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AuthorTopic: A500 Rev6A 1MB RAM  (Read 705 times)

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Offline scuzzb494

A500 Rev6A 1MB RAM
« on: October 05, 2020, 10:32:27 PM »
Were any of the A500 6A boards fitted with 1MB out of the factory. The FAT AGNUS is the 1MB version. Just that I have an A500 without the trapdoor card [ without ] and it is reading 1MB CHIP and 1MB slow [ FAST ]. The vacant slots have been filled with 4 x W-GER TC514256-10R chips. The computer has the card on the GARY that connects via a wire to JP2 and there was a Power Computing 1.5MB upgrade card that I found not to be working. The GARY was wired to the memory card but I removed the card.

With the card connected the computer wouldn't work, or I should say the floppy didn't work. With the card connector disconnected from the GARY it would work but only hold one disk in memory. Any further disk would be destroyed and render checksum errors needing reformatting. So I pulled the card. The memory on the card is shot and serving up gobledegook. If that makes sense.

So without a RAM card I have 2MB on the A500 and puzzled cus I thought all 500s only came with 512K even with the 1MB FAT.

https://www.scuzzscink.com/amiga/scuzzblog_october20_1/car_sbd_051020_30.jpg
« Last Edit: October 05, 2020, 10:34:19 PM by scuzzb494 »
 

Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #1 on: October 06, 2020, 06:08:02 AM »
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 :)

Offline scuzzb494

Re: A500 Rev6A 1MB RAM
« Reply #2 on: October 06, 2020, 12:41:06 PM »
I actually don't believe it myself. Below is what I posted yesterday and trust me the 2mb is being reported without, that is no card fitted.

The computer will not work with the card so I had to pull it anyway. Removed completely. I checked this morning again and with the computer powered and firing up AVAIL I get 1mb chip and 1mb fast ram. The total RAM in the top WB screen is 1.9MB or thereabouts. I can see no cutting of the jumper at J2 or J7A. There is the wire from the jumper to the GARY on the raised podium. The GARY IS NOT connected to anything.

I also checked the card on another machine this morning and it is not working. Same yellow screen.

The board looks butchered with the hot glue, but it is not.

https://www.scuzzscink.com/amiga/scuzzblog_october20/scuzzblogdoctober20_0501.htm

I checked the added RAM that was fitted to the empty slots on the A500 and they appear to give a total of 1MB. The base chips look like the original. Which can't be the case.

« Last Edit: October 06, 2020, 12:42:17 PM by scuzzb494 »
 

Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #3 on: October 06, 2020, 09:51:32 PM »
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?
 

Offline scuzzb494

Re: A500 Rev6A 1MB RAM
« Reply #4 on: October 06, 2020, 10:20:48 PM »
There is nothing on the underside of the board. The card gives a yellow screen on another Amiga so its fried. The battery killed something.

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.

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.

Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #5 on: October 07, 2020, 04:40:05 AM »
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. :)

Quote
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).
 

Offline scuzzb494

Re: A500 Rev6A 1MB RAM
« Reply #6 on: October 07, 2020, 02:40:08 PM »
UPDATE. The computer refused to load anything beyond the first 512K limit. If I leave a disk in the drive it will corrupt the disk and make it so I have to format it again. So with Wb1.3 in disk DF0: and inserted if I try to run DOpus on DF1 it will kick out after 500K is used. The disk becomes unreadable and I have to format. If then I leave the disk write protected the computer freezes and the power light flashes and I get a GURU.

I reinstalled the card and amazingly it booted up but didn't show any memory changes. And sadly the same thing happens. Only 512K is available but the computer still tries to use the busted memory and locks up.

When this problem first became apparent to me I had concluded with the damage to five disks that the extended memory either on the board or the meotherboard was sending out garbage information to the machine and sending it into a spin out destroying the disks.

My problem is that with all the hot glue on everything I gotta first unglue the chips to check anything out. And then there is the chips he added which I am convinced are 512k only. He was trying something adventurous and its possible the computer was broken when it came here. I only used the other in the pair that I got that day. The second works without a problem but then it doesn't have this GARY board. I'm just about done for now. I think the soldered Fujtsi chips he put on the board are the issue. I think the OKI ones are the original 512K ones that were on the board. The 4256 ref I find misleading cus it kinda feels like that adds up to 1MB but  it doesn't mean that. If you look at the chips on the Rev 6A on the Big Book they are 4256 also as are a number of my machines. OKI is quite common.

Thanks for all the help. I would like to get to the bottom of this GARY issue. These cards are a bit rare and recently someone with a similar configuration wanted to know info about them. Neither Big Book or Resource have any information on these cards. There has to be a guide somewhere.  All I have is the guide that came with the MegaChip that explains what is needed to mod the A500 motherboard.

I concur with your earlier view that there is 512K extra RAM only on the board and the 2MB is a double reading of the same memory which is 2 x 512K. Or not. Who knows.


Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #7 on: October 07, 2020, 04:32:01 PM »
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 :P).
 

Offline scuzzb494

Re: A500 Rev6A 1MB RAM
« Reply #8 on: October 07, 2020, 07:41:14 PM »
Hi

RESOLVED:

I removed the GARY and found that cus he had a sticky back pad on the back it had caused the chip to rock on the small pad and bent it so it was bowing. I did try without the small board but failed. So I tried the card without the main RAM card and that resulted in the same issue. So I then reconnected the main card and I got a flashing light, a green screen and then a grey screen. That was different.

NEXT:

I flicked the switch at the back of the computer to the up position. INCREDIBLE: She booted but this time I only had 1.4MB of combined RAM. I did the AVAIL and I had 1MB Chip and 500 Slow FAST. Next I was able to fire up DOpus and memory available was 800 K. SysInfo gives 508 Slow Public FAST and 1MB Chip.

WE DID IT: She is working a treat and can fire up two disks etc.

There is 1MB of CHIP RAM that the GARY board configures and this is a combination of the trap door card and the memory on the machine. The other 520K is the filled in memory on the Amiga 500. That is how I read it. I don't think the computer has been modified to make it 1MB I believe the GARY card and the expansion card give the 1MB of CHIP. The filled in chips are the extra memory. The problem was that the GARY was not seated properly. That was causing issue with the floppy.

ANYHOO: She is working. I will update the blog tonight.

Many thanks..... The guy was just bonkers about sticking stuff down. On the other A500 I had from him he used such sticky plastic to hold down a ROM switcher it oozed out all over the motherboard when hot. Took me an age to clean off. I still think he travelled around with his Amiga.

Onwards

Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #9 on: October 07, 2020, 10:36:35 PM »
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  :)
 

Offline Matt_H

Re: A500 Rev6A 1MB RAM
« Reply #10 on: October 13, 2020, 06:55:05 PM »
@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.
 

Offline scuzzb494

Re: A500 Rev6A 1MB RAM
« Reply #11 on: October 18, 2020, 12:18:44 AM »
I will keep an eye out for that. I do have another Power Computing card that is similar. Sadly doesn't have any information included. The GARY would sit upside down which is odd.

https://www.scuzzscink.com/amiga/scuzzblog_january18_6/car_sbd_170118_01.jpg

https://www.scuzzscink.com/amiga/scuzzblog_january18_6/car_sbd_170118_03.jpg

That 1501 is strange cus its label suggests  a memory expansion for the 1500. I'm pretty sure the 1500 wasn't even released at that time.

In the end I decided to use the Amiga 500Plus with the ROM switcher for 1.3 and 2.04. Way more stable a machine. I have a project on the go at the moment where I am trying to build a home made 2.04 Workbench disk using only files from cover disks and old software disks. It will be a variant of the actual Workbench disk as I have already incorporated the facility to read MSDOS disks without the aid of CrossDOS. I dunno how legal it would be to distribute it but I like the idea of having my own Workbench disk for the Plus that isn't a Commodore product.  I call it PlusBench.

https://www.scuzzscink.com/amiga/scuzzblog_october20_2/car_sbd_161020_04.jpg

An actual Workbench disk was not used at any time in the creation. And it works fine. It incorporates MultiDOS. So I can read PC formatted disks.

https://www.scuzzscink.com/amiga/scuzzblog_october20_2/car_sbd_161020_21.jpg

Keeps me busy.