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Old 30 January 2016, 22:26   #1
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Ram chip testing/compatibility

Today I've been trying to get what I believe to be a Zydec A500 ram expansion working, it's a 1.5mb expansion card with gary adapter, to give a total of 2mb ram.

With the card and gary adapter plugged in, but the switch on the board off I can load workbench and no extra ram is recognized, as expected, but the clock is not recognized either. With the switch on it won't boot, just a green screen. With NO card plugged in but the gary adapter is plugged in, with the switch off it boots no problem, no extra ram, with the switch on I get an error message. With a separate 512kb expansion plugged in and switched on, and the gary adapter from the Zydec board still attached, it will boot to workbench and show 2mb, with the switch on the Zydec board on or off!

I'm assuming the extra 1mb is purely from the gary adapter, but it's only detected if there's an expansion plugged in and turned on. Because 2mb is detected with a separate ram expansion I'm assuming the problem is with one of the 4 ram chips that are connected in the usual manor, i.e. through the expansion port.

Is there any way to test ram chips with a multi meter to see if any are faulty? If any are faulty, can they be replaced with any DIP-20 256k DRAM chip? The exact model are expensive, but I have an A500+ motherboard that has been donating parts to good causes, so I could take some from that if they will work?

If the ram chips from the A500+ will work and there's no way to test ram chips with a multi meter, I'll start by swapping the 4 that are connected to the expansion port, one at a time.

The battery had leaked so I removed it and cleaned the board up, I checked all the traces/vias/solder joints and made sure everything looked ok before testing, I thought it could be a bad connection to the expansion port because of the clock circuit not working, I've got a home made tool to clean them, made from a sharpened and bent pin header from an old pc motherboard soldered onto a bolt to use as a handle, using it didn't make any difference, maybe there's too much corrosion to get off like that, I can't really see inside the holes. Is there a source for the pin headers for expansion cards?
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Old 31 January 2016, 08:03   #2
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The ram test is very silly, if you have the gary adapter with a normal 512k expansion, the mem test routine will see the same RAM for every memory bank and think you will have the full amount present.

The gary adapter doesn't have any RAM on it, it just fools the Agnus into selecting the trapdoor for three separate memory banks instead of one, and the actual RAM is on the Zydec.

So to me this sounds like either the cable between the gary board and the Zydec is plugged in incorrectly or broken or the Zydec has some broken memory chips on it.

A multimeter is not suitable for testing RAM, but using a memory chip with the same internal size would suffice. The Zydec probably has four chips per 512k so the A500+ motherboard chips should be the same size.

If your 512k expansion has a four chip configuration and the chips are socketed, then it would be a good candidate for memory testing. You could of course install one or more sockets onto it.

Last edited by Jope; 31 January 2016 at 08:09.
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Old 31 January 2016, 09:55   #3
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I understand now, I didn't understand how the gary adapter worked, I knew all the ram chips were on the expansion card and none on the gary adapter, I didn't expect any more than 1mb total with the gary adapter and regular expansion, I was just testing the expansion slot and didn't see any harm in leaving the gary adapter in place. I thought it was somehow accessed by the wires between the two, I was puzzled as to how only 2 wires could access a whole 1mb!!

The Zydec does indeed have 4 chips per 512k, as does my regular 512k expansion, neither have sockets, I'll see if I have any old motherboards with with sockets I could use, if not I'll get some ordered.

The wire from the gary adapter to the expansion card is soldered, not plugged, and is in good condition.

Thanks for your help, these amigas will be the death of me, in the last few weeks I've gone from not having one to having 2 A500s, an A500+ and an A500+ that I gave up on repairing!
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Old 06 February 2016, 00:24   #4
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The sockets I ordered came yesterday, today I finished removing all 12 ram chips from the Zydec board, fitted a socket to one of the ram chips on the known good 512k expansion, then tested the chips one by one. For each one I loaded workbench to check it was all detected, every one worked fine.

I've already ordered new caps and found a source for some pin header sockets, now that all the ram chips and caps are off I might as well remove the rest of the components and give the board a good clean so it can go back together with all fresh solder and a few new components.
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