Using a cheap PC Power Supply for a NeXTstation

I performed the above procedure now twice, but my power supply only lasted 5 mins the last time. Either something else is broken, too, or the little chopper ceramic plate is cracked too much and too uneven now to make proper heat contact. Since Sony, who made the original power supply, decided to use these thick film modules, further repair is made impossible, especially since I could not find any source of these very special replacement parts. Therefore I decided to use a cheap (< DM 100) PC power supply for my NeXTstation. Obvious drawbacks are:

it won't fit into the NeXT housing and has to be placed outside
it does not support the electronic power on/off (but see below!)
less obvious is that the b&w-NeXT needs at least 2A @ -12V, during startup up to 3.3A @ -12V are consumed for a few seconds (for the monitor!), while most PC power supplies only supply 0.3A @ -12V. This does not apply to the color NeXTs, as the huge current on -12V is only consumed by the black&white monitor, which is powered by the NeXT directly (the monitor is also the one part that increases the +12V current to 4A).

Before you start, be warned: there are lethal voltages inside a power supply! Only do this if you know what you are doing! I am not liable for any damages you might incur. This is only information for the expert, i.e., a trained TV service repair man.

How to increase the -12V current rating

So first you have to increase the rating of the power supply at -12V (only for b&w NeXTs, not color NeXTs, where the monitor has its own power supply). Luckily, sometimes the same coil of the transformer is used for +12V and -12V and only the rectifiers are the current limiting parts.

I should be very clear here: to increase the 12V current rating for b&w-NeXTs you must have a type that uses the same coil for +12V and -12V!! Otherwise you could ruin your NeXT! And I learned that these (older) PC power supplies are hard to find these days. So be warned, not all PC power supplies are usable!

It is hard to give general instructions here since the PC power supplies vary a lot, so you are basically on your own. Replace the -12V diodes with stronger Schottky-type diodes. I used those in a TO220 housing (MBR1030) and mounted them to the wall of the metal housing of the power supply, for cooling. Of course you must mount them isolated as otherwise you would shorten out your new power supply. In Germany the company Bürklin has isolation kits, but I am sure Radio Shack will have them, too. You can also use axial Shottky diodes, e.g., SB530 should do. I my case I also had to bridge a low-Ohm resistor in the -12V path. It is a good idea to ramp up the -12V smoothing capacitor: use a 1000F, 16V capacitor in parallel to whatever is already in there.

Next, test your new power supply using lamps of the appropriate rating. Test it at full load (see specs of the old power supply above) and for a long time, I recommend one hour. Measure voltages and currents.

If all is fine, you have to cut the power connector from the old power supply, remove the old supply from the NeXT, solder new, strong (1.5mm^2) wires to it (use the hole of the former AC connector to get the wires outside), and connect it to your new supply. Make the wires as short as possible, best less than one meter. For isolation of the soldering joints I recommend "Schrumpfschlauch" (called "shrink wrap" or "heat shrink tubing": black rubber sleeves that you put on the wire, and then shrink down with heat). Watch out for the color coding of the wires: the NeXT uses a different scheme than most PC-type supplies:

line	NeXT	PC
+12V	red	yellow
+5V	yellow	red

Do not connect equal colors but equal voltages!! Look at the pinout of the power supply connector.

Reset/PowerGood for the NeXT

The one thing left is the PowerGood pin of the NeXT. I tried connecting it to the PowerGood outlet of my new power supply but it did not work. The NeXT expects a long (1 sec) low signal here while the PC power supply sets PowerGood to high much too fast. I am still working on this, but I hope that a simple 555-reset timer will be good enough. ...YES, indeed I tested it and it works just fine! Have a look at the circuit diagram (also as GIF if you can't handle tiff!).

Adding electronic on/off switching capability

Since I was tired of switching the supply on and off manually, I devised a simple circuit with triacs to switch the new power supply on and off electronically from the NeXT power button. Here is the circuit diagram I am using (works like a charm). The MUC3043 is a special zero-crossing opto-triac. This works for 230V AC line voltage, but could also work for 110V (I am not sure about the current rating of the triac).

Now I have the same functionality (keyboard power on/off, no manual intervention required) as with the old power supply. I leave my NeXT running almost all the time and the PC-type supply works without any problems now for over a year. Highly recommended!

The following might be interesting for NeXT-owners in Germany or Europe:

In case you are not the die-hard electronics fixer-upper with lots of experience, a friend of mine might be able to help you: if you send him your defective NeXT power supply, he will cut-off the DC connector, connect it to a PC power supply which he will modify so that it can be used for a NeXTstation (i.e.: -12V with 2A current, a reset signal suitable for the NeXT, electronic switching on/off via the NeXT keyboard). However, that new PC-type power supply will have to be placed outside the NeXT housing. I asked him how much he would charge for that and he said around DM 220 plus shipping charges (this includes the PC-type power supply, of course). If you are interested, send e-mail to me and I will relay your request to him.

Good luck!!

Helmut