Hack in an external power supply - Page 1
Forum section: electronics / projects

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: October 31, 2017, 02:30:27 am
content: I have an Arris branded cable telephone modem (used to connect POTS phones to the cable company's telephone service). It's power supply is utter crap and radiates tons of RF interference, both on the AC line and the incoming coax, negatively impacting my amateur radio station. (I see birdies across 3.5MHz up through 15MHz or more.)I could try to choke the RFI off like I did with the cable modem, by using 17 turns through a mix 31 ferrite torrid on both the AC line AND a separate 17 turns through a separate torrid on the incoming coax line. This did succeed in getting rid of all the noise from the data cable modem (the other black box from the cable company to get internet out of the cable connection). I am out of torroids, though. I figured the better solution would be to replace the power supply in the phone modem. I am not as concerned about modifying it, as I don't have to worry about it impacting data rates, just the phone.I am trying not to permanently modify this too much, as it technically belongs to the cable company.I was able to measure the voltage on the output of the power supply (on the filter cap) at 10.4V. I tacked some wires onto the main filter cap and am feeding 10.4V in there. It seems to be working fine. I was just curious if I needed to worry about somehow disconnecting the built in SMPS. I obviously am no longer using the AC line. I do not have a schematic, and it is a bit of a pain to reverse engineer because it has a four layer PCB which is very opaque. The only connections over to the mains part of the board are a single transformer and a couple optocouplers. The board has isolation slots around the HV section.Can you think of any problems feeding D.C. onto the output filter caps without disconnecting any of the original supply components? So far it is working great.Thanks.

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: November 01, 2017, 10:55:32 pm
content: Still working. Though I think I will switch to an isolated supply.

author: grifftech
author_rank: Frequent Contributor
timestamp: November 02, 2017, 09:05:06 pm
content: can you just unplug it when you are using your radio?

author: jbb
author_rank: Super Contributor
timestamp: November 02, 2017, 10:51:06 pm
content: I recommend external toroid chokes. It’s boring but won’t get you in trouble.

author: jbb
author_rank: Super Contributor
timestamp: November 02, 2017, 10:53:24 pm
content: Oh thank goodness.I had a horrible feeling that my phone might have “corrected” that to “torrid.”

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: November 03, 2017, 02:05:37 am
content: Can't really unplug it when using the radio. That would mean disconnecting the phone line.It would take at least $15 worth of torroids (2.5" ID mix 31) to choke it off, and even that doesn't get all of it. It radiates on all bands, and that is 17 turns in order to get meaningful impedance at 3.8MHz. It radiates both on the incoming AC line and the incoming cable tv coax (common mode on the outside of shield). If I add re torroid (2 on the coax) that might get it all, but now we are at $21 (plus $5 shipping from Mouser) worth of ferrite. My hack took five minutes and two cents worth of wire. I had the external 12V power supply already.It is still working great. So I guess this solved it.Still interested in thoughts on the technique of just feeding voltage onto the filter cap without disconnecting the internal SMPS.

author: jaycee
author_rank: Regular Contributor
timestamp: November 03, 2017, 10:16:15 am
content: It should be fine. There will be rectification diodes on the transformer's secondary, and these will effectively be reversed biased so the supply is "isolated"I note though if it is emitting this much EMI, there is probably something wrong with the design... especially if it's radiating noise back down the CATV cable!

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: November 03, 2017, 04:14:27 pm
content: There definitely is something wrong with the design, but this is common today, especially with SMPSs. Designers and/or manufacturers cut costs and do not properly shield or filter their designs. Once you get into Radio you come to find just how many Part 15 devices should be thrown in the trash. I never knew there were so many bad designs.The data cable modem from SMC has the exact same issues, poor RF isolation and filtering with an internal SMPS. In fact, having taken it and the Arris cable modem apart I wouldn't be surprise if they were made by the same company-- many similarities. It also radiates hash down the incoming coax.Technically no user may operate such devices if they interfere with a licensed station (the onus is on the user, which is why everything comes with that little Part 15 disclaimer these days). However, it takes a lot to get the FCC to start threatening people the $10,000 a day fine that goes along with that. They have been known to do it, but I don't know if they have ever actually collected the fines.Also, just because you presumably pass the FCC testing for the product doesn't mean the product can be used by the consumer in every particular installation. It can, and often does, still interfere with stations.