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Old 05 July 2019, 18:50   #1
RiP
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Exclamation A500 Rev6 fries any memory expansion

Hi all,

I've just found a fully working A500 Rev6 but the problem is that it fries any memory expansion I connect to it quickly
Actually, it burns the four 514256 RAM chips on the card that smells very bad

How can I fix it? what to check?
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Old 05 July 2019, 19:01   #2
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The computer works otherwise? ;-)
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Old 05 July 2019, 19:05   #3
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The computer works otherwise? ;-)
Yes, it works fine without the memory card =/
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Old 05 July 2019, 19:06   #4
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Can you do a mockup installation of the memory card and take a photo. Please don't power up the machine.
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Old 05 July 2019, 19:14   #5
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Here you are:

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Old 05 July 2019, 19:15   #6
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Looks fine to me. How many mem expansions have you blown with the machine?
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Old 05 July 2019, 19:23   #7
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Two till now
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Old 08 July 2019, 18:44   #8
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Well, I connected one chip like this but it still gets hot.
How is it possible while the Vcc +5V leg is disconnected?

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Old 08 July 2019, 21:02   #9
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Have you checked for shorts? maybe the isolation material between the board and metal shielding has failed in some way and 5V is going into other pins?
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Old 09 July 2019, 09:24   #10
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I traced the 5V but it had no contact with other pins.
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Old 09 July 2019, 12:34   #11
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Have you checked the 12V lines as well?
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Old 09 July 2019, 19:35   #12
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Yes, those are isolated.
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Old 10 July 2019, 01:51   #13
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Are the expansions you've tried the same model and from the same source? could it be fake chips? I noticed they look a bit scuffed and no markings? I'd still try separating the the board from the metal shielding to see if it helps.

I've also noticed a bit of yellow where the connector is soldered to the A500 motherboard, is it excess flux? previous owner possibly did a repair and didn't bother to clean?

Last edited by Mick; 10 July 2019 at 02:26.
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Old 10 July 2019, 14:41   #14
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Since you have a desoldered board - what is the voltage on the 5V pad for the IC?


And how does the shield look on the underside of the motherboard below the expansion connector? If someone soldered on the connector with the shield+insulation in place, it could have melted the insulation and created an intermittent short (so you wouldn't be able to measure it all the time).



If the IC was already fried, it could get hot simply from the current running through the other pins.
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Old 11 July 2019, 10:09   #15
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Are the expansions you've tried the same model and from the same source? could it be fake chips? I noticed they look a bit scuffed and no markings? I'd still try separating the the board from the metal shielding to see if it helps.

I've also noticed a bit of yellow where the connector is soldered to the A500 motherboard, is it excess flux? previous owner possibly did a repair and didn't bother to clean?
You were right. The chips were fake and I wasn't tested them years ago



After replacing the chips:

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Old 11 July 2019, 10:31   #16
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Haven’t seen a A501 card like that before. Did it have a battery on it?
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Old 11 July 2019, 11:32   #17
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Looks like a third party/homebrew card, not an official Commodore one.
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Old 11 July 2019, 16:58   #18
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You were right. The chips were fake and I wasn't tested them years ago

Those are logic chips, not RAM. Look at the bottom one, you can still see the 74HC marking on it.
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Old 11 July 2019, 17:39   #19
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Are the expansions you've tried the same model and from the same source? could it be fake chips? I noticed they look a bit scuffed and no markings?bother to clean?
Spot on!
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Old 11 July 2019, 19:01   #20
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those are logic chips, not ram. Look at the bottom one, you can still see the 74hc marking on it.
lol !
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