[SCCC] Incredibly Noisy Power Supply

[Steve]

Pour yourself a cuppa and sit back, as this is kinda long...

I could ask about this on some computer forums; but those guys likely 
wouldn't have the vaguest idea what kind of noise I'm talking about, 
nor why I'm bothered by it. I'd likely get all kinds of suggestions 
and questions about my software, operating system, amount of RAM, and 
what-not, none of which figures here anyhow.

I have a fairly-new HP laptop (that my brother gave me) that uses a 
ubiquitous 19.5V 2.31 amp switching supply. When this switcher is 
plugged in and charging, the radiated noise level is so high that my 
S-meter reads S5 on 80 or 40 and S7 on 20. I haven't tried 160 or 15 
but I'm not nearly as concerned about those bands, as the bands I'm 
most likely to use at any given time are 80, 40 or 20.

The laptop connector isn't simply a two-pin coaxial type; it 
apparently has a battery sense connection inside the shell 
surrounding the center pin. I built myself a nice little regulated 
19.5 volt linear power supply and after testing and being satisfied 
it wouldn't go apes and kill my new laptop, finally got around to 
cutting the DC power cord between the switching power supply and 
laptop so I could splice in connectors to plug it into the new linear 
supply, only to discover there are THREE wires in there, not just 
two. That's when I carefully examined the insides of the connector 
and noticed the inner coaxial shell surrounding the center pin. After 
temporarily splicing the cable back together, I measured the voltage 
on the third blue wire (the two main DC wires are conveniently red 
and black), I found the blue wire to snap up to 19.5V and sometimes 
down to 0, but also sometimes some value in between, apparently 
depending upon what I was doing with the laptop and how much current 
it was drawing from the switcher.

Looking on epay, I find these power supplies available for a few 
bucks and extremely common. My brother's newer Dell laptop has the 
same power connector and voltage/current ratings on his switching 
power supply (which looks identical to mine except with different 
markings; it even has the same design strain relief on the DC cord). 
The connector seems to be 4.5 mm OD, 2.5 mm ID, according to the epay 
vendors. It's similar to, but not like, the power connector on my old 
Sony Vaio laptop, which DID have only the two power wires in its 
cable (and whose center pin is much thicker, as well as not having 
that center shell connection).

Of course, I've tried ferrite beads on both the AC input and DC 
output supply cables, to no avail. Thus, I conclude the switcher is 
likely radiating all by its lonesome (the laptop is almost dead-quiet 
in the radio(s) when not plugged in, with just a little bit of 
scratching noise when I scroll the screen).

Other than mounting that dang switcher inside a metal box then 
filtering the heck outta all wires, I can't think of what else to do 
right now. I'd love to use my custom-built linear supply but am now 
reluctant to plug it into the laptop, in case the laptop NEEDS some 
sort of signal on that third blue wire and otherwise goes apes or 
dies, itself. The switcher's plastic case is solidly glued together. 
I suppose I could buy an epay-special then break apart the plastic 
case to try to see whether that blue wire's connections might be easy 
to emulate; I guess that's my next step. I have little hope of being 
able to reverse-unjinere that switcher, however.

Would anybody happen to know what that third blue wire actually does, 
what signal it carries, and how to emulate it on a homebrew linear 
supply? Or maybe even know of an available linear replacement power 
supply?? Surely, I'm not the only character who's come across a 
filthy switcher for these newer laptops.

Thanks,
SteveH K0XP