Amiga Power supplies, diagnosing problems and upgrading

Version 1.06
2nd August 2008 by Ian Stedman (ICStedman@techie.com)
Now with information on purchasing an ATX adaptor board for A500/A600/A1200.

 

Introduction

 This document details how to diagnose power supply problems on the Amiga series of computers. It also details how
to use a PC PSU as a higher power replacement for the Amiga and the necessary steps to make your own connecting cable.
This document has been updated to include details on the A2000/A3000/A4000 and some PCI/Zorro bus boards,
I have not personally verified the information in these categories, they were written from information obtained and cross
 referenced from numerous sources.

This document primarily focuses on how to adapt an ATX PSU for use on your Amiga with reference material on the older
AT PSU, this is for good reason, if you go out to buy a new PSU today it will be an ATX form factor. I have used ATX PSUs
 a number of times outside of a PC and have always managed to get them to work, so I will attempt to pass on some of my
experience here.

Contents

PC Power Supplies as replacements/upgrades
 AT PSU
 ATX PSU
Amiga ATX PSU adaptor for A500/A600/A1200
ATX PSU colour guide
 Rewiring your Amiga power lead (with Photos) for the A500/A600/A1200
Adding a new PSU to a CD32
Bypassing the CD32's power switch
Connecting a new PSU to an A2000
Connecting a new PSU to an A3000
Connecting a new PSU to an A4000
 Power Supply FAQ
 Dealing with power supply issues
Stock Amiga power supply ratings
 Power consumption figures for various Amiga models
Diagnosing power supply problems
References
Glossary of terms

 

PC Power Supplies as replacements/upgrades

 It is quite common now to replace the original, under rated original Amiga PSU with a PC one, with
PC PSUs power ratings from 135W to over 500W it is easy to see why.

There are minor differences in using an AT or ATX PSU to power an Amiga and later on I will
explain the wiring.

 

AT PSU

 Not really used in PCs since 1998/99 due to the PC requirement that software can switch
the PC off (or a Microsoft plan to stop their software crashing).

Normally found in old/dead PCs (they have some uses). To hook it up to your Amiga you will
need to note wiring of the square DIN plug (check the underside of the PSU) and then cut it off.
Commodore never seemed to use the same colour code in all Amiga models so you will need to check
your wiring with a multimeter. See the power connector section for more details.

Once you have wired up the Amiga cable, connect a 21W car bulb across the +5V (RED) and 0V (BLACK)
wires. This will provide a suitable load to turn on the PSU and you can then ensure you have the
right voltages going to the right pins before plugging into your Amiga.

If all seems well, connect to your Amiga and switch on!

Important note. Modern PSUs have what is called crow bar protection, if you short it out or
wire up backward, to cause a large power surge, it will switch off. Don't count on this, double
check your wiring.

Some AT PSUs had the mains switch inside the PSU housing, that is fine. Some had a mains lead
that went to the power switch on the front of the PC case. If possible avoid this type unless
your Amiga case has a suitable power switch.

Note: from my research I found that you need to connect the Orange power good signal, to one of the
 +5V wires for this PSU to work properly.

 AT PSU wiring colour code

Colour	Voltage
RED	+5V
BLACK	Ground
YELLOW	+12V
BLUE	-12V
WHITE	-5V
ORANGE	Power good, +5V

 Final note, do not open a PSU case. Due to their operation, a typical computer power supply can have
up to 400V DC stored within, for a considerable time after power off. If you inadvertently touched the
wrong part you could be killed!You have been warned, if in doubt, consult a technician .

ATX PSU

This is the type of PSU found on all modern PCs and within cases. There are 2 important differences
to the AT PSU. The following conditions must be met.

1) You need to have about a 1 Amp load on the 5V supply for it to work.
2) You need to connect the PS-ON line to ground for the PSU to switch on.

Condition 1 is easily satisfied, an Amiga uses enough current on the 5V supply.

Condition 2 gives you 3 options.
1) Connect a toggle switch or similar latching between the PS-ON wire and ground. This may not suit you if your Amiga
  is in a tower.
2) If your PSU has a mains power switch, hardwire the PS-ON and ground wires together and use the mains switch.
  This is what I do with my A1200.
3) Use the circuit shown below to utilise the circuit shown below.

It is possible to create a simple circuit to use the push to make power button of
an ATX case. This is the circuit you need:

 
How to implement this

Amiga ATX PSU adaptor for A500/A600/A1200

I have designed and built a small PCB with the above circuit on it. It contains the 7474 D-type flip-flop and the ATX power connector together
with suitable screw terminal connectors for the Amiga PSU lead.

Click here to go to the order page

Here is a picture of the PCB, it is a simple board.


To use this simple connect your Amiga PSU to the screw terminals and the ATX case power switch to the pin header.



The feet are self adhesive for sticking the PCB inside the Amiga.

Schematic of this adaptor, click image for full size,

The PCB design and schematics are available here ATX_adaptor.zip 83K .

ATX PSU wiring colour code  

Colour	Voltage	Pin(s)
RED	+5V	4,6,19 & 20
BLACK	Ground	3,5,7,13,15,16,17
YELLOW	+12V	10
BLUE	-12V	12
WHITE	-5V	18
GREEN	PSU-ON (active low)	14
ORANGE	+3.3V	1,2,11
GREY	Power OK	8
PURPLE	+5V standby	9

ATX PSU pinout as viewed looking at the plug

Obtaining connectors

 With the exception of the A500/A600/A1200 with their specific square DIN plugs you can still purchase
power connectors for the other Amigas. In this section the relevant part numbers and more importantly suppliers, are listed.

Suppliers:
RS Components http://rswww.com
Rapid Electronics http://www.rapidelectronics.co.uk

ATX connectors
The ATX connectors are from the Molex Mini-Fit Jr. range.

If you are making a wired lead (with no PCB) to adapt an ATX PSU to an Amiga, you will require this connector.
20 way dual row panel mount plug  RS 418-0150
Molex part number 3901-2201
You will also require the crimp contact, RS 324-8003 or Rapid 22-3850
These are expensive (£11 for 100)!

If you are connecting into a PCB or wish to (carefully) solder onto the tabs, the part is:
20 way dual row vertical header RS 172-8923

I have personally bought and used all these connector types successfully.

Amiga A500/A600/A1200 connectors

As this is a connector unique to Commodore, I advise you to chop the lead of your old PSU and wire it up to your new
 PSU using the guide later on.

Amiga A2000 connectors

Information required

Amiga A3000 connectors

AMP  part. 172171-1 (Plug), and 170366-1 (Terminals)


Amiga A4000 Mate-N-Lok connectors
The A4000 power supply  connectors are from the Amp Mate-N-Lok range.
To make up a new power supply lead you will require:

6-way Mate-N-Lok housing      RS 848-874
                                                 Rapid 22-4608
AMP part number is 1-480704
Mate-N-Lok pin 14-20 AWG   RS 849-085     
                                                  Rapid 22-4672
AMP part number is 926894-1
Note: you have to buy the Mate-N-Lok connectors in packs of 100 pieces.
You will require the above items to make a new power connector for your A4000 desktop motherboard.

If you want to fit a new motherboard connector you will require this part:
Mate-N-Lok 6 pin PCB            RS 129-5221
                                                 Rapid 22-4644
AMP part number is 350711-1

ATX to AT power adaptor

I found this whilst hunting through the RS catalogue, an ATX to AT adaptor, perfect for the A4000T and
some bridgeboards, RS part number 214-739 and at £3.50 cheap too. It converts the ATX power lead to
the P8/P9 leads required by your system.

 

 Rewiring your Amiga power lead

You have your replacement PSU but how do you connect it to your Amiga?
Where do you get one of those funny square DIN plugs?

I can answer the first question but I do not know where to get a square DIN plug so I will explain how to use your existing PSU lead.

 

For A500/A600/A1200 users.

Required tools & parts
A pair of wire cutters,
A pair of pliers,
A small flat blade screwdriver,
A terminal block connector,
A multimeter/tester.
Optional, a Soldering Iron + solder

 

1. You will have to cut the lead off your Amiga PSU. This is easily done with a pair of cutters. Strip the ends of the wires
2. As Commodore never used a standard wiring scheme for their PSU's you need to ascertain which PSU pin connects to which voltage.
   This picture of the power plug should help.
    
   
3. To ascertain which pin connects to which wire, either use the continuity function of your meter or use a couple
   of batteries, 3 pieces of wire and bulb to work out the connections.
4. To make the connections to the PC PSU you have two options.
   i) Cut off the PC motherboard connector and connect the power wires from there. If you choose this method, be sure to insulate the
   unused wire.
   ii) Use one of the peripheral power connectors (4 way type) that would be used to power your CD-ROM, HDD etc, I prefer this method.
   I like to take a spare plug or socket and wire it up to the Amiga's PSU cable.
5. With reference to either the AT PSU or ATX PSU colour code cross reference it to that of your power lead.
   
   On the power lead of my Amiga PSU, it had the following colour code:
    

Amiga Colour ATX Colour Voltage RED RED +5V BLACK BLACK 0V/GND BROWN YELLOW +12V WHITE BLUE -12V

NOTE: It appears that Commodore did not use a consistent wiring scheme for all PSUs so please check your lead.
 

1. Cut the wires from the PC PSU as appropriate and connect them, via the terminal connector block to your Amiga PSU lead.
2. Do not worry about the Shield of the Amiga PSU cable, it will do nothing if you connect it to ground.
3. Power up the PSU but without your Amiga connected, check the voltages that come out of each pin to ensure they match those shown above.
4. If the voltages are correct, you can connect the lead to your Amiga and power the system up.

 For reference, this is how I wired up my Amiga PSU. I used a blue wire to connect the -12V from the Motherboard connector to the Amiga PSU cable.

 

For reference, this is the pinout of the Amiga power socke t

Adding a new PSU to a CD32

 If you own a CD32, you can easily make a power lead up. All you need is a 4 pin DIN plug and to wire it up as shown here.
 


The easiest way to make the connection to the PC PSU was to buy a 'Y' power cable and cut of one of the sockets.
Solder the wires as shown and it is done. Here is my completed cable:

 

For reference here are the connector pinouts of the CD32 power socket
 

Bypassing the CD32's power switch

If you have issues with your CD32 not powering up correctly or intermittently, it may be due to the
poor quality power switch fitted.
I bypassed this switch on my CD32 with some 18 AWG wire links as shown here:



You need to bridge the the two columns of contacts closest to the rear of the PCB. doing this eliminated a few power
related problems I was experiencing with the CD32.

Fitting a new PSU to an A2000

The main issue issue you will encounter with the A2000 is connecting to the Motherboard. You can either cut off your original power lead
and re-use the existing lead or buy a new one. In addition you need to change jumper J300 to cover pins 2-3, so that the TICK signal is
 provided internally by the VSYNC signal.

NOTE: Originally the information I had suggested J200 should be connected across pins 2-3, this was incorrect, as J200 control the lightpen input!
The information above has been corrected to U300, which does in fact select the clock source for the TICK input.

An alternative method to generate the Tick signal is by using this circuit on Aminet: Tickit.lha, be warned this circuit connects directly to
the 220V AC mains, do not build unless you are competent in working with the AC mains, if in doubt do not build it.

After some research I found that the +5V_USER supply can be safely connected to the normal +5V supply as it is only used to provide
+5V to the mouse and joystick ports.

Control the PS-ON (ATX pin 14) either by using my circuit shown above or by connecting it to ground via a switch, which ever is easier for you.

A new connector is an TBD 14 way connector.

Here is the wiring list from an ATX plug to your A2000

Amiga Pin # Description Amiga colour ATX colour ATX pin # 1 +5V 4 2 +5V 6 3 +5V 19 4 +5V 20 5 GND 3 6 GND 5 7 GND 7 8 GND 13 9 +12V 10 10 KEYWAY No connect N/A 11 -12V 12 12 +5V USER 4 13 -5V 18 14 TICK No connect N/A

Fitting a new PSU to an A3000

The first choice you will face when adding a new PSU is how to connect it to the Motherboard, you can either chop off the original lead or
buy a new connector. The A3000 desktop PSU uses a an AMP #172171-1 connector with size TBD inserts.

Similar to the A2000, the +5V_USER/+5V_VID supply can be connected to the sytem +5V wiring.

To ensure that the A3000 works, connect jumper J350 across pins 2-3 to ensure that the machine generates the TICK
 signal from the VSYNC signal.

Control the PS-ON (ATX pin 14) either by using my circuit shown above or by connecting it to ground via a switch,
which ever is easier for you.

The A3000T takes a power FAIL signal from the original PSU which indicates when the power rails are operating out of tolerance,
when connecting to an ATX PSU you must connect this wire to the PWR_OK (pin 8) of the ATX PSU or your A3000T
will not power up.

Wiring list to connect an ATX PSU to an A3000D

Note: do not rely on just the colour codes, check the voltages on each pin as Commodore were not consistent with all their power
supply leads, you have been warned.

Amiga Pin # Description Amiga Colour ATX colour ATX pin # 1 +VID 4 2 +5V 6 3 +5V 19 4 +5V 20 5 No connect No pin No pin N/A 6 GND 3 7 GND 5 8 GND 7 9 GND 13 10 GND 15 11 -5V 18 12 +5V_USER 4 13 TICK ????????? No pin N/A 14 -12V 12 15 +12V