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Reviews For: MFJ-251 dry switchable dummy load

Category: SWR & Wattmeters & Dummy Loads

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Review Summary For : MFJ-251 dry switchable dummy load
Reviews: 1MSRP: $210
Description:
A dummy load with three switchable resistors that can make 50 ohm, but also 16.6 ohm (3 resistors parallel), 25 ohm (2 resistors parallel), 100 ohm (2 resistors in series), 150 ohm (3 resistors in series). Max 300 watts / 30 seconds. Pretty useful to test antenna tuners
Product is in production
More Info: https://mfjenterprises.com/products/mfj-251
# last 180 days Avg. Rating last 180 days Total reviews Avg. overall rating
0014
PE1HZG Rating: 2023-07-27
Needs a little love on arrival, then a nice piece of equipment Time Owned: 0 to 3 months.
We've all heard the nasty comments some people make on MFJ products. Personally I think MFJ makes innovative products at a good price and being hams we've passed exams to understand technical work and we can be expected to take on small matters ourselves. Does this device match? Read on.

The MFJ251 has 3 50-ohm resistors that can be switched parallel or in series to create a number of different load resistance. This is useful to test antenna tuners with different loads and see how autotuners behave, or how to operate a manual tuner. Inside the box, there are 3 50-ohm resistor tubes we know from MFJ, 4 relays and a little switching circuit to make it all work. The documentation comes with a schematic, which I didn't need.

My MFJ251 arrived today and when I took the MFJ box out of the shipping carton, I heard some rattling screws and I could feel the PCB inside was not tight. I opened up the unit and found that 3 out of 4 screws had worked themselves completely loose and hence the PCB was not fixed.
I also found that the resistors were clamped-in using clamps and the clamps had been bridged-over with a jumper wire. Since one of the screw holes was below one of the resistors which I would need to remove to put the screw back, I had to wait for MFJ to wake up (I'm in Europe) to ask permission to work the unit and keep the warranty.
I did measure the dummy-load resistance in various settings and it was a bit high. When looking close, I found that some of the resistor clamps was not fully tight and I was able to rotate the ceramic resistor tubes in the clamps. There is a picture of the inside of the box at https://en.classicinternational.eu/_productfiles/producten/450_2/mfj251_binnen.jpg.
I wanted to change the jumper wire construction.

Upon calling MFJ (a quick phone call, no waiting) I was approved to make the adjustments and I would keep the warranty so I started.

When removing the jumper wires ("to replace with something better"), to my surprise, I found that the clamps actually switched on tight and were much tighter w/o the soldered jumper wire - interesting!

I removed the jumper wires and temporary removed one of the resistors to have access to the four screw holes and tightened the screws (with a small dab of locktite). I then put the resistor I had removed earlier, back in place and found that the clamps actually were pretty tight but finally managed to put it back.

When I subsequently had put everything together and again measured the resistance and now found the values spot-in. The jumper wires causing the clamps to not fully engage was the reason for the slightly-high resistances earlier, now solved.
I found I didn't need any jumper wires to keep the resistors in (to the contrary!) and simply removed the wires and the clamps were on pretty tight.

After this little bit of rework I used the tuner to test a number of antenna tuners. I found that one of my automatic tuners had a broken relay, easy to test with this tester. That is what I had purchased it for and it worked nice for that.

So, after the initial rework I now have a nice piece of kit to test antenna tuners and I am happy I bought it for that. It can do testing a normal dummy load cannot do.

Is the rework a problem? I don't think so, I knew I might have to do some touchup but is was simple and brief, and I would not be able to buy something similar from another brand (want to buy a Drake for this?) and certainly not for this price.

Hence four stars. Would I buy again? I already have one, but otherwise "yes".

One final comment: some product pictures show a 6-position switch with an "off" position suggesting a "dummy load open" setting. The actual product has a 5-position switch and does 16.6 - 25 - 50 - 100 - 150 ohms, and doesn't have "open". Also, the unit uses relays and hence needs a 12V power supply - w/o power, the delays default to 50 ohms, which I think is a nice touch.