A5000 memory chips required

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jubber
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A5000 memory chips required

Postby jubber » Tue May 30, 2017 2:24 pm

Hi there. I've picked up an old and battered A5000 that appears to have suffered a battery leak. The OS gets as far as posting an error through the disk drive before giving up. I think one issue is the ram chips because pin 15 on chip 1 doesn't connect all the way through to pin 15 on the last chip. This happens in a few places where I think the connection should be continuous. So either the board lines are damaged or a couple of the chips are.

Does anybody have either an old A5000 board or its ram chips for sale?

The chips are Samsung KM44C256BJ-7 (20-pin 70ns dram) chips, if that helps.

Cheers,

Robin

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danielj
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Re: A5000 memory chips required

Postby danielj » Wed May 31, 2017 5:18 am

Robin, have you tried utsource.com?

d.

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Wed May 31, 2017 3:31 pm

The diagnosis that the memory is faulty based on continuity issues on the memory pins confuses me. These are not light bulbs in a christmas tree; even if the memory is faulty the address lines should be connected. #-o

The memory on the A5000 is located relatively far away from the battery. Do you have a detailed picture of the board?
Paul

dp11
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Re: A5000 memory chips required

Postby dp11 » Wed May 31, 2017 4:09 pm

Pin5 isn't connected to all the ram chips, but all the other address lines should be connected. A ram chip faulty can't cause a disconnect on the PCB

jubber
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Re: A5000 memory chips required

Postby jubber » Wed May 31, 2017 5:49 pm

Hi chaps - it's quite possible my diagnosis is incorrect. I'm pretty new to electronics. I was told all the pins connect through to all the other pins like with the roms. Quite possibly bad information as pins 18 and 19 don't seem to connect any of the chips together (assuming pin 1 is bottom left hand corner when chip label is the right way up). I was hoping to get hold of an old mobo to test against.

The disk drive reports a fault with ram/cmos (00020141) and looking at the board, it's more than possible all the corrosion around the battery means that is where the fault actually lies.

IMAG1485.jpg

IMAG1482.jpg

dp11
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Re: A5000 memory chips required

Postby dp11 » Wed May 31, 2017 6:16 pm

In the image of the ram chips pin 1 is top left.

jubber
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Re: A5000 memory chips required

Postby jubber » Wed May 31, 2017 7:26 pm

Hi dp11 - thanks for the advice -

you mean the following?

Image3.jpg
(48.48 KiB) Not downloaded yet

jubber
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Re: A5000 memory chips required

Postby jubber » Wed May 31, 2017 7:29 pm

and danielj - thanks for the electronics store tip - interesting site

dp11
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Re: A5000 memory chips required

Postby dp11 » Wed May 31, 2017 7:30 pm

Yep, corner pin is 20 not 15.

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IanS
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Re: A5000 memory chips required

Postby IanS » Wed May 31, 2017 7:49 pm

Looks like Acorn number the missing pins on the TRM diagrams, so there is no pin 6, 7, 8, 19, 20 or 21and the pin mubers go up to 26.

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Thu Jun 01, 2017 5:51 am

IanS wrote:Looks like Acorn number the missing pins on the TRM diagrams, so there is no pin 6, 7, 8, 19, 20 or 21and the pin mubers go up to 26.

That is slightly confusing (but true). It is odd that the diagram doesn't follow the numbering in the data sheet.
Paul

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Thu Jun 01, 2017 8:23 am

A few more questions...

Is the jumper on LK24 connecting ok (not corroded?).

I assume you are testing without a memory upgrade?

Do you have the tools & knowledge to check the tracks around the CMOS chip (next to the battery, the chip seems to have been replaced)?

Do you know if it was working before?

Are you using an old CRT vga monitor to test (not LCD flat panel)?
Paul

jubber
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Re: A5000 memory chips required

Postby jubber » Thu Jun 01, 2017 9:32 am

jumper LK24 seems okay (far bottom-right side of board, bottom two pins are connecting, top pin is bare). The jumper itself is connecting the two pins - tested with multimeter. Underneath the board I could follow the bottom pin to l36, which makes a beep. The other trace seems to go under what looks like a newly replaced chip - but I couldn't find a leg or connection in that area that made a beep.

The memory upgrade is disconnected and the ram jumper set to 2Meg. I think the A5000 has 2Meg as default.

I will do my best to check the connections around the cmos chip and get back to you on that one. Not sure I have the knowledge or skills! I do have a multimeter, a hot air station and some insulated tweezers in the same way you could equip a gorilla with an operating theatre.

The A5000 was apparently working many moons ago. I suspect before battery damage came calling. The last owner was the one who replaced the two chips before giving up.

I used the same vga flat panel my A3010 works with. I do have a vga game studio monitor with aerial input that my Beeb uses - I can try with that. I don't own an actual CRT any more. I'll give that a go this evening with delete held down.

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Thu Jun 01, 2017 10:52 am

An excellent guide with the relevant part of the circuit diagram can be found on retro-kit.

Booting with the Del key won't work if the CMOS circuit is malfunctioning; the Del key boot sets settings to the default but no settings are stored.

It will most likely have the monitor setting set to Auto, but that does not detect LCD VGA screens very well; hence the question about a CRT screen. So you may get to a supervisor prompt but not see it.

Also please note that with CMOS issues it can take over 35 seconds before getting to the supervisor prompt, so you'll have to be a little patient...

These battery acid repairs require some endurance. :D
Paul

jubber
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Re: A5000 memory chips required

Postby jubber » Fri Jun 02, 2017 4:05 pm

That's a great site - thanks for the link. I've been prodding around with the multimeter, and it seems pretty much all the connections are dead. I'm going to desolder the battery and remove the cmos chip next - and give that bit of the mobo a more thorough vinegar scrub. Due to all the corrosion it does increasingly look as if I'll have to try a breakout board or something.

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Fri Jun 02, 2017 7:15 pm

Or you can cheat and get one of these.
Paul

jubber
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Re: A5000 memory chips required

Postby jubber » Sat Jun 03, 2017 12:20 am

Because the cmos circuit seems to be very badly damaged I might have to consider that option. From the instructions it seems you can snip the legs of the clock chip to disconnect the entire circuit from the rest of the computer. If I used a hot air work station to desolder the clock chip, would it achieve the same thing? I ask because then I could test if the machine will power on without the cmos circuit being part of the problem. If it still won't boot then I know the ram circuit is also a problem.

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Sat Jun 03, 2017 8:42 am

jubber wrote:Because the cmos circuit seems to be very badly damaged I might have to consider that option. From the instructions it seems you can snip the legs of the clock chip to disconnect the entire circuit from the rest of the computer. If I used a hot air work station to desolder the clock chip, would it achieve the same thing? I ask because then I could test if the machine will power on without the cmos circuit being part of the problem. If it still won't boot then I know the ram circuit is also a problem.

Desoldering the CMOS/RTC chip would do the same (in a neater way than snipping the legs).

The trouble with powering on without a CMOS chip is that the A5000 VGA auto detect does not handle LCD panels very well. So you could have a semi-functioning machine without seeing it.

From the picture of the CMOS circuit it looks like it would be possible to fix with a few bypasses; most of the PCB pads for the CMOS chip seem to be intact.

Note that sometimes the battery damage extends to other areas; the IDE interface is close and so it the floppy drive.

RAM errors can be hard to fix, could be damaged PCB tracks but also failing capacitors or other components (in order of likeliness).
Paul

jubber
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Re: A5000 memory chips required

Postby jubber » Sat Jun 03, 2017 6:42 pm

Do you have some advice about how to bypass any failed connections? I assume I need some solder paste, maybe a flux pen, the hot air work station and incredibly steady hands (plus a magnifying glass, from the looks of it). Teeny tiny wire as well?

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DutchAcorn
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Re: A5000 memory chips required

Postby DutchAcorn » Sat Jun 03, 2017 7:35 pm

Well, the page on the retro-kit site gives some advice for removal and fixing. Very thin wire is useful (30awg Kynar wrapping wire) and generally use lots of flux.

In the A5000 I fixed I used some normal thin wire, but the result is not as nice. See this post for pictures.
Paul


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