and every one was at least a 9+ condition. It was surreal.

That's kind of how it was at Dayton.  There were a bunch of KW matchboxes for sale and all looked pretty good.

Yeah, when my filter charred, (and I saw a few coils on the way to charring) I simply pulled it out and tore it apart to see inside and now operate without.  There's a point when you can have too much in a feedline  :D

someone must have set up shop redoing the matchboxes. The ones I saw save one looked literally brand new. I didnt ask prices.

MFJ >>> Mighty Fine JUNK

Ooh, there's a worse version than that.  :o

I learned the same lesson back in the early 90s when I shoved a Drake MN-2000 in line with the KW-1 to alleviate the only RFI complaint I ever had. But...but...it sez it's rated for 2 KW!!! *zorch*

Those look like nice EF Johnson rollers, Rob. Couldn't tell if they were the 'flat strap' models.

Johnson sold those matchboxes into the 80s, didn't they? Interestingly, one of the three companies that resulted from the split up of Millen back in the....early 90s(?) was still making their big tuna in the late 90s, maybe still today. Prices had changed considerably. They were also selling a lot of the Millen components and a few products last time I checked.

Is there, was there, ever a ham tuner that would handle AM from a "1KW DC input" rig of the olde school days? I am not sure the KW Matchbox even would do that.

Saw an ad for the KW Matchbox in my 1957 Handbook this morning and the ad copy does say handles 1 KW input.   Doesn't say if that's CW, AM or what.  Since it doesn't limit itself by mode, I guess they claim it can handle 650 or so watts output carrier AM i.e. 4 times that.  So the question is whether or not E.F. Johnson is for real, or like MFJ  :D  Probably more like for real but I guess we don't have to worry on account of the current power limit  ;)

Rob said:

Hmmmm, I thought they were essentially designed to go with the Desk KW. Just like the smaller version was to be (somewhat) mated with the Valiant.

Got it right here.  here's what it says (1957):

Viking Kilowatt "Matchbox"--Bandswitching 80, 40, 20, 15, and 10-11 meters -- self-contained.  Use with transmitters up to 1000 watts input--handles unbalanced line impedances from 50 to 1200 ohms and balanced line impedances from 50 to 2000 ohms.  No coils to change, no "tapping down" on the inductor.  Transmit/receive relay grounds receiver antenna terminals in "transmit" position.  Adjustment for matching antenna to receiver input.  Fully shielded.  Provision for RF probe.  Dimensions:  17 1/4" x 12 1/8" x 10 7/8".  Cat. No. 250-30 Wired and tested...Amateur net $124.50

Oh yeah this is kind of interesting, the same ad says the small Matchbox handles 275 w. (I always thought it was 250) and the Z range is different, balanced range 25 to 1250 ohms; unbalanced 25 to 3000 ohms.  Other features same as KW model.  $49.85.

The big Millen Transmatch (92200) is built like the proverbial brick shithouse inside and looks like it would easily handle a KW input. Can't find the book for it atm, but I seem to recall it says it will handle over 3KW, Can't remember what the class is. I've got one here that I've toyed with selling a few times, but it's just so well made and purty, inside is all shiny copper. Sadly, it only covers 10-80 and is built for coax. That's not a bad thing unless you're using balanced feeders. ;)

Remote it with some motors in a plastic tub out at the vertical feedpoints.

There's one for sale right now on eBay.  I'll see if I can find it.
da da..up to $153 now with one day left.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320431318598

I agree that Vectronics is light duty. However, I got that exact same unit at a ham fest for $5.  I ran it for about 6 months at 1500+++.  If the match through the unit is flat, the signal is clean, We are dealing with the connectors and board traces. If you put alot of out of band energy through it or high SWR, You will smoke it.

I dont use TVI filters anymore.  I have a balanced antenna and the transmitter is flat to the tuner. Or I dont key.

C

I like that millen too  :)


73
Jack.

I've learned alot about lousy matches and that a decent KW size tuner will tune them, but something may arc even with a 100W radio! I've got one I cobbled together and I am confident it will take 1KW CCS or PEP all day and it has done so into a 1700 ohm load I have cobbled up, but once the load gets too screwy, the voltage ratings are exceeded. I am saying that I get a 3/16" RF arc at 100W because of the rotten little antenna I have. It is depressing. That is the point where some hammies will put an 1:4 transformer in place. I'm not going to bother, it is the antenna that's wrong. great SWL antenna, absolutely wrong cut for any ham band.

Maybe this winter I can put up the Cushcraft 23FT marine vertical I've got, and connect the Sunair autotuner and GSB-900DX and use that. I can just screw the ant. to the roof of the shack, throw up 3 guys of nylon cord, and bolt the tuner to the metal rafters right under it. The shack is an all metal 30x40 building, should make a fine ground plane. I hope.

I'm still going to run a LPF on the outpoot of the NCL2000 leenyar, just an old Johnson one. I want to take the MFJ and shove it squarely where it belongs. The way I see it, I was cheated and stolen from.

Patrick,

The output from your NCL-2000 should be quite clean so you shouldn't need to use the lowpass filter.  It is just one more thing to add loss (and go wrong) and you do want to be kind to the 8122 finals since they are a bit pricey.  I doubt that TVI is going to be an issue for you due to the NCL-2000.

You metal building should make a good ground plane and I used a vertical for a while on my metal roofed barn.  One thing you should be cautious about is you are putting a lot of RF into the vicinity of some potentially bad metal to metal contact areas.  If your luck is not good, one or more of these will form little rectifiers and you will get all sorts of RF garbage created that cannot be reduced at the transmitting end (except by reducing your power).  Hopefully this won't be the case for you. 

Rodger WQ9E

Agreed. Unless you are operating on 10 meters, that LP filter will do you little or no good.

A friend of mine and I were talking about the Millen but as good as it is, you can still do better with some shopping in a few decent (AM, lotsa boat anchors, maulish parts) hamfests.  Get a thick slab of pine or plywood and bolt everything down to it; two big roller inductors and a 1000 pF vacuum variable.   Or roll the coils and tap them manually.   But yeah even the widest range most scrot tuner that matches anything won't help you get out if the antenna stinks.   I learned the hard way that if you can't get a match it is sometimes better to mess with the antenna size and/or feedline length than mess with the tuner.