How to repair a broken SMA cable without replacing it? - Page 1
Forum section: electronics / repair

author: Bicurico
author_rank: Super Contributor
timestamp: May 26, 2022, 06:58:08 am
content: Hi,I have a field meter where an internal SMA type cable is broken. The center conductor is broken at one point and the shielding/ground around it, is broken too.The cable cannot just be replaced, because it comes out of a sealed switching box (which selects if the input goes to tuner A or tuner B). Opening this box would be a pain, as it is soldered on all the corners and has point soldering of the plates on top of it.So, my idea is to clean cut the cable, strip it, solder the center core, put some shielding and the solder the GND mesh around it with a blob of solder.Has anyone a better idea?Thanks,Vitor

author: Berni
author_rank: Super Contributor
timestamp: May 26, 2022, 07:23:21 am
content: Just buy a new SMA connector, cut the cable and put the connector on the end. If you need the cable longer you can add a SMA extender cable on the end of it.Splicing coax cable in the middle using soldering is going to have poor RF performance because you mess up the impedance of the line.

author: Bicurico
author_rank: Super Contributor
timestamp: May 26, 2022, 05:42:46 pm
content: The cable seems to be ruined at more than one point.I will need to completely replace it and for that the tinned switching box will need to be opened.After soldering the broken point on the cable I got mainly noise.Thanks,Vitor

author: Bicurico
author_rank: Super Contributor
timestamp: June 03, 2022, 09:43:43 am
content: Finally got to do the repair.I figured out that something was wrong/broken in the "RF switch box". This is a module in a metallic box where the RF input is soldered to and where two cables come out: one for the SAT-Tuner and one for the terrestial tuner (DVB-T, DVB-C, analog TV and FM radio). This box switches the RF input between the tuners and provides output power for LNB and antenna amplifiers.Connecting the external signal cable directly to either tuner worked, but inline power was missing, so an LNB had to be powered externally.The repair took so long, because I did not know how to open the RF switch box. It was soldered all around. Last night I figured that I could open it with a knife and basic brute force. The solder would give in and I finally got the case open.I found that the original cable to the terrestrial tuner was actually not broken (just the shielding was broken and I could have repaired it). The problem was something totally stupid: the RF input connector is soldered to the PCB inside the switching box and this solder was actually broken! By bridging it again with a new clean bit of solder, the device started to work again.But by this point I had to replace the cable going from the switching box to the terrestrial tuner. I sacrificed a cable and soldered it in, replacing the original one. After assembling all together again, the device is 100% working again!Conclusion: this device has a bad design. Because the input connector is directly accessible to connect cables to the field meter, it suffers stress and this ultimately cracked the solder point. There should have been an internal cable from the switching box input connector to a RF input connector on the device with a robust connection and which is easy to replace.Anyway, I am happy, as I have yet another field meter in working condition in my collection. This was actually bought broken for a low price, so that makes me twice as happy!Cheers,Vitor