View Full Version : Modem power supply specs??

March 5th, 2006, 11:46 AM
I have a few old modems by USRobotics that don't have power supplies. Does anyone know the voltage, polarity of it? I have two different kinds, the 28.8 and the 33.6 speeds, but they seem to be identical in appearance. They look just like this one:


The modems seem pretty common but I can't find the power specs for it. Anyone have one of these???


March 5th, 2006, 02:18 PM
I have that same USR 28.8 Sportster modem. The AC adapter output is 20vac. You read that correctly, it is AC.

Don't have a 33.6, but I do have a 14.4. The AC adapter output for that one is 9vac.

I also have a USR Courier HST 9600, and though I don't have the adapter readily at hand, I'm pretty sure that it has AC output as well (on a DIN plug).

If you can't get a definitive answer about the 33.6, I'd try the 9vac first. If that doesn't quite work, then I'd bump the voltage up in small steps until it did work, but I wouldn't exceed 20vac.


March 5th, 2006, 11:16 PM
20v AC! I just wonder what is the purpose of having such high voltage in a device that'll run at 9. Thanks, got it recorded, now time to go dig one up. Would they all be AC?

March 6th, 2006, 07:35 AM
Found it! Someone had conveniently posted the voltage and amperage on ebay in a sale fo the same item.

9VAC, 1000mA

There's a 195 page manual that goes with this modem, it's big and I can email it, 2mb, to who wants it. It'll come from my yahoo address so I can send it from school. 2mb is a pain on a dial-up.


March 6th, 2006, 10:29 AM
I went to the US Robotics web site. They have some legacy support, but nowhere in their specs or manuals (yes, a lot of install manuals for those 28.8 modems still are online) was it mentioned about the power supply specs, so it was good that you found it elsewhere.

Micom 2000
March 7th, 2006, 05:23 PM
20v AC! I just wonder what is the purpose of having such high voltage in a device that'll run at 9. Thanks, got it recorded, now time to go dig one up. Would they all be AC?

I've found that many telephone devices use AC; modems, Fax', answering machines, as well as full-featured phones. There must be a reason ?

As for the 20v DC you must remember that only 1/2 of that is available as DC 10v+ or 10v- due to the changing polarity. Perhaps there is also some V-drop as well. If you use a multimeter measuring DCV on NA home circuits for example you only see 1/2 the 110v standard because of the set polarity of your measuring device. Otherwise the resultant would be 0v. I imagine most wall-warts use some form of diode-blocking or coil or capacitor coupling.

If you examine most(all?) sterios they use some sort of coupling and isolating device to run the speakers. The huge coils in older quality Amps are reflected in the cheaper capacitor coupling of the small transistor radios.
Most audiophiles swear by the greater fidelity of coils and of course analogue devices like tubes(valves in the UK).
An admitted can of worms debate. :^)


March 8th, 2006, 03:01 PM
I was toying with the idea that phone signals have to be modulated with the same phase as the electricity, therefore AC input would allow the unit to sample the frequency. But of course, I know nothing about this stuff, so it was just a crazy guess.

March 8th, 2006, 07:27 PM
I do believe that all USRobotics external modems use AC power in, there must be some underlying logic to it. Perhaps it made the adapters cheaper or more reliable to leave out the rectifiers. Or maybe they wanted to ensure that you used only the genuine USR adapter, which could explain the AC adapter AND the DIN power plug on the Courier HST line of modems.

Micom 2000
March 9th, 2006, 11:01 AM
Well I have a couple of Bell wall-warts from a PBX installation that I scooped out of a dumpster many moons ago and they were also 18v. AC. There must be some reason for it.


November 5th, 2006, 07:14 AM
The reason for AC is simpply that it is cheaper to provide an AC power supply. As soon as the power gets into the unit it is imidiately converted to DC. Electronics do not run on AC!!

when a user looses the power supply which is basically a simple transformer it is less costly to replace that than a switching dc powersupply like what you would see powering a laptop for example which can cost upto $100 or more to replce.

Hope this helps.