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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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Old 7th Sep 2019, 10:49 am   #1
budkor22
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Default 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
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Old 7th Sep 2019, 11:47 am   #2
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Default 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?
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Old 7th Sep 2019, 12:01 pm   #3
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Default 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.
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Old 7th Sep 2019, 1:18 pm   #4
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Default Re: MFJ Dummyload faulty?

If it's a used one, someone may have overheated it.

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Old 7th Sep 2019, 1:18 pm   #5
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Default 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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Old 7th Sep 2019, 1:20 pm   #6
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Default 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
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Old 7th Sep 2019, 1:22 pm   #7
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Default 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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Old 7th Sep 2019, 1:49 pm   #8
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Default 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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Old 7th Sep 2019, 2:03 pm   #9
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Default 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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Old 7th Sep 2019, 2:15 pm   #10
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Default Re: MFJ Dummyload faulty?

Quote:
Originally Posted by Herald1360 View Post
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.
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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Old 7th Sep 2019, 2:19 pm   #11
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Default 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.
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Old 7th Sep 2019, 2:23 pm   #12
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Default 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 budkor22; 7th Sep 2019 at 2:33 pm.
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Old 7th Sep 2019, 2:38 pm   #13
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Default Re: MFJ Dummyload faulty?

If you want a reasonable dummy load, you should be shopping for Bird, not another MFJ.

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Old 7th Sep 2019, 2:56 pm   #14
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Default 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.
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Old 7th Sep 2019, 3:26 pm   #15
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Default Re: MFJ Dummyload faulty?

Spec VSWR on that model is 1.3:1 so they are inside that.
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Old 7th Sep 2019, 3:30 pm   #16
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Default Re: MFJ Dummyload faulty?

Quote:
Wow, great advice. Thank you. I have a 220 ohm heat resistor somewhere in the draw. Maybe give a try with that?
Don't know exactly what that is, but needs to be non inductive. If it's an aluminium case wirewound type, it is no use.
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Old 7th Sep 2019, 3:37 pm   #17
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Default 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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Old 7th Sep 2019, 4:26 pm   #18
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Default 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.

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Old 7th Sep 2019, 6:36 pm   #19
budkor22
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Default Re: MFJ Dummyload faulty?

Quote:
Originally Posted by robinshack View Post
Quote:
Wow, great advice. Thank you. I have a 220 ohm heat resistor somewhere in the draw. Maybe give a try with that?
Don't know exactly what that is, but needs to be non inductive. If it's an aluminium case wirewound type, it is no use.
Rob
It is square pillar shaped ones, I used them to restore 1940s all valve radiogram once, and leftovers from the work. Like this one in the link.

https://www.digikey.co.uk/product-de...kaApcREALw_wcB
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Old 7th Sep 2019, 6:37 pm   #20
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Default 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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