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Amiga A500+ graphics problem

 
Jayceebee
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Any Amiga experts on here? I resurrected this A500+ with ECS chipset some time ago when searching for something else in the loft, my daughter was interested in me reviving it and playing the old games we still have although many have succumbed to bit rot. Unfortunately when powered on it was quite obvious all wasn’t well, an inspection of the innards revealed what is now a well known issue with the A500+, leakage of the NiCad rechargeable battery for the RTC. After a thorough clean-up life was restored, although one issue remains i.e. corrupted graphics with some but not all titles. Most play without any issues and this problem was apparent before it went into storage.

The pictures below give an idea of the problem, in picture 3 you can see that a lot of the background is not rendered correctly but the sprites in the foreground are fine. The screen asking for a disk swap shows corrupt graphics from earlier game play in the background.

Research suggest the Fat Agnus or Denise is a possible cause, an OCS Denise has been tried and provided no change. The Fat Agnus, an 8375 version is fitted in this machine. They are now scarce, expensive and only available from abroad. Anyone care to comment and make suggestions? I have a request in the wanted section if anyone has an IC going spare.

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John.

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Topic starter Posted : 13/04/2020 10:05 pm
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hamid_1
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It does look like the graphics memory is bad. Does your Amiga have a memory expansion board fitted? I had the standard Amiga 500 which came with 512Mb RAM built-in, but I added an extra 512Mb RAM underneath the bottom of the machine. This came on a circuit board with a Ni-Cd battery on it. I'm thinking this could have damaged some of the RAM when it leaked. You could try removing the RAM expansion if fitted, then see if that makes any difference, though some games may not run without it.

There must be a memory test / diagnostic disk available for the Amiga, though if you downloaded one using a PC the difficulty would be writing it to a floppy disk. PCs can't write Amiga formatted disks. Back in the day, one way I got round this was by installing CrossDOS on the Amiga 1200. The Amiga could then read 720k PC disks. I copied the wanted files onto a PC disk, read them in the Amiga then wrote them back on the Amiga in Amiga format. There are also ways of transferring files by serial or parallel port. I had to make a special parallel cable and use some PC - Amiga file transfer software on both machines.

Another problem I had with the Amiga 500 was the square socket that held the Fat Agnus chip became intermittent. I kept pressing all the chips down in their sockets to temporarily revive the Amiga, but eventually it failed completely. I had to get the socket replaced at a computer shop as I didn't have the equipment to desolder it myself. You might actually find that it's the socket, not the chip that's faulty.

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Posted : 14/04/2020 10:44 pm
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Jayceebee
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I too suspect that memory could be the issue but have no means to remove the ICs without cutting each leg and unsoldering rendering the original useless, there is also the worry that replacements have a high probability of being fake. The 8375 replacement is the easiest and is not destructive.

I suspect their may be some diagnostic tools on the Workbench disk supplied with the machine but as with a lot of floppies they have the dreaded bit rot. A Gotek FDD emulator might be an option as there are several sites with all the old software converted to .ADF which can be handled by a PC.

The A500+ comes with 1meg of ram on board so no extra memory board  in the trapdoor. This graphics issue has been apparent before the battery leaked in storage and could even be a factory fault, the machine wasn’t new when I obtained it.

If I do bite the bullet and swap the memory 80nS chips are much cheaper than 100nS, I’m presuming the faster memory will be ok to replace slower or am I mistaken?

John.

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Topic starter Posted : 15/04/2020 10:30 am
Nuvistor
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Different memory speeds. These are just my thoughts I know nothing about Amiga internals.

If the mem bus in the Amiga is Asynchronous then the faster memory should be ok, the data will be on the mem data bus and be accessed by CPU when the CPU is ready. This is presuming the Mem signals the data is ready and waiting.

If the mem bus in the Amiga is Synchronous, I.e, The CPU timing being the same as the mem then a different speed mem module could produce timing errors.

Frank

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Posted : 15/04/2020 12:40 pm
hamid_1
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Since the RAM is soldered in place, I would want to be really sure it's faulty before trying to replace it.

A bit of Googling found a diagnostic disk "Advanced Amiga Analyzer" : http://www.l8r.net/install/misc/aaanalyzer2.DMS

I've run it in an Amiga emulator on a PC. It can test the RAM for you. The file is a DMS disk image compressed using Disk Masher System on the Amiga. The WinUAE emulator on a PC can load this file directly, but to use it on a real Amiga you'd have to transfer it to one along with an Amiga program that can write DMS images to floppy, if you don't already have one. After writing the DMS image to a blank disk, simply boot the computer using that disk just like a game disk. When the main menu appears, select 'Memory' then 'Run Test'.

I realise it's not going to be very easy to transfer a file from PC to Amiga if you don't already have the necessary Amiga software - it's a chicken and egg situation. I might be able to write it to a disk for you and send you a copy in the post, though I haven't actually used my Amiga for quite a long time and I'm not sure if my floppy disk drive still works, but I can certainly try. To be honest, the Gotek USB floppy emulator might be worth buying in your case, as you said a number of your existing floppy disks have already deteriorated. I'm not sure if the Gotek supports .dms files as well as ADF's. If not, you can easily convert the DMS file to ADF on a PC, then you can run it on your Amiga and test the memory.

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Posted : 15/04/2020 4:26 pm
Jayceebee
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Posted by: @nuvistor

If the mem bus in the Amiga is Asynchronous then the faster memory should be ok .. ...

If the mem bus in the Amiga is Synchronous, I.e, The CPU timing being the same as the mem then a different speed mem module could produce timing errors.

Hi Frank, I'm lead to believe the memory is synchronous and there is no separate bus. The odd thing is this evening I've just come across a video on YouTube of an identical machine which uses a mixture 70 & 60nS ram, the circuit diagram I have shows 150nS and as the CPU clock speed is a paltry 7.09Mhz maybe there is a bit of leeway?

Posted by: @hamid_1

To be honest, the Gotek USB floppy emulator might be worth buying

Hi Hamid, I've took the plunge and ordered a Gotek emulator and thanks for the link to AAAnalyser. I'll give it a try and report back.

Pic attached of the damage from the NiCad.

rsz ram

John.

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Topic starter Posted : 15/04/2020 10:51 pm
Red_to_Black
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Hi JC,

just looking at the corrosion from the leaking NiCd, is it worth trying to 'stitch' the vias (pth) from both sides of the Pcb ?

I did manage to repair to repair a few Amigas, and Atari STs back in the day, but then of course scrap/donor chassis were readily available back then for trial and error substitutions.

Most Amigas I repaired back then were for trashed floppy drives for some reason, (relatively expensive from CPC), some were for memory upgrades, and most of the Atari STs were for smpsu repairs (start up resistor ?). There also seemed to be a shortage/long lead time for Commodore spares around that time too

I did repair quite a few Amiga RGB to cvbs/modulator converters as well if I recall, Motorola MC1377P chip failure IIRCC, this chip was starting to become hard to get hold of and expensive even then.

Actually everything CBM supplied was expensive!!. I also remember Donald Bullock of TV mag fame putting a request out once for a 'fat Agnus' chip out for his youngest son (Paul was it ?), this would have been in the early 90's maybe around '91/'92.

"This is my multimeter. There are many others like it, but this one is mine. My multimeter is my best friend. It is my life. I must master it as I must master my life. Without me, my multimeter is useless. Without my multimeter, I am useless."

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Posted : 16/04/2020 12:05 am
Jayceebee
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Hi Baz, this issue has affected the machine since it came into my hands as it was obtained second had, the problem was there long before going into storage and battery leakage, only affect a very small number of titles. The worst affected area was in the audio stages, this has no issues since the cleanup. I've resisted the recommendations on YouTube and the like to neutralise the effect of the battery electrolyte with white vinegar, just giving it several thorough wash downs with IPA. I have made a continuity check of all the address, data lines, RAS, CAS etc. to every device involved with memory, Agnus and Denise all appear intact.

Searching around the net I've found someone has kindly made available an .ADF version of AAAnalyser although I did find some info on ADFopus and using a Null Modem cable to connect from PC to the Amiga to to do the transfer and conversion from .DMS. Just waiting for the Gotek to arrive and see if it's as good as it's supposed to be.

John.

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Topic starter Posted : 16/04/2020 5:01 pm