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From: burkert1@platon.fmi.uni-passau.de (Klaus Burkert)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Help!  My 2000 motherboard is dead
Keywords: dead motherboard megachip
Message-ID: <1991May15.144853.25645@forwiss.uni-passau.de>
Date: 15 May 91 14:48:53 GMT
References: <1991May13.031522.28822@mtu.edu>
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In article <1991May13.031522.28822@mtu.edu> cdh@mtu.edu (Chris Hooper) writes:
>My Amiga is dead.
>
>Machine:
>   Amiga 2000, AmigaDOS 1.3 in ROM, Rev 4.3 motherboard
>   1M Agnes, Supra 8-up (with 2M), IVS Trumpcard
>
My configuration:
Amiga 2000, AmigaDOS 1.3 in ROM, Rev 4.3 motherboard
1M Agnes, GVP-Series I Hardcard, 4 Meg Fast-RAM

>Problem/Symptoms:
>   A diode was burned out on the motherboard (actually burned clean
>   through).  It was replaced (1N4001) located near the power supply
>   connector to the motherboard.  A technician at the company that
>   sells the Megachip said diaode D400 is on the +12 power input.
>   He then told me that the particular revision motherboard I have
>   (B2000 Rev 4.3) is missing a capacitor (C908) which is needed for the
>   Megachip board to work.  He stated this capacitor is listed as 100pf,
>   but recommeded 70pf.  I still have yet to put this capacitor in.
>
I don't know, how or why You burned that diode.
Also I don't anything special about the MegaChip, I simply saw it
at the Amiga'90 expo in Cologne, Germany , but it was too expensive
(DM 600.- (german currency, ~350.- US$) _without_ 2-Meg-Agnus),
so I can't help you with the capacitor.

[ stuff deleted ]

>What I have already tried:
>    Replacing the diode that was burned clean through.
>
>    Swapping every pullable chip on the motherboard
>    [68000, Agnes, Paula, Denise, Gary, ROM, the two 8520
>     chips, etc].
>
>    Probing around randomly with a logic probe.
>
| >    Praying to the Magic Montra
| >
| >    Sacrificing the neighbor's cat.  (Just kidding, all you
| >                                      animal lovers out there)
| does this help ? :-)

>
>Anyone with any ideas or suggestions, please help.
>

Due to the high pricing mentioned above, I decided to do a hack by 
my own. (Too much to post or mail, a lot of trace-cutting, de- and
re-soldering, piggy-backing, etc. I took me about three or four days
before working perfectly).

During this hack, I ran into some similar problems:
Sometimes, the machine booted, sometimes not, or it booted perfectly
from floppy or HD but screen remained white.
I got the moving-bits-display, too.
The reason was a floating (i.e. bad contact) Address-Line at Agnus.

I think,there are only few possible defects:
 - Agnus is dead (you swapped it already?)
 - some RAM-chips are dead (*)
 - the RAM-buffers are dead (**)
 - Agnus' socket is damaged

(*): the RAM-area is split into two 512K banks. If the lower bank
would be defective, you'd get only a _green_ screen (error in first
256K of chip-ram). If the upper bank would be flaky, you should
try switching the jumper J101, when it helped then there is/are
one/several bad RAMs in the upper bank.

(**): when the buffers are dead, you'd get only the green screen, too.

I think it's the last possibility, because you already mentioned how
hard it has been to pull agnus out of it's socket.
So here's my advice: replace the socket and all should work well.

>Thanks.
>   -Chris

I hope this helps and you can "reanimate" your Amiga!

>- Chris Hooper      Computing Technology Services Consultant
>  cdh@mtu.edu       Michigan Technological University
>
>Also: cdh@mtus5.mtu.edu CDH@MTUS5.Bitnet and cdhooper@symmetry.cs.mtu.edu

-------------------------------------------------------------
Klaus Burkert        email: burkert1@platon.fmi.uni-passau.de
Brandweg 11          voice: +49-851/83993
D-W-8390 Passau   /  Federal Republic of Germany