A1200 Expansion port not working

[john1979]

I decided that I really needed a spare A1200. So I bought another one from ebay, this one was pretty cheap and yellowed so I knew it might want a bit of love.

Several of the keys were not working, the metal shielding was filthy and going black in places, and the disk drive was a converted PC floppy and did not seem compatible with certain games/demos.

After a bit of money and time all that is sorted now However the memory expansion port doesn't want to know. I have a good A1209 8MB expansion that works fine on my "clean" A1200 but on this machine the board is listed as faulty or not there at all on boot.

Finally, one oddity I see is graphics corruption in Battle Squadron. At the start of the game, just before your ship comes on screen there is a fast bit of scroll. On a good Amiga its the first bit of the playfield before the action starts, on this Amiga it's completely corrupted, as soon as the ships appear then everythings fine

Any ideas on what's up with her? Once I get this sorted she can be retrobrighted and back to her former glory

[hooverphonique]

the graphics corruption could be due to the floppy problems not being completely resolved?

[john1979]

Sorry, I should've also mentioned that its currently using the CF WHDload installation from my good Amiga. The crappy floppy has been replaced with a known working one from an A600.

[kipper2k]

is the motherboard version 2b ?

[john1979]

Hi Kipper, yes it's a revision 2B, the case has the Commodore Amiga (not escom style) branding.

Just checked over the caps and they all appear fine, with no motherboard corrosion that I can see.

[kipper2k]

version 2b motherboard is bad for timing issues, with or without accelerators attached. there is a fix (es) out for it, can't remember link atm

[amigasith]

Hi john1979,

I too have a 2B board. On mine, I applied Stedy's fixes and it works flawlessly, even with my ACA-1231/42 and my Indivision AGA Mk2 plugged in. The link to Stedy's fix is here: http://www.ianstedman.co.uk/Amiga/am..._mobo_fix.html

And here is a complete list of Jens' findings for the ACA series of accelerators:
http://eab.abime.net/showpost.php?p=...postcount=2453

The last fix on this list is the one applied by the folks from Amigakit as far as I know.

Hope this helps,
amigasith

[john1979]

Originally Posted by kipper2k

version 2b motherboard is bad for timing issues, with or without accelerators attached. there is a fix (es) out for it, can't remember link atm

Without accelerators? I'd heard about the timing problems but they seem to be always in conjunction with accelerator cards.

So a timing issue could be screwing up my GFX and causing the Expansion port to fail on a bare motherboard?

@amigasth
Thanks for the links. On the last one down, Schoenfeld is able to reproduce GFX corruption via heating Alice artificially and using an ACA. Again I'm using a barebones board, so I'm kinda shocked that it's the infamous timing issue raising its head.

[prowler]

Originally Posted by john1979

I'm using a barebones board, so I'm kinda shocked that it's the infamous timing issue raising its head.

Hi John,

When you know the full circumstances which gave rise to the timing issues associated with Revision 1-D-4 and 2B A1200 motherboards, you won't be at all surprised.

When the A1200 was designed, there were - as is common in designs of that complexity - some "bugs" in the chip designs. Many of these were corrected by the time the main production run started, but some were not, and in fact several bugs still remained in the latest revisions of the AA chip sets.

The main production board issued in the UK - Rev 1-D-1 - had onboard provision for hardware workarounds to fix the main bugs, notably bus timing problems in the Gayle and Budgie chips. In general, these boards work fine with most combination of expansion hardware and were the boards used to test A1200 expansion products before shipment.

The later revisions of motherboards - particularly the Rev 1-D-4 and 2B - are a different story, however. These board revisions were designed to take corrected versions of the Gayle and Budgie chips - and therefore were designed without provision for the earlier hardware workarounds. However, revised versions of the custom chips were never produced and, as a consequence, all but a few of the last AT/Escom-manufactured motherboards were shipped without either bug-free chips or hardware workarounds.

Component tolerances make the degree to which the timing issues will affect individual A1200 motherboards, whatever their revision, even more uncertain.

See what I mean?

[john1979]

Thanks Prowler, that all makes sense.

I can see how it would cause some GFX corruption.

Although I spent some time last night scouring eab for reports of timing issues causing the expansion port to not work at all, and there isn't anything. Not one reported case. In all cases it causes stability issues/gfx problems.

So I think the non working expansion port is something else. Before I get the timing fixes done, is there anything else that might go wrong to break the expansion slot?

[demolition]

How does the expansion connector look? If it's dirty/worn, that could give some problems. You could try using a soft pencil eraser to clean it.

I also agree that the timing problems should not cause the expansion connector to not work at all, so there must be some other problem.

[kipper2k]

Originally Posted by demolition

How does the expansion connector look? If it's dirty/worn, that could give some problems. You could try using a soft pencil eraser to clean it.

I also agree that the timing problems should not cause the expansion connector to not work at all, so there must be some other problem.

Dont forget, he mentioned corrupted graphics in games which has nothing to do with ther expansion port so there may be multiple issues. time to get out the meter and do a check on all traces

[john1979]

Okay, I spent a couple of hours today working on the 1200.

Stripped the whole thing to bits, cleaned the motherboard and case completely with alcohol, used a soft rubber on all the pins of the expansion connector, and rebuilt.

Now all the GFX corruption is gone!

But the expansion problem remains. I used to get sometimes just black screen but now it is consistent with a red screen and message saying defective expansion.

How do I test the expansion slot with a multimeter. Any tips?

edit:
My multimeter experience goes as far as testing c64 and amiga power supplies. Something I've got rather good at, but no more experience than that.

[desiv]

Probably not the same, but I know my 1200 expansion doesn't like the card to be plugged all the way in.
I have to plug it in all the way, and then back it out just slightly.
I had to do the same thing with my 8M RAM card and with my ACA card when I got that.

desiv