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!

Given the fact that the Sony power supply inside the NeXTstation seems to be a real weak point you might be better off using a PC-type power supply. 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 (see below: -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). At the moment he has about 10 power supplies rigged ready for the NeXT, so delivery should be fairly fast (once he gets your old power supply). If you are interested, send e-mail to me with your phone number/address and I will relay your request to him.

2) Used Sony power supplies

A lot of used NeXT stuff can be bought at http://www.deepspacetech.com/. The price plus shipping ($50 to Europe) was $49 for a used replacement power supply.

3) Samuel M. Goldberger

He once had the cheapest offer and in general was very nice and helpful!
New power supplies are $135 each, plus shipping ($50 to Europe).

Spherical Solutions    
47 Myrtle Avenue
Mill Valley,
CA  94941
E-mail: smg@orb.com

4) Pixelated Technologies

1-800-749-3563 & 310-459-6831

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:

USA hardware service has been purchased by Bell Atlantic. They will be supporting the Authorized Service Centers and are selling extended warranty contracts.
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
I called them, but their price was about 700 Deutsch Marks, which is PRETTY expensive.

7) dancing bear:

no longer in the NeXT business

So, instead of just buying a new one ($135 from Sam sounded very reasonable to me!) I thought, oh well, I can't more than break it and so I decided to open it up and have a look for myself. If I can't repair it myself, I can always buy a new one (the power supply itself is very easy to replace; that can be done by anyone). I found a couple of interesting facts which I would like to share with all of you:

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 while 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.

0) First you have to open it: 
   Unscrew several screws to carefully remove the top lid. 
   Attention: the low voltage power lines are held with a little plastic 
   thing, 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, but you have to hold down the restrainers.
   If you were not careful and ripped of the three wires that connect
   the little daughterboard, you can see in my drawing 
   in which order you have to attach them again.

1) 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.

2) 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 (be very careful when take 
   away the top lid, as you can easily rip off the wires from this 
   little top board!!)! 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.

3) The PCB closest to the 110/220V connector (I will call it 
   the "line board") holds the fuse, some capacitors, and the 
   rectifier. 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.

4) The rectified 110V/220V voltage is then routed through a 
   black (minus) and red (plus) wire to the main circuit board. 
   De-solder these wires from the line board to be able to 
   further disassemble the power supply.

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

6) The choppers are mounted on a little ceramic board, 
   which itself is attached to the ground aluminum plate. 
   Now the following I found out the hard way and actually 
   broke my ceramic plate:
6a) The MOS FETs are *soldered* to copper areas on the ceramic plate. 
6b) 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.
6c) There are copper connections on the ceramic plate 
    (source pin of one chopper to drain of the other).

7) 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!):
7a) bend away the black thick film module to gain better access 
    to the FETs.
7b) de-solder the pins of the FETs from the ceramic plate.
7c) with a very strong soldering iron try to de-solder the 
    ceramic plate from the 6 points where it is soldered to 
    the copper wires on the aluminum plate. This is the most 
    difficult step!!
7d) Once you have it 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).
7e) cut the excess metal fins from the new IRF740s to match 
    the shape of the old ones.
7f) 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).
7g) 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.
7h) solder the 6 connections of the ceramic plate to the 
    copper wires on the aluminum plate. Use a STRONG iron!

8) 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.

9) Before you reconnect everything to your NeXT, I suggest to 
   test the power supply: connect lamps of the appropriate 
   ratings (e.g.,  12V 5W lamps to +5V, +12V, and -12V and 
   ground, respectively). Connect the power supply to your 
   line voltage, then 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 :-)

10) 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. 
Good luck, you are completely on your own, I decline any warranty or any responsibility. You should have soldering practice, a calm hand, and patience. You risk loosing your NeXT completely, may be even electrocuting yourself or other persons. If you are not completely sure you know what you are doing, buy a replacement module, that is MUCH easier and you don't risk anything.

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 400V) 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!!!

Here now the drawings:

Pinout of the power supply connector.

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.

Read here how to avoid all of this hassle by using a PC-type power supply for your favorite NeXT :-)