The A500 reported 1.5 MB when I last reseated
Previously on scuzzblog: The A500 PAD - Chip, Fast or Slow memory.
ScuzzBlog: Diaries December 2024
Entry 11th December 2024: Post 1: Amiga 500 - That hot glue mystery Pt II.
Amiga 500 - That hot glue mystery Pt II.
I can never leave things well alone, but hey, that's half the fun.
So back to the mess that is the A500 covered in hot glue. I did
some further reading on the processes involved in upgrading the
AGNUS to use the 1MB and discovered a couple of items that I want
to emphasise, as I think they are not clear.
The first is the need to insulate Pin 41 on the AGNUS if you want
the machine to use PAL and not boot into NTSC.
Second, and the more significant, once you snip the traces on the
the two jumpers JP2 and JP7A you will render the trap door RAM
for anything other than providing the additional 512k to increase
the current 512K to give the 1MB, useless.
The point being that if you modify a Rev 6A by soldering 512K RAM
to the vacant slots on the motherboard then you will see 1MB CHIP
and only that. Expansion cards become redundant and will lock the
machine up in truth.
The MegaCHIP manual has more clear images of the two jumpers that
need to be modified. JP2A needs the trace between the two jumpers
linked on the diagram on the board to be snipped and then the two
upper pads soldered together. Jumper JP7a needs only the trace to
be snipped.
I repeat that carrying out the snip limits your machine to either
512K additional onboard or 512K trapdoor RAM to supplement the
512K you already have to give 1MB CHIP RAM. Many of the RAM cards
are incompatible with the change so do your research.
Never confuse the trap door with the side expansion. It is still
possible to use the side expansion port for up to 8MB RAM.
So to my conundrum and the hot glue fiend. I have concluded that
the processes here were in two stages.
Stage 01: Modifications were made to the board and 512K soldered
on to give the 1MB of RAM. This rendered the trap door useless
from this point on.
Stage 02: The guy bought the Power Computing card and I sense that
because of the GARY mod there was no need to have the original
mods to jumpers 2A and 7A or just maybe 2A. I say this cus if you
look at jumper 7A the two pads have been bridged with solder and
the trace connection reinstated.
At this point the 512k soldered RAM went back to running on the slow
bus. The Power Computing board is banked in 1.5MB made up of 512k
and another 8 chips at 1mb. What I sense from closer inspection
is that the board is damaged and has suffered corrosion from
battery damage. This may be the root cause of the problem.
With the RAM faulty and even removed from the machine I think the
GARY is double stepping the same RAM to fool the machine into
reporting 2MB of RAM available. Not only is it not there but I
seriously believe what is there is faulty.
Today I again fired up the machine and soaked with SID by copying
it into RAM and the machine locked up. I ran SID and it locked
and I tried copying the whole of a cover disk and it locked. In
addition the power socket is unstable. I decided not to venture
any further for fear of damaging the machine.
The guy hot glued all the chips so they are locked in place. I
cannot get between the chip and the socket to remove the glue. He
really did completely bugga this 500 up. I can do no more than
place in store. AND LEAVE IT AT THAT. No more lying in bed
thinking 'what if?' like I tend to do , and not getting sleep.
In conclusion a 1MB AGNUS chip upgrade will give you 1MB CHIP
RAM and little else. Also 2.04 works on a 1.3 machine without
the AGNUS upgrade. There are four options to upgrading the A500
in respect of ECS: changing the ROM to 2.04: changing the Denise
for the ECS version: fitting a Fatter Agnus if you don't have
the 1MB chip: and modifying the board to use 1MB CHIP RAM.
On that I will shut up except do read the last link regarding PAD.
Amiga 500 - That hot glue mystery Pt II.
the GARY which may be the correct memory. Dunno.
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Last updated 11th December 2024
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