Help! A500 minimegachip chipram mod gone wrong!

[Pat the Cat]

Hi all.
I think I've done something wrong/bad....
So, I thought I'd fire up the ol' A500 and see what state she was in.
current spec.
Rev5 board, 3.1rom, ECS Denise, MiniMegachip, 512k+RTC trapdoor, IndivisionECS, Jens 68k socket clockport board. GVP A530 8meg ram, 4GB CF via usual curmudgeon of adapters.

so. fired her up without the GVP, trap door ram or clock port adapter attached. flashing power light, and eventually get a green screen (chipram fault). reset, guru meditation, click left mouse, rom screen. boot off floppy, get the workbench, 512k ram. ok fine. power off. plug in the trapdoor slot expansion. nope. no screen. power light, no floppy click. reset seems normal as in the caps lock light comes on and goes off... flick the switch to disable the trapdoor expansion, all ok again. shes a bit wobbly from cold, but she gurus out eventually, and then left click resets to the insert floppy screen.

now. for the minimegachip, the JP2 lower solderpad has been cut, but the upper pad has not been joined. the side trace from near the trapdoor slot has been cut, but a quick poke with a multimeter tells me its still live? accidentally touched the metal shielding and got a connection sound. not sure that's right?
the fly lead from the megachip board is connected to pin 36 of gary, and that's a good connection. also get a good connection from pin 48 on the 68k without moving the lead.

i've never got the minimegachip working right. even with a jumper on the upper patches of JP2. had 1.5meg chipram at best once, and I know fat agnus is working.

seems something nasty is going on with the trapdoor slot but I don't know where to start looking.

OK. Rev 5 boards shipped with either 8371 or 8372 Agnus. You don't have an 8371 Agnus with more than 512K of chip RAM ever, IIRC.

Now, broadly, you have an issue with chip RAM. It seems to be with the memory board bringing the system down, but it could also be a problem with the mod to give Rev 5 A500s more than 512K of memory too.

A500 expansions come in all kinds of exotic variations, but they all do pretty much the same thing - add 16 bit memory. How that is configured depends partly on what Agnus revision you have fitted, as well as the card.

Post hires pics of the area around Agnus for any mods (bridges with solder between tracks cuts into tracks). Some of the expansion card to understand your issues with it would help.

If you can't photograph, examine and describe. It's probably a faulty component on the expansion board, or it could be something else too. Most all expansions have at least one strange behaviour.

It could also be the power supply is dying, or you have a local brown out for power. A530s can be greedy beasts. Disconnect it.

Examine the expansion and connector THROROUGHLY to see how well they connect, and if any bits of metal are touching where they should not.

[Pat the Cat]

Rev5 board, 3.1rom, ECS Denise, MiniMegachip, 512k+RTC trapdoor, IndivisionECS, Jens 68k socket clockport board...  4GB CF via usual curmudgeon of adapters.

That's a fair bit of hardware for many Amiga standard power supplies. Is there a floppy connected too? And how is the CF wired, to the SCSI bus, or to the floppy, or some other way?

[Pat the Cat]

Could just be the trapdoor RAM doesn't like working as chip RAM. A501s don't, the first version with 16 chips. Chips are too slow.

EDIT: BS, you just fixed it by bypassing the usual chip RAM mod. But it can be an issue with upgrading A500 chip RAM.

Great stuff!

[Pat the Cat]

Could just be the wire to Agnus flapping about. Try shaking it when working properly.

Wire wrap mods like that usually go on the back of a board, to keep them out of harms way.

If it just won't start with the expansion, check the chip temps. It's probably gone short somewhere and gets hot really quick.

[Pat the Cat]

If you want to build a new one from a blank board, it can be done.

http://eab.abime.net/showthread.php?t=85395&highlight=A500+RAM
EDIT: Hmmm, extra 512K of chip RAM or a Vampire... no contest. You might lose a litltle compatibility, but you get huge CPU power, and the HDMI output can give you way better resolutions than the native A500 ones.

[Pat the Cat]

maybe if I can get the 1meg chip thing working that'll ease my stress in that regard...

That's the way I'd run it. But then again, I don't have much need for more than 1Mb chip, and am probably very untypical in that respect.