A500 Rev6A 1MB RAM

[Matt_H]

Can you post a photo of the board(s)? I don't quite understand the configuration. The system is reporting 2MB even though there's 1MB onboard and no other expansions connected? That doesn't sound right. I wonder if the system is falsely reporting an extra 1MB that isn't there as a result of incomplete uninstallation of the Power Computing card. And if the Power Computing card is 1.5MB, I assume it's supposed to be 512K Chip plus 1MB Fast. That will definitely conflict with the extra 512K on the motherboard.

I don't believe any rev 6s were shipped with 1MB onboard. My guess is that adding the capability to the motherboard was a cost-savings attempt to eliminate the need for the A501 trapdoor card (or to simplify it to be clock only), but engineering never got approval to manufacture any that way. They finally did with rev 8 and bringing the clock onboard, but the cost savings were partially nullified by the need for the A501+ for the second meg of Chip RAM. Typical Commodore

[Matt_H]

Wow... some stuff has been done to that board! The extra RAM chips are definitely post-purchase. If they weren't, all 8 chips would be the same part. There were tutorials kicking around BBSes and magazines back in the day that explained how to add the extra RAM chips as a slightly cheaper alternative to a trapdoor card (for those with the technical capability). I'm guessing that's what was done here. I'm no expert at reading part numbers, but all eight chips contain the code 514256, so I take that to mean they are functionally the same, but from different manufacturers, of course. The aftermarket chips are 80ns and the originals are 100ns.

I'm guessing the original owner did that mod and then laster wanted some "Fast" RAM, hence the Power Computing card. Did you remove the Gary adapter when you took the trapdoor card out? If not, that might explain the false reporting of an extra 1MB of RAM - some address line being triggered by the adapter that shouldn't be. I wonder if there's also some jumpery stuff going on, since either the extra 512K onboard or the bank of 512K of Chip on the trapdoor card would need to be disabled. Enabling them both would cause a conflict - maybe that's the yellow screen you got?

Anything on the underside of the board?

[Matt_H]

My soak for tomorrow is to fire up DOpus and open the interface and just keep adding more stuff to RAM till the thing falls over. It'll either kick out at around 850K with DOpus and Workbench loaded or it'll carry on past 1MB.

Good plan. That's exactly what I was going to suggest.

So what'dya think...My money is on flipping out at around 850K. There is no way that computer has 1MB chip RAM. Maybe The memory is on the same bus so I guess you are right. I was going to pull the Gary but I gotta put it back in the motherboard or find another.

I have another one of these Power Computing GARY cards plus expansion but reading the guide for the Mega Chip there is always some board hacking that needs doing to Jumper 2 and 7A. I can't see if any of that was done. The previous guy has stuck every single chip down. I still can't work out the switch. MAYBE.. just a thought he took the A500 on his motorbike to Amiga Demo Parties and the chips kept popping out. Either that or he was a nutter. Gotta be some reason. There is also the possibility that there is a peice of the jigsaw missing like an A5000 or B5000, Spirit, Viper or Midget board that sat over everything and was stuck down.  Who knows.

Modifications to JP2 and JP7A are normally required in order to add the trapdoor 512K to the onboard 512K in order to get a contiguous 1MB of Chip RAM.  See this. (The default is for 512K Chip + 512K Slow.) If there's 1MB onboard, however, I imagine the settings for 1MB of Chip RAM would be different since you'd want to tell the system to find that extra 512K onboard and not in the trapdoor. I'm not sure what those settings would be, however. I'm trying to find that article for you - I'm fairly certain it's in one of the old Amazing Computing issues. I'd guess, though, you'd want to keep the change to JP2 described in the file I linked, but I don't know about JP7A. Either way, it looks like there's some funny business going on with both your JP2 (connected to the Gary adapter) and JP7A (appears to have been re-soldered by hand).

[Matt_H]

I found this thread which seems to suggest that the process for 1MB of Chip onboard is the same as the process for 1MB of Chip via 512K onboard + 512K via trapdoor. Meaning the instructions for the latter process should work.

So my suggestion would be the following:
Leave trapdoor empty.
Remove the Gary adapter (it looks to be the one component not glued down?).
Clean up JP2 and JP7A to match the above instructions.

This way you don't need to deal with all the glue up front. In theory that should give you a stable 1MB Chip system. If that fixes the floppy problems, great. If not, then you've at least determined that the floppy problem is a separate issue. Then you can start examining the other components (and deal with the glue ).

[Matt_H]

Strange... the switch looked like it was hooked up to the audio filter. But I guess it adjusts the RAM config.
It looks like the system has a full 1MB of Chip as well as 512K Slow? I didn't know that was possible, but with the funny business of the jumpers and the fully populated RAM sockets maybe it is. Still, the system should work (in different configurations) with the switch in either position, right? So there may be more troubleshooting ahead to get it 100% operational.

The trapdoor card RAM is in 2 different banks - 512K (4 chips) of "true" trapdoor RAM, which would be either Chip or Slow, plus 1MB (8 chips) of Fast (addressed via the Gary adapter). You didn't have the 1MB of Fast when you got the system to boot - perhaps the switch is in the disable position. And maybe the system wouldn't boot in the other (enable?) position because one of the chips/address lines in the bank of Fast RAM is toast? Possibly due to battery damage - I did notice corrosion on the pins of the trapdoor card.

Glad it's at least partially solved. Keep us apprised 

[Matt_H]

@scuzzb494

I uploaded a file to Aminet last night about a hardware project called Rascal, which is a circuit to use both the full 1MB onboard and the regular 512K of trapdoor slow RAM (although the author calls it Fast RAM) at the same time. It looks similar to your setup. Not exactly the same, of course, but perhaps employing some of the same principles. You might want to have a look when the file appears in a day or two.

This thing also looks very similar to your board. Perhaps an earlier/later revision not licensed to Power Computing. No additional info on it, unfortunately.