Amiga 500 Rev6 RAM Mod

[Xamiche]

I just found an article which says the Amiga 500 Revision 6 can have RAM chips soldered into the blank places on the PCB to increase the RAM to 1MB.



I have a few questions regarding this.

1. Does this really work? I've read a lot of hack/mods for the Amiga 500 only to hear from someone who has tried them that they cause other issues. eg. the 68000 @ 14MHz hack causing floppy drive step rate issues.

2. If it does work, can I still use a trapdoor 512KB RAM expansion?

3. I have a dead A500 Rev 6 board I'm thinking of salvaging the RAM chips from to attempt this mod. If this mod does work, would there be any reason the salvaged chips wouldn't be suitable?

[mongo]

1. Yes.

2. Not without making other modifications.

3. The chips should be fine.

[Xamiche]

Would the use of a trapdoor expansion on top of the RAM chips require the Fat Agnus chip to be upgraded to say the Fatter Agnus chip?

[cv643d]

Interesting I have a couple of A500 motherboards over, what do you have to do to keep 2 MB expansion in trapdoor?

[countzero]

doing the 1mb hack and keeping the trapdoor RAM is not easy. check this thread over at EAB for more information.

[Xamiche]

countzero wrote:
doing the 1mb hack and keeping the trapdoor RAM is not easy. check this thread over at EAB for more information.

Thanks for the link. Seems like an easy enough mod to me. I'll try it out and see if it works out.

[Xamiche]

Ack! My solder sucker is dead. :-(
A two day wait for a new one to arrive.

I was just thinking and I have a few other questions.

1. If I add the chips to the empty spaces, am I going to need to add caps at C20, C21, C22 and C23?



2. On the mod description I have, it says to enable the added RAM as chip RAM you need to set J2 to 2-3, and J7A to 2-3. If this works for the RAM added to the board, would it work for RAM added at trapdoor expansion?

3. Is Santa Claus real?

4. Will Fat Angus 8372A allow for 1MB Chip RAM?

Thanks to all who have helped so far, and any who may help with these questions.  8-)

[countzero]

1. If I add the chips to the empty spaces, am I going to need to add caps at C20, C21, C22 and C23?

I've seen people get away without those caps. But your mileage may vary. do the hack and see how it performs. If you have memory access problems, do the caps too.

2. On the mod description I have, it says to enable the added RAM as chip RAM you need to set J2 to 2-3, and J7A to 2-3. If this works for the RAM added to the board, would it work for RAM added at trapdoor expansion?

No. You can't have more than 1mb chip ram with 8372A.

http://amiga-hardware.com/showhardware.cgi?HARDID=1448

3. Is Santa Claus real?

yes.

4. Will Fat Angus 8372A allow for 1MB Chip RAM?

yes.

tip, if you want more than 1mb chip ram through trap door, save yourself the trouble and get an A500+ ;-) 8375 has a different pinout, and I haven't seen any hack that would allow to use a 8375 on a plain A500 (aside from 2mb chip ram boards of course).

[Xamiche]

Thanks countzero. :-)

I'm not really after more than 1MB chip RAM. I'm happy just to get the 1MB Chip and maybe 512KB Fast.

Question two was kind of a side question not referring to the mod, but just as a separate 'on it's own' question. To clarify, I don't mean 1.5 Chip RAM.  What I meant was if I were to change the J2 and J7A to 2-3 would that allow a stock Amiga 500 to see a trapdoor expansion as chip RAM, on its own, without having added extra RAM to the motherboard?

So in other words; take one stock a500 Rev6A with standard 512KB chip RAM, change J2 & J7A to 2-3, add trapdoor expansion 512KB RAM. Would this now see the expansion RAM as chip, or would it still just be fast RAM? The reason I ask is on one of the mod descriptions, it said that adding RAM to the spaces in the board was just like adding trapdoor RAM. So I figured if this is correct, why not forget the RAM chips and the soldering, and just encourage the Amiga to see the trapdoor RAM as chip RAM.

Edit:
It's okay. I just found out the answer to this question. :-D

Thanks for the help. :-)

[orange]

yes, it would see trapdoor RAM as CHIP. I did that (using aminet instructions) and it works. That way you keep the clock.
BTW, Amiga will still work OK if you remove trapdoor expansion after the modification.
Interestingly the on/off switch on my trapdoor expansion stopped functioning, A500 would always see 1Mb CHIP..

[Xamiche]

I think when you cut J7A 1-2 that disables the RAM expansion switch, at a guess. Actually now I think about it, it doesn't seem so likely. :-?

I just did the mod to my Amiga 500 and it seems to have worked. I'll put it through it's paces and see if it runs stably.

[Chain]

if anyone got it working (1mb chip onboard +512k fast/slow in trapdoor) please let me know. I still have problems with it.

[Xamiche]

Chain wrote:
if anyone got it working (1mb chip onboard +512k fast/slow in trapdoor) please let me know. I still have problems with it.

The next thing I'm going to try is the 1MB chip + 512KB fast. I'll let you know how it turns out. Gotta wait for some parts though. Will be a couple of days.

[Xamiche]

Got a question for you Chain. I read your thread about this same topic, from back in Nov '07. Do you, or anyone else here, have a copy of the scanned article referred to in this thread? I only have the below schematic to work from and it would be cool to see some pics of the actual mod.

Also, in my digi electronics units so far we have only worked with 74LS chips. What's the difference between 74ALS and 74LS? Is it just the propagation delay, and if so would I be better off using 74F chips instead of 74ALS?

[rkauer]

 Actually, the 74ALS is a family "upgrade" from the old 74F, but with less consumption (and less heat dissipation, of course).