White Screen on an A500

[Maximilian]

From what I remember, you get a yellow screen if an exception occurs before the exception handler is installed.
(otherwise you would get a Guru meditation)
The question is what is causing this exception.

it can literally be anything, a broken line, a bad ramchip, logic chip, Gary.
Does any of the ram get hot?

Otherwise, if you cannot find any other corrosion damage, I would burn a Diagrom rom and use it to further diagnose:
https://www.diagrom.com/index.php/download/
It can also output on the serial connection, but I think that your machine will display the normal diagscreen and there will be no need for a serial connection.

[russellnash]

I have eliminated the yellow and green screens by reconnecting the RAM extension, putting the switch on it in the right direction. I removed it in order to get the board out and didn't think about it afterwards that it could cause problems.
With the JP2 settings on a Rev 5 board for 1mb RAM I get a bit confused, is it correct that pads 2 and 3 from the bottom must be bridged and the connection between pads 1 and 2 stays intact? There are guides where they cut the link between 1 and 2, but that is for Fast RAM with a different type of Fat Agnus and what I have is slow RAM, I think. :D
I will keep bashing at it, so far not a single corroded trace or via has failed a continuity check. As a matter of interest, which component needs -12V?
Thank you for all the help so far.

[rubber_jonnie]

I have eliminated the yellow and green screens by reconnecting the RAM extension, putting the switch on it in the right direction. I removed it in order to get the board out and didn't think about it afterwards that it could cause problems.
With the JP2 settings on a Rev 5 board for 1mb RAM I get a bit confused, is it correct that pads 2 and 3 from the bottom must be bridged and the connection between pads 1 and 2 stays intact? There are guides where they cut the link between 1 and 2, but that is for Fast RAM with a different type of Fat Agnus and what I have is slow RAM, I think. :D
I will keep bashing at it, so far not a single corroded trace or via has failed a continuity check. As a matter of interest, which component needs -12V?
Thank you for all the help so far.

If the machine was working before with the RAM expansion installed, then there should be no reason to check the links other than for continuity.

-12v is needed for serial, nothing else really. I think it would run on 5v only if you weren't bothered about serial.

Have you checked the basics, is 5v getting everywhere it needs to and are there clocks present at the CPU? Do you have an oscilloscope to check these things?

I think a Diagrom is going to be a good way forwards TBH.

[JezC]

I think a Diagrom is going to be a good way forwards TBH.

One thing to check here is that you mention it's a rev 5 board, so an EPROM (which I think covers all DIAGROM ROMs) probably won't work without either an adapter or modding the motherboard (just 2 track cuts & 2 mod wires apparently).

I found this out when trying to fault-find one of my rev 5 A500 boards...
:roll:

[rubber_jonnie]

I think a Diagrom is going to be a good way forwards TBH.

One thing to check here is that you mention it's a rev 5 board, so an EPROM (which I think covers all DIAGROM ROMs) probably won't work without either an adapter or modding the motherboard (just 2 track cuts & 2 mod wires apparently).

I found this out when trying to fault-find one of my rev 5 A500 boards...
:roll:

Yes, good point and worth remembering.

[JezC]

One thing to check here is that you mention it's a rev 5 board, so an EPROM (which I think covers all DIAGROM ROMs) probably won't work without either an adapter or modding the motherboard (just 2 track cuts & 2 mod wires apparently).

I found this out when trying to fault-find one of my rev 5 A500 boards...
:roll:

Yes, good point and worth remembering.

There is also the option to lift pins & add wire links on the EPROM itself if modding the motherboard or buying/making the adapter are not the way you want to proceed...kind of depends which approach you want to take.

I think I'm going to mod the motherboard on my rev 5 as I have a couple of other A500/A500+ boards (different revisions) and also want to fit a different/later KS ROM image to my rev 5, so modding the board will meet my immediate needs.

[russellnash]

The reason I asked about about JP2 is I found a very short piece of wire with a blob of solder on it loose in that area which made me think it was the link between pads 2 and 3. Looking at the photo I took it is hard to see but I think there was a link there. I also had to flip the switch in the other direction on the expansion, so I reckon it is running on 512k now.
I have started doing the voltage checks, the schematic is very helpful in that respect in that it doesn’t show the power connections. I think Commodore deserve credit for making a 68000 that worked without power. :D Much better than those Atari losers and way ahead of their time.
I have a logic probe.
Thanks for letting me know about the Diagrom, I am quite pleased that it doesn’t work on a Rev 5, then I don’t need to do it. I am going to drink a couple of beers outside in the sun and then have look later. Cheers.

[matt020]

Google how to use 512k ROM's on Rev 5 boards. The info is everywhere. Diagrom will work on a Rev 5 board if you Google search...

[rubber_jonnie]

The reason I asked about about JP2 is I found a very short piece of wire with a blob of solder on it loose in that area which made me think it was the link between pads 2 and 3. Looking at the photo I took it is hard to see but I think there was a link there. I also had to flip the switch in the other direction on the expansion, so I reckon it is running on 512k now.
I have started doing the voltage checks, the schematic is very helpful in that respect in that it doesn’t show the power connections. I think Commodore deserve credit for making a 68000 that worked without power. :D Much better than those Atari losers and way ahead of their time.
I have a logic probe.
Thanks for letting me know about the Diagrom, I am quite pleased that it doesn’t work on a Rev 5, then I don’t need to do it. I am going to drink a couple of beers outside in the sun and then have look later. Cheers.

AFAIK JP2 is a 3 pad jumper and only 2 pads at a time should be connected. What would help is if you could do a continuity check between the centre pad and both outer pads.

By default I believe the middle and bottom pads are linked Looking from the front of the board), so if you aren't reading continuity there or with the top pad then you may well be missing a needed link.

If the middle and bottom pad are connected that means you're using upper RAM as chip RAM, but the middle and top being connected would mean upper RAM becomes slow RAM.

So first thing check to see if any pads are connected, just buzz out with a multimeter.

Can you get a photo at all, well lit and as high rez as you can please.

[russellnash]

I have looked on the original photo before I made it 800 pixels wide for the forum and on very high zoom I can see that all three pads on JP2 were linked, that means that must have been how it was as it was working, so I have restored it to this state and checked continuity between all pads.
All the main chips have power, are not halted or in reset. The RAM data lines are pulsing but on pins 2 and 14 there is no signal at all.
After doing these checks the board now boots into a green screen. I did no alterations, only touching pins with a a probe, so I have no idea why that happened. I will keep going, I expect it to be a long process as my previous experience with board troubleshooting has taught me.
By the way, I have looked at PCB explorer, and although it only has the one Rev. version, it is very helpful at showing where the traces go to, which the schematic isn't at all clear about. Thank you.