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MFJ-4275MV Switching Power supply RF hash

Moleculo

Administrator
Staff member
Apr 14, 2002
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I did a review a while back on the MFJ-4275MV switching power supply, which has a lot of nice features. This power supply is advertised as having no RF hash, which is a common problem with switching power supplies used in an HF radio environment.

Unfortunately, this supply does exhibit quite a bit of hash. I've made sure the that the supply is properly grounded and even changed the AC power cord to a EMC Eupen ferried lined cable (which helped) but I still see and hear the hash.

Here is a screen shot of the hash visible on the panadapter screen, evidenced by the vertical lines that the red arrows are pointing to.

mfj-4275hash.jpg

They say a picture is worth a thousand words, but here's a video so you can hear what the has sounds like:

MFJ-4275 Power Supply Hash - YouTube

It's worth pointing out that this problem is fairly common with switching power supplies and is not unique to the MFJ products. Often I find that the hash is not strong enough to disrupt normal operations, but sometimes it is a real hassle to deal with. On my station, I'm going to move this power supply to mostly VHF/UHF and VHF amplifier duty, while returning the HF gear to a regulated linear power supply. I hope the information I've proved helps others make their own decisions.
 

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  • @ ButtFuzz:
    on the connector with the green wire, if you short that to a nearby black wire, the power supply will turn on. (for use as a bench supply or whatever)
  • @ ButtFuzz:
    And IIRC, the purple wire is -12VDC, but that is from memory and it does not serve.
  • @ ButtFuzz:
    So if you need, say, 7v, connect + to the Yellow wire and - to the Red wire.
  • @ ButtFuzz:
    or or you need 9v connect + to the yellow wire and - to the orange wire. note: not all PSU's have the orange wire nowadays, as many manufactures had the voltage regulation on the motherboard, and all the power supply gives is 12 vdc
  • @ BJ radionut:
    Thanks ROB will check it out!~!! got to clean the work bench a little make room for that big case :)