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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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7th Sep 2019, 10:49 am | #1 |
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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MFJ Dummyload faulty?
Hi All
I just received a used MFJ dummyload via the post. When I read the resistance from the dummyload, it reads 61 ohms instead of 50 ohms. Surely this is faulty dummyload? Is there way to adjust the resistance on the dummyload? Or should it be returned as faulty item? Thanks |
7th Sep 2019, 11:47 am | #2 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: MFJ Dummyload faulty?
Almost certainly impossible to adjust and might contain beryllium (should be marked). Dangerous to open if so.
61Ohms is not ideal but it is not terrible. VSWR is 1.22:1 if perfectly resistive. Is the reading stable? Frequent failure with dummy loads is at the connection between pin and resistance element or resistance element to body. Check to see if it changes with slight pressure on the centre pin or light tapping on a wooden block. Maybe post the model number and someone might have another to compare readings? |
7th Sep 2019, 12:01 pm | #3 |
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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Re: MFJ Dummyload faulty?
Thank you for your info and advice
Yes, the model is MFJ 260B. it looks very old the casing is all scuffed. I opened it up, and checked all the connections and screws, and they seem OK. I tightened the screws further. But even reading resistance across the element in the casing, it is 60 ohms. All the connection is tightened up, so it seems the element which is giving 60 ohms true. I hooked it up to TS-130SE, and fired up on 14Mhz with 100 W CW. The SWR reading is 1.1 on the meter. I have used another meter for the PWR output and SWR just to confirm, and they were the same 100W and 1.1:1 SWR. I have another dummyload from ZETAGI DL50 and it reads 52 ohms. but it is rated for 50W max. |
7th Sep 2019, 1:18 pm | #4 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: MFJ Dummyload faulty?
If it's a used one, someone may have overheated it.
David
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7th Sep 2019, 1:18 pm | #5 |
Heptode
Join Date: May 2015
Location: Chatham, Kent, UK.
Posts: 964
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Re: MFJ Dummyload faulty?
Hi i have mfj 989c this reads 57 ohms my home made one using eb** surplus telecoms resistor bolted to heat sink 4 inches square ex audio amp gives 50 ohms and 1 to 1 up to at least 430mhz its rated at 150w and 5ghz cheep i think £3.50 for 5 if memory serves me right just looked they have gon up ti 2.56 each https://www.ebay.co.uk/itm/RF-termin...YAAOSw0~pcEz~V
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7th Sep 2019, 1:20 pm | #6 |
Nonode
Join Date: Oct 2010
Location: Tintinara, South Australia, Australia
Posts: 2,340
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Re: MFJ Dummyload faulty?
It's possible it has been run either above it's rating or very close to it's rating for an extended duration.
This will cook the resistive element somewhat and as we all know too well, this can result in the resistance changing. Bingo David |
7th Sep 2019, 1:22 pm | #7 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: MFJ Dummyload faulty?
If it’s MFJ it was probably broken when it came out of the factory like my ATU was. Not getting out was because there was a wire hanging off inside the tuner ffs.
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7th Sep 2019, 1:49 pm | #8 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,288
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Re: MFJ Dummyload faulty?
It's said that MFJ denotes "Made From Junk", but I've never had any trouble with my MFJ-948 AMU.
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7th Sep 2019, 2:03 pm | #9 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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Re: MFJ Dummyload faulty?
If there's room inside you could always add a parallel 300R non inductive resistor or resistor combination rated for 20W or so. That'll get it back to 50R.
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7th Sep 2019, 2:15 pm | #10 |
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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Re: MFJ Dummyload faulty?
Wow, great advice. Thank you. I have a 220 ohm heat resistor somewhere in the draw. Maybe give a try with that?
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7th Sep 2019, 2:19 pm | #11 |
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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Re: MFJ Dummyload faulty?
But yes, the MFJ used to be regarded as real poor product back in the early 1990s. Their ATUs used to cost a pittance, and no one respected them.
But they seem to have come a long way now. Some of their products such as antenna analysers 259 269s sell like hot cakes at high prices and their ATUs seem getting grabbed for good price too. |
7th Sep 2019, 2:23 pm | #12 |
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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Re: MFJ Dummyload faulty?
I had an MFJ atu a few year ago, and it worked OK, but when you open the case and have a peek inside, you always feel the quality of the parts are a bit low grade stuff.
This 260B is from long time ago, and made with bare lowest grad parts, it feels. I am wondering if I should return it for full refund for out of spec = 60 ohms is a bit too far off from the nominal spec on the paper for the product = 50 ohms. And maybe have to go for a brand new one even at double the price. I think newer model 260C covers higher frequency albeit same power handing at 300W peak, and could be bought for about £50 brand new. Last edited by ORAWA01; 7th Sep 2019 at 2:33 pm. |
7th Sep 2019, 2:38 pm | #13 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: MFJ Dummyload faulty?
If you want a reasonable dummy load, you should be shopping for Bird, not another MFJ.
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
7th Sep 2019, 2:56 pm | #14 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: MFJ Dummyload faulty?
Bird stuff that works is expensive. I’m using a microset CF300 with a homebrew W7ZOI power meter and tap. The microset has flat VSWR to about 700MHz as it uses flanged RF resistors not those horrid carbon poles with a horrid tempco.
When I got into amateur radio I bought a diamond antenna “100W for ten seconds” load. It lasted two seconds at 100W then proceeded to emit smoke and now measures at 58 ohms. Thus you get what you pay for. |
7th Sep 2019, 3:26 pm | #15 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: MFJ Dummyload faulty?
Spec VSWR on that model is 1.3:1 so they are inside that.
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7th Sep 2019, 3:30 pm | #16 | |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,859
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Re: MFJ Dummyload faulty?
Quote:
Rob
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7th Sep 2019, 3:37 pm | #17 |
Guest
Posts: n/a
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Re: MFJ Dummyload faulty?
A dummy load is just that, a load, near enough for farm work as they say. A termination on the other hand should be quite accurate.
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7th Sep 2019, 4:26 pm | #18 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: MFJ Dummyload faulty?
Anything that works properly in this field is expensive.
Bird stuff is OK, but not precision. The measurement grade stuff from Weinschel, Narda, etc is even dearer. I use attenuators rather than loads so I have an output for a spectrum analyser, or a precision power meter. For something to check a transmitter is working, 60 Ohms will do, but it's disappointing. I'd suspect damage. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
7th Sep 2019, 6:36 pm | #19 | ||
Hexode
Join Date: Apr 2011
Location: Stirlingshire, UK.
Posts: 407
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Re: MFJ Dummyload faulty?
Quote:
https://www.digikey.co.uk/product-de...kaApcREALw_wcB |
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7th Sep 2019, 6:37 pm | #20 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
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Re: MFJ Dummyload faulty?
Its not pretty, but look at this....
https://www.ebay.co.uk/itm/Ex-MOD-Bi...kAAOSwSlBY0qAM I have something very similar here, ad it has worked flawlwssley for over 20 years....
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