From: pete@bignode.equinox.gen.nz (Pete Moore)
Subject: Re: Installing Fatter Agnus (Me too!)
Newsgroups: comp.sys.amiga.hardware
Organization: Equinox Networks
Reply-To: pete@bignode.equinox.gen.nz
X-Newsreader: TIN [version 1.1 PL8]
Date: 16 Dec 92 13:00:28 +1200
Organization: Equinox Networks

WALLY@UKCC.UKY.EDU wrote:
 
>I have an AMiga 500 with the regular Agnus, and I would too like to replace
>it with the Fatter Agnus...  I have a revision 5 motherboard, and would
>also like to know what else needs to be done to get 1 meg of chip ram.
> 
>I tried swapping my agnus with a fatter agnus once before, but could not
>get my machine to work at all.  (I posted, but got no responses.)

This is what I used (I've lost the name of the helpful person who posted it
originally). Step 4 is probably the trickiest bit - let me know if you
have trouble locating the _EXRAM line, and I'll send an ASCII diagram.

              -= Installing the 8372a to Revision 5 A500's =-

1. Remove the 8371 from its socket. If you are an NTSC user who does not wish
to use the PAL mode, go to step 3.
2. Pin 41 of the new Agnus controls whether the machine starts up in PAL or
NTSC modes. When it is tied low (to ground), the machine boots in NTSC mode.
When the line is tied high or left floating (ie unconnected), it boots into
PAL mode. On the Rev. 5 board, pin 41 is connected to ground by tracks on BOTH
sides of the board, making it virtually impossible to cut the track without
desoldering the socket. This means if you just plug in the new Agnus, you will
be locked in NTSC mode !! There are a couple of ways to go about circumventing
this problem; I will detail here the one I undertook on my own Rev. 5 board. I
will assume you want to install some sort of switch to toggle PAL/NTSC mode.
If you only want to run in PAL mode, you can simply bend out pin 41 of the
Agnus socket (don't bend the pin on the chip) to prevent it from making
contact with Agnus. If you do this, proceed to step 3. Otherwise, get yourself
an SPDT or SPST toggle switch and two lengths of telephone (or other thin,
stiff single-core wire). Solder wires to the switch :

                            +---+
    To a convenient ground--+-o |  BACK VIEW OF SWITCH
                   +--------+-o |  When held as shown, UP=PAL, DOWN=NTSC.
                   |        | o |
                   |        +---+
 Uninsulated --> | | <-- This side is insulated
                 \-/ <-- Fold the wire thru 180 degrees

3. Insert the new Agnus. If installing the switch, you should now poke the
loose end of wire (see above) from the switch into pin 41 of the socket so
that the bare copper side touches the pin on the chip and the insulated side
touches the pin on the socket.

4. If you wish to install a memory mode switch, go to step 5. Otherwise,
locate JP2 (between 68000 and ROM). This consists of three pads. The center
and lower pads are joined by a thin track. Cut this track and place a drop of
solder between the center and upper pads. Now locate the _EXRAM line coming
from the A501 port (it comes from pin #32 and runs on the upper side of the
motherboard parallel to the A501 connector). Cut this line. You have now
finished installation.

5. Complicated. Locate JP2 and cut the track which joins the center and lower
pads. Locate the _EXRAM line from the A501 port (see step 4) and cut the
track. Solder wires to all three pads of JP2 and one wire to either side of
the gap you cut in the _EXRAM track. Wire a DPDT switch as follows :

                   +-----+  BACK VIEW OF SWITCH
Upper pad of JP2 --+-o o |  When held as shown, UP=512K CHIP, DOWN=1M CHIP
Center pad of JP2 -+-o o-+- To one side of the gap in the _EXRAM track
Lower pad of JP2 --+-o o-+- To the other side of the gap
                   +-----+
You have now completed installation.
--
 +-------------------  pete@bignode.equinox.gen.nz  -------------------+
 | The effort to understand the universe is one of the very few things |
 | that lifts human life above the level of farce, and gives it some   |  
 | of the grace of tragedy   -  Steven Weinberg                        |
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Newsgroups: comp.sys.amiga.hardware