MFJ 260CN dummy load needs RF to work it?

Discussion in 'General Technical Questions and Answers' started by 2E0LPL, Oct 14, 2019.

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  1. 2E0LPL

    2E0LPL Ham Member QRZ Page

    So I bought an MFJ-260CN 300w dummy load...not to expensive.

    Got it from a major UK dealer...when it came before using it I thought I would see if it gave 50ohm with my multi tester.

    Put the probs into the antenna socket...reading...nothing...open circuit so it looked....I have a dud I thought.

    Rang the dealer...oh sorry about that...I did ask am I missing something...I dont think so he said...send it back we will test another one and send it out.

    New one arrives today...same test with my milti tester...nothing again...I ring them again...same guys...OMG...we must have a bad batch I will refund you.

    Hang on he says...just thought on...the engineer mentioned put RF into it.

    Goes on...hook up to you rig low power...see what happens...Mmmm not keen on that in case it open circuit...be ok just blip your key.

    Sure enough, perfect reading 1-1 show my 5w...bit more power 10w 1-1.

    Anybody else come across a dummy load like this one...I did have another but it was only 10w...used to show 50 ohm with my meter.

    Does this MFJ one sound right...and safe to use.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    1. Sounds like you didn't make good connections with your Ohmmeter...or...
    2. If there's a capacitor in series with the load resistor, then of course it would read "open circuit" with an Ohmmeter but be fine for RF.

    I really can't think of anything else it could possibly be.
     
  3. WB6CXC

    WB6CXC Ham Member QRZ Page

    You are not probing the N-connector properly with your DMM. This dummy load has no capacitor, it's just a 50-Ohm resistor. I have the MFJ-260C (the "UHF" connector version) and it measures 50 Ohms (more or less) with my DMM.
    Go ahead and open it up to take a look. There are eight screws that attach the cover to the body of the dummy load, and you can easily see how the internal connections are made (all two of them).
     
    Last edited: Oct 15, 2019
  4. 2E0LPL

    2E0LPL Ham Member QRZ Page

    One probe to inner center brass contact...one probe to outer on N type socket....hard not to do it right.

    The shop I bought it from tested it in the same manner as me as we chatted on the phone...they sent it to there tech guy in the backroom who its seems to have told the guy on the shop counter...you need to send RF into it.
     
  5. K2WH

    K2WH Premium Subscriber Volunteer Moderator QRZ Page

    That's a real dummy.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    "You need to send RF into it" makes no sense.

    It's very possible the center contact has some oxide film so using an Ohmmeter probe on it won't really make contact. You cannot actually "see" this, the oxide can be transparent. Plugging in a male coaxial connector, the center pin can scrape away that oxide and make fine contact that your Ohmmeter probe may not.
     
  7. KA2IRQ

    KA2IRQ Ham Member QRZ Page

    You sure you're using your meter correctly? Correct setting (you're not on AC Volts)? Meter batteries good? We've all accidently mis-used a meter.

    Are your meter probes good? They usually have very thin wires in them and can easily open. Been burned by that one too. Measure across a known, good resistor.
     
  8. AA7QQ

    AA7QQ Ham Member QRZ Page

    In the "Ohms" position, short the leads to check if all is working properly.

    Ed
     
    WA9SVD likes this.
  9. W0IS

    W0IS Ham Member QRZ Page

    Yeah, the only other explanation is that MFJ put a capacitor in series with it, but I can't think of any good reason why they would have done that.

    Instead of putting the meter probes directly on the socket, I would try using the known good cable that you hooked to your transmitter and measure with that connected.
     
  10. 2E0LPL

    2E0LPL Ham Member QRZ Page

    Yes meter was used correctly...always touch probs to check operation before use.

    Ok tested it again today...surprise...its reading as normal....well 53 ohms.

    Seems the RF woke it up....no response from MFJ yet...be interested in their slant.

    I'm 100% sure I was getting good contact with the probes...very odd.
     
    KA2IRQ likes this.

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