Inside a cheap Chinese-Made RF Attenuator - Page 1
Forum section: electronics / rf-microwave

author: xrunner
author_rank: Super Contributor
timestamp: January 23, 2018, 11:57:29 pm
content: I bought this 40 dB / 100W attenuator a couple of years ago from a Chinese Ebay seller, it was brand new. I used it to attenuate 100W transmitters before viewing on my spectrum analyzer. Yesterday I used it to attenuate a 100W 6 meter (51 MHz) ham transceiver I was repairing for a friend for very brief periods of time. But it wasn't working correctly, and I finally had to take it apart. The Majik Smoke smell was there, and I found a burned resistive element inside. That was bad enough, but to my horror I found some of the crappiest construction I've seen in a long time.Take a look for yourself - and BEWARE of what you buy on Ebay!

author: xrunner
author_rank: Super Contributor
timestamp: January 24, 2018, 01:15:25 pm
content: I'm going to re-purpose the thing. I took out all the insides and cleaned it up. I put back in two of the cleaned strip boards. I have two 100W 50 ohm RF loads available that I'm going to install for both connections - they happen to fit the tapped holes already, all I need to do is file the ends off a little since they are a bit too wide.This will make a good dual-RF dummy load, which has either a female N or male N connection available to use.

author: xrunner
author_rank: Super Contributor
timestamp: January 27, 2018, 12:00:52 am
content: And the saga continues ...So I ordered a used JFW 100W 40 dB attenuator from a US Ebay seller who had a big lot of them. When I got it today, I found that the female connector has a broken part as you can see in the picture. I messaged the guy and he gave me apologies and said his wife packed it and she got it out of the "checked bad" area. He said to toss this one and he's sending me two for my trouble.But I'm wondering if I can make a repair to this one. As you can see, the female center connector is missing one of it's fins or tabs. Of course I do not have another part, but I wonder if I could solder a tab on if I can get it off of another N connector?What do you think?

author: edpalmer42
author_rank: Super Contributor
timestamp: January 27, 2018, 04:17:42 am
content: I wouldn't try to replace the finger. Your solder joint will be under stress whenever the connector is mated and it won't hold. As it is, it might still work at low frequencies. You'd have to test it to see where VSWR starts to go bad.If you really want to try to repair it, think about cutting off the broken connector and solder a new one onto the post. Extra points if you drill a hole in the end of the post and grind down the outside of the new connector to form a pin that goes into the hole. Carefully clean up the joint after soldering so that there's no excess solder on the outside of the post.Remember that everything must beperfectlyaligned, both side to side and forward and backward or you'll damage any other connector you plug into it.Probably not worth the effort.One other thing I want to say is that runninganythingat 100% of its ratings would make me nervous. Even the terminators you're planning to install might not appreciate that. Does the datasheet spec the size of heatsink they need to reach their rated power dissipation? Does the body of this unit meet those criteria?Ed

author: xrunner
author_rank: Super Contributor
timestamp: January 27, 2018, 02:28:30 pm
content: Quote from: edpalmer42 on January 27, 2018, 04:17:42 amIt might not be worth the effort - true. I managed to take apart the connector further and obtained a spare pin from some junk parts. I don't think I can replace the entire connector accurately enough with the tools I have. But what I was thinking was cutting off one finger from the spare part I got. You are right in that it would be susceptible to cracking off eventually without reinforcement. What I want to try is to wrap around it with small copper wire about 6 - 8 times so that the joint is completely covered with this reinforcement. I think it will be a good mechanical bracing when soldered to the pin.QuoteWell it all depends on the duty cycle. I don't have data sheets for the RF load resistors, but I've used them before. They are rated for 100W if properly attached to a heat sink. You can see the final installation in the attached pic with the loads attached w/heatsink compound underneath. I have tested them with an HF transmitter outputting 100W, for periods of 20 - 30 sec or so, and the heat sink barely gets warm. It sinks quite a lot of heat. Keep in mind this is for testing for short periods of time - not for 100% duty cycle for hours at a time.

author: xrunner
author_rank: Super Contributor
timestamp: January 29, 2018, 01:09:49 am
content: Well, the repair was a FAIL. It just wouldn't have held up. But ... I asked the Ebay seller if he might be able to send me some out of his bad pile, that had failures other than the N connector being bad. He actually agreed, so if he sends me a couple of bad ones I hope to scavenge a female N connector from another one. Stay tuned.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 29, 2018, 01:51:33 am
content: That sounds like the best possible solution. He's probably ecstatic that he can make you happy by shipping stuff that he was going to spend time repairing.Ed

author: xrunner
author_rank: Super Contributor
timestamp: January 29, 2018, 03:00:24 am
content: Quote from: edpalmer42 on January 29, 2018, 01:51:33 amThis guy is really a good seller. He's really upset that I got a bad part. I offered to pay for extra shipping and a price for the bad parts units but he didn't want to hear it - he insisted that I get the parts units for no charge.I wish all the sellers were that conscientious.

author: Co6aka
author_rank: Unknown
timestamp: January 29, 2018, 05:08:39 am
content: Quote from: xrunner on January 23, 2018, 11:57:29 pmWhat does eBay have to do with "cheap" junk made in a dark alley on the wrong side of the tracks somewhere in China, or wherever? There's lots of quality stuff for sale on eBay, even some stuff made in China, but typically it costs quite a bit more; is that perhaps the problem?

author: xrunner
author_rank: Super Contributor
timestamp: January 29, 2018, 01:14:31 pm
content: Quote from: Co6aka on January 29, 2018, 05:08:39 amBecause it's sold on their site. They are empowering the selling of it, just as they are empowering the selling of very good items. So beware of cheaply made things sold on Ebay - because it a fact that it is there to buy. That's simply an objective fact.QuoteYou can't always tell by the price though. Sometimes you have to look inside and that's not always possible to see by the pics they post. So again my statement is good advice "BEWARE of what you buy on Ebay!"

author: xrunner
author_rank: Super Contributor
timestamp: January 30, 2018, 11:11:59 pm
content: I got a really nice package today. The seller sent me the two good 40 dB attenuators, plus two Mini-Circuits UNAT-10 1W 10dB attenuators and a JFW 50DR-046 rotary attenuator. Very nice of him to send these things.

author: edpalmer42
author_rank: Super Contributor
timestamp: January 30, 2018, 11:26:03 pm
content: Oh, wow! What a score! So, come on, share a name! We hear so much about sleazy Chinese suppliers, I'd love to have a good supplier that I could check first.Ed

author: xrunner
author_rank: Super Contributor
timestamp: January 30, 2018, 11:47:58 pm
content: Quote from: edpalmer42 on January 30, 2018, 11:26:03 pmBe happy to!f4techno