My very first modem was an Avatex 1200 connected to my trusty C64. Unfortunately, the power adapter has long since been, let's say, disassociated from the hardware. I probably still have it, but the trick is that there are no markings on the modem, nor can I find any details in the user's guide, that explicitly state what voltage and polarity the modem expects.

Usually this information is documented somewhere, but the only thing in the user's guide is "Power: 120 VAC, 60Hz, 8W max, with supplied power adapter which is UL/CSA approved".

I do have an adapter that I think has an above average chance (say, 55%) of being the right adapter, mostly because of it's location where I found it, and because it does physically fit the modem. It outputs 9v 200mA center positive, which is very possible, but I don't want to fry this modem giving it the wrong voltage or polarity.

Does anyone have one they could look at to confirm the specs? Or even a good photo of the adapter that is readable?


I did find one very grainy photo of the power adapter where I think I was able to make out "Output: 12v AC". Working on that info, I went back to where I found the 9v DC supply and indeed, found a suspicious 12VAC supply that would also fit the modem. So now, I have two possibilities.

  • What about the good old "Find the device first and look at the bottom and/or near the plug?" method?
    – Raffzahn
    Commented Sep 13, 2023 at 23:32
  • Yes, I looked all over the modem, especially near the input power jack and nothing. It's only labeled 'adapter'.
    – Geo...
    Commented Sep 13, 2023 at 23:35
  • Dir read a bit like the modem is as well not available. Well, then opening and looking about polarity of diodes etc. might be the fastest way, wouldn't it? Also: thecomputerarchive.com/archivemain/…
    – Raffzahn
    Commented Sep 13, 2023 at 23:45
  • This is just a guess, but a modem of that vintage, I would say that for many reasons, it is likely to use an AC supply than DC. It was just so much simpler to build a power supply that takes in AC and generates the internal positive and negative DC rails from that, than to use a DC input and generate internal positive and negative rails with a DC/DC converter. It would help if you open the modem and take pictures from the power supply and RS-232 level shifter section.
    – Justme
    Commented Sep 14, 2023 at 7:55
  • Thanks guys. I got lucky and found the answer, the details are in my post below.
    – Geo...
    Commented Sep 14, 2023 at 10:09

1 Answer 1


I did some googling and didn't find much of value, and then I got very lucky. I found a for-sale listing for very close variation of my modem (1200 vs. 1200hc) that showed the power adapter.

Having found a suspicious 12v AC supply, and a photo clearly showing the supply as providing a 12v AC output - I decided to go for it. The modem powered right up with no problems.

I am posing the photo of the rear of the modem and the power supply for future reference.

Here is a rear view of Avatex 1200/1200hc modem where you can see the power input is simply labeled 'adapter'.

Rear view of Avatex 1200/1200hc modem

and here is a cropped image taken from the appropriate power adapter which clearly details the output is 12VAC.

Power adapter details

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