Power Supply Repair

I just resurrected my black NeXTStation power supply and would like to share the info that I have on this subject with others, as to make it a bit easier for you if yours breaks down. (At the moment there are many posts on the net from people with failing power supplies. It seems that this is a really weak point of the NeXTstations. Given this fact, you might be better off if you do NOT just replace your broken power supply with a new Sony-type one, but use a more reliable PC-type power supply instead. Look at solution 1 below for more info!) All info is provided "as is", no warranty, no guarantee, no liability on my side for anything (e.g., electrocuting yourself or frying your NeXT).

I have to WARN you: I heard of people who followed these instructions and seriously damaged their whole NeXT computer! You can even damage your life, even kill yourself! These voltages are lethal! So, if you don't have lots of electronics experience, DON'T DO IT YOURSELF! Better buy a new power supply from one of the addresses below!

Here is my story: while I was working on my NeXT (black hardware) it suddenly turned itself off! Bummer! Needless to say: I could no longer turn it back on. Hmm.

First I thought it is the well known infamous boot parameter error, so I opened the station and removed the 3V Lithium battery over night to erase the boot parameters from the NOVRAM. I also made sure that the battery still has 3Volts (the power-on sequence is powered by this battery!), it was fine.

But this did not help: no power! So I suspected a fault in the power supply and I asked around the NET for help. I came to find out that the power supply was made by Sony Japan specifically for NeXT to their specs. For your info, mine was:

NeXT part number 1477
Sony model number APS-21
Sony part number 68-1120-51
and its specs are +5V: 7A; +12V: 4A; -12V: 3A.
I measured the actual consumption at -12V to be 2A.

It is 100V to 240V continuous input range and can be switched on and off electronically. So you won't be able to buy one at Radio Shack. And Sony could not help me either (I was looking for circuit diagrams and such and called them up. They told me that only NeXT has info, i.e., circuit diagrams, etc., on that power supply.). I found out that it is still possible to get replacement power supplies from various sources:

1) Use a PC-type power supply instead!

2) Used Sony power supplies

3) Samuel M. Goldberger

Spherical Solutions    
47 Myrtle Avenue
Mill Valley,
CA  94941
415-383-2919--voice
415-381-9556--fax 
E-mail: smg@orb.com

4) Pixelated Technologies

He can sell you the power supply for $150.00

James Moosmann
E-mail: moose@moose.pdial.interpath.net
255 Camelot Rd.	Salisbury
NC 28147
Phone/FAX: (704)633-8885

5) Bell Atlantic:

Bell Atlantic Computing Technology Services
Voice:	800 499 6398, or 800 848 NeXT
Fax:	510 732 3078

6) Sorbus Deutschland - NeXT Hardware Support

Willstätter Straße 13
40549 Düsseldorf

Tel. 0211 - 52 61 - 01
Fax. 0211 - 52 61 - 142

7) dancing bear:

Actual repair instructions

I just recently learned that the procedure outlined below is not applicable to all NeXT power supplies. Apparently there are some revisions which use an aluminium plate instead of the ceramic plate for the copper MOSFETs. In that case the MOSFETs are covered with a thick white blob which is almost impossible to remove. If you own that type of supply, you will have a very hard stand, to say the least.

1. First you have to open it: Unscrew three screws (two "golden" ones and the one on top, see red arrows) to carefully remove the top lid.
   
   Attention: the low voltage power lines are held with a little plastic thing (green arrow), which notches into the lid part. Careful with the PCB which is attached to the top lid! You can slide it out of its holding brackets while you hold down the restrainers. If you were not careful and ripped off the three wires that connect the little daughterboard, you can see in my drawing in which order you have to attach them again.

2. Remove the two plastic covers.
   

3. In my view the power supply is built very shitty and its construction makes repairs very hard: There are 4 PCBs inside, one even attached to the top cover! One of the PCBs is the aluminum ground plate; due to its (intentional) high heat conductivity it is almost impossible to solder on it with a normal (60 Watts) soldering iron. In addition there are 4 thick-film circuits, which are sealed with thick, black stuff. And most of the circuity is covered with ugly grey stuff which is very hard to get off (and you have to remove it to exchange parts). And then several parts (resistors) are GLUED in various places. Argghh.

4. There is a fuse in the power supply. It is a 3.15A T fuse, but it is hard to replace (mine was blown). Due to the 100-240V input range, this fuse is just a last resort to protect the main fuse in your house from blowing. Therefore, once this fuse is blown you can suspect that something else is wrong with your power supply.

5. The PCB closest to the 110/220V connector (I will call it the "line board") holds the fuse (red arrow, fuse is missing in picture), some capacitors, and the rectifier. The fuse is usually blown and needs to be replaced.
   
   You have to remove the PCB to get to the fuse because it is held in a position with all its parts pointing "inward". Oh well.

   To remove the line board (but you do not necessarily have to do this, see alternative method below!) unscrew the two screws (blue arrow) on both sides of the power supply.
   
   Then you have to move the board upward and at the same time unsolder the two connections to the lower PCB board: these can be red/black wires or two metal posts, see red arrows in image!
   
   You don't have to unscrew the "golden" screws or unsolder anything else.

   Alternatively, you unscrew the four screws at the bottom of the lower board (the aluminium heat sink plate) and the two on the low voltage end, bend the two metal sides apart, unsolder the two connections to the main board and slide the main board just out. You then end up with this configuration
   

   One afterthought: While I always thought the reason for the failing of the MOSFETs is insufficient cooling, a reader of this page, Gerrit Heitsch, suggested that the primary capacitors (those that are charged to about 155V (USA) or 325V (Europe)) might be the culprit. He says the electrolytic capacitors tend to dry out and loose some of their capacity, which will then increase the ripple on the rectified line voltage. So while you are at it, either check their capacity or use an oscilloscope to assess the ripple on the primary DC. Again: these are very lethal voltages and the capacitors hold their voltage for quite a while - enough to kill you, even if the power supply is unplugged!!!

   I just measured my C (nominally 150 uF, 400 V) and it has 145 uF and 0.15 Ohm ESR. I think these values are not too bad and I would thus not assume that the C is the culprit. Since this time my NeXT died while running continously for several years it is probably a design flaw that overloads the IRF740 MOSFETs. But it could also be a cooling problem, as I noticed that my power supply was not really flush in contact with the aluminium base: when I mounted it the last time, I didn't hook one of the two hooks in properly. So it could have been my fault!

6. The rectified 110V/220V voltage is then routed through a black (minus) and red (plus) wire (or through two metal posts) to the main circuit board. If not already done, de-solder these wires (see red arrows above) from the line board to be able to further disassemble the power supply.

7. I pretty soon found that the two chopper MOS FETs (type: IRF740, UF740 shozld also do) were blown. They are easy-to-get standard parts (about $3 each). The problem is how to remove the defective ones?!

8. The choppers are mounted on a little ceramic board (red arrow),
   
   which itself is attached to the ground aluminum plate.

   If you want to remove the low voltage plate (you do not have to do this!) then you have to unsolder the 9 connecting rods (green arrows) and the 3 ground pins (blue arrows). You do this with a suction pump to remove the solder. For reference, you see here
   
   the resulting low voltage PCB with the dangling PCB.

   Now the following I found out the hard way and actually broke my ceramic plate:

   1. The MOS FETs are *soldered* to copper areas on the ceramic plate.
   2. The ceramic plate is soldered on 6 points to copper strips on the aluminum plate; there is heat conducting compound between the ceramic plate and the aluminum plate.
   3. There are copper connections on the ceramic plate (source pin of one chopper to drain of the other).

9. To remove the MOS FETs I suggest the following (remember: I tried a different approach, since I did not know all that yet, and broke my ceramic plate. Try to avoid this!):
   1. bend away the black thick film module to gain better access to the FETs.
      
   2. with a very strong soldering iron try to de-solder the two connections marked with red arrows in the above picture of the ceramic chopper plate and bend it slightly upwards. This is the most difficult step!!
   3. Once you have it partially removed, de-solder the broken FETs from the ceramic plate (this is now easy since the aluminum plate is no longer removing the heat which you try to supply with your soldering iron). Put some spacer (I used a piece of wire, see 3rd picture) under the ceramic plate to keep it from touching the aluminium plate (and loose heat to it).
      
   4. cut the excess metal fins from the new IRF740s to match the shape of the old ones. If you have it in a TO-220 package, you also have to cut off the fin with the mounting hole. Use sandpaper to smoothen all edges and to remove oxid layers so that later the solder will creep between the MOSFET and the ceramic plate.
   5. clean the ceramic plate as good as possible! Use alcohol to get rid of flux and dirt. Use a Q-tip to apply it. Remove old solder with a desoldering wick.
   6. re-solder the new IRF740s to the ceramic plate, the same way the old ones had been placed. Connect their pins (cut the middle (drain) pin; that connection is made through the body of the FET).
   7. apply additional heat sink compound to the area on the aluminum plate where the ceramic plate goes, before you place the ceramic plate in again.
   8. solder the two connections of the ceramic plate to the copper wires on the aluminum plate. Use a STRONG iron!
      

10. Now bend the thick film circuit back, reassemble everything, don't forget the plastic covers, re-solder the black and red wire, screw everything together.

11. Before you reconnect everything to your NeXT, I suggest to test the power supply: connect the power supply through a current limiting device (e.g., a 60W incandescent lamp) to your line voltage and make sure nothing happens when you power it on. Then connect lamps of the appropriate ratings (e.g., 12V 5W lamps to +5V, +12V, and -12V and ground, respectively) and remove your current limiting device. Apply a +3V voltage (e.g., from two 1.5V batteries in series) for 1 to 2 seconds (not too long since now the cooling fins and the fan are missing!) to the startup pin, usually the pink or brown wire, pin 11. Look at the pinout of the power supply connector. Use correct polarity: plus on the pin, minus on ground! The light bulbs should light up and go out after you remove that voltage. Nothing should blow :-)

12. Apply heat sink compound to the outside of the aluminum base plate and to the mating part of your NeXTstation to improve heat conduction here (and to avoid loosing your chopper MOS FETs again). Reassemble the power supply into your NeXT, pray, and switch it on.

Well, I have done this now to two power supplies (PS), but I was not as lucky on the second one: there is an active power factor correction circuit in there as well. And the diode is likely to also get shot (short circuit) when the MOSFETs go. This was the case here. You see the red arrow pointing to the defective diode. I had to clip it where the green arrow points to.

I replaced it with a very fast (3A, 600V, 50ns) recovery diode MURS360 SMD from Reichelt for 0.23 EUR.
Interestingly, the PFC starts to work at about 50V AC input. It keeps the big capacitor charged to about 382V all the time.

Now the power supply seems to work again, but it gets quite hot already with minimal load.

Here now the drawings:

Just for clarification: pin 11, the +3V startup voltage, comes from the lithium battery inside the NeXT (via the keyboard power-on switch). Once the power supply is running, the NeXT then increases this voltage to standard TTL +5V level. This pin is always an input to the power supply.

The "power good" pin is normal TTL level: +5V if power is good, 0V to reset the NeXT.

Please note: all the colors mentioned in the drawings are as I found them on my black&white NeXTstation. On PC power supplies the color coding may vary: on PC supplies the +5V is usually run through red wires and the +12V through yellow wires (just opposite as on the NeXT and opposite to the drawing)!! You have to use caution and common sense (and a volt-meter!) to make sure you connect the correct voltages to the correct pins on the NeXT - don't simply rely on the colors of the wires!

Ceramic chopper plate.

Using a cheap PC Power Supply for a NeXTstation

• 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 NeXT needs at least 2A @ -12V (for the monitor!), while most PC power supplies only supply 0.3A @ -12V.

How to increase the -12V current rating

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 1000uF, 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

Reset/PowerGood for the NeXT

Adding electronic on/off switching capability

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!

Adding a fan supervisor

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

Good luck!!

Helmut