Old Modem AC 9v Power Supply.

[mwd]

Hey Guys,

Hope this is ok to post.

I have an old external 56k modem I want to get working. I don't have the power adapter.

It says it takes 9volt AC but I feel a 9volt DC adapter may work, something about old modems had AC adapters because it was cheaper but they still run DC internally.

I can open up the modem and attach photos.

Thanks

[Buriedcode]

Yeah, post a quick photo of the board - preferably where the power connector is.  A bunch of power diodes and caps, or an integrated rectifier would pretty much answer this.  There are some cases with AC input is preferable - such as generating higher voltages from diode multipliers, but its certainly not common.

[SeanB]

9VAC because it half wave rectifies it inside, to get the higher positive and negative voltages to drive the RS232 interface. Then a regular bridge rectifier, that is then regulated down to 5V with a 7805 regulator or two in parallel. The AC is needed to operate. Look inside you will find the AC input, the power switch, then a bridge rectifier and a 1000uF 16V capacitor, plus another 4 470uF 25V capacitors and diodes, used to do a doubling of the voltage, and a negative doubling, to get a pair of 18V rails, that are regulated to + and - 15VDC rails with another pair of voltage regulators, or in some a pair of zener diodes and transistors.

[mwd]

Hey guys,

Thanks for the replies! I haven't worked out how to get into it yet but I've found these docs:

https://fcc.report/FCC-ID/H52PT-3020/80190.pdf - Block Diagram
https://fcc.report/FCC-ID/H52PT-3020/80188.pdf - Inside Photos (bad quality)

https://fcc.report/FCC-ID/H52PT-3020

EDIT: I've ordered an HT802 ATA with the plan to setup 2 modems to communicate with each other. The other modem I have is a Netcomm MyModem (previously Banksia MyModem) which is very similar to the first modem I ever had, so that is cool.

[Zero999]

It'll probably work from a 9VAC or 12VAC square wave. Use a 12VDC power supply and an a self-oscillating circuit made with a few BJTs.

Here's a circuit origionaly designed for 24V, but it will work with 12V.

[magic]

Sometimes a low power audio amp can be used to power such devices in absence of a suitable transformer
At least on the bench. For long term use, it shouldn't be hard to find a suitable wall wart.