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The cable technician installed my cable modem today, but brought the wrong adapter. He managed to force it into the modem, but I looked at the ratings and the modem says 12V 1.5A while the adapter says 12V 1.25A.

It's working, but could this damage the modem?

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The adapter may not provide enough current to the modem, which while unlikely to harm it could result in problems ( crashing etc. ) –  Lamar B Jan 12 '12 at 15:27
    
First rate technician, if it does not fit, Force It! It most likely damaged the dc jack in the modem. –  Moab Jan 13 '12 at 4:03

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up vote 5 down vote accepted

Using a correct-voltage slightly-lower-current power supply is unlikely to actually damage the equipment, but it may cause intermittent operating problems depending on the modem's power draw during various scenarios. After all, if the modem really only needed a maximum of 1.25 A, then why state a 1.5 A requirement on the modem? The main question here is how the power supply will behave when the current draw exceeds its rated maximum (poor rectification, outright shutdown, overheat, ...), which is nearly impossible to know without testing and some test equipment (a pair of multimeters and an oscilloscope will get you a long way), as well as how the modem will respond to that (which is very difficult to know beforehand - some RF equipment, which a cable modem is, can start to self-oscillate when fed unstable voltage, which can cause line disruption or in the worst case a blown-out final amplifier stage).

One important factor to consider is if the modem downloads new firmware over the network and installs it. What if the power supply acts up during that? While it likely won't physically damage the modem, it may cause EEPROM corruption leading to a bricked piece of hardware in a worst-case scenario.

Quite frankly, I would have been more concerned about the technician physically forcing the power plug into the modem receptable. That may have caused physical damage to the relevant circuit boards, which almost certainly is going to come back and bite you at some point.

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I see. Yeah, he really jammed it in there. I don't think I can even remove it. I think I'll try to get a new set sent out. –  Louis Jan 12 '12 at 15:42
    
I would have them send out a new modem, then send the one you have now back in basically as it is hooked up now (particularly with regards to the power supply cable). Include a note with the name of the technician who installed it, when he did the installation, and any relevant customer details (your customer number, full name and address, and/or whatever else may help them identify the case). Make sure to do it as soon as possible so he cannot reasonably claim to not remember doing it. –  Michael Kjörling Jan 12 '12 at 15:48
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Good points, and thanks for making me look at the work order. I see I was also charged double the stated installation fee! –  Louis Jan 12 '12 at 15:59
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@Louis That double charge was for the extra effort he expended forcing the adapter into the modem. –  Tyler Faile Jan 12 '12 at 16:10

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