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Caleb Hansberry
February 11th, 2017, 01:12 PM
I got a Mac 128k with most all accessories. It boots to this screen:
http://i.imgur.com/DNxZDKe.jpg
So I think it is saying it has two bad RAM chips. I opened it up, it appears to have never been opened before and has all the original Apple RAM chips, so I'll just replace them all. Attached is a picture of the board. Whats the best way to get 16 of this kind of RAM chip? I'd like to get them as fast as I can, so is there anyone here who would like to ship me some from the US, is there a reasonably priced eBay listing, or should I go with this site, which looked decent but not super cheap and I'm unsure how fast the shipping would be: http://www.jameco.com/z/4164-150-Major-Brands-64KX1-DIP-16-DRAM-150NS_41662.html ?

And later I'll recap it. I'm guessing traditionally I'll need to replace all the capacitors in the logic board and the analog board?

links about what capacitors I need: http://macfaq.org/plusanalogue.html http://www.jagshouse.com/analog.html

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yuhong
February 11th, 2017, 06:52 PM
You should be able to use any 4164 or 41256 DRAM chips you can find. Using 41256 DRAM chips will allow an upgrade to 512K RAM with some additional modifications.

Caleb Hansberry
February 11th, 2017, 07:35 PM
Alright, so really there's no reason to not use 41256, right? These look fine, there's a bunch and they're 41256 - what do you think?
www.ebay.com/itm/302164834599 (http://www.ebay.com/itm/302164834599)
I don't wish to do the modification right now, but maybe I could later, while keeping the RAM in place? Might as well if it's all the same to the computer.

Caleb Hansberry
February 15th, 2017, 08:51 PM
I attempted to replace the bad chip with one of the ones I just got, the TMM41256CP-12. However, it still did the same thing (RAM error) - which doesn't surprise me since it's not the same capacity RAM chip, but I don't have any idea what it should do. I tried shorting address pin 8, the highest, to ground, to emulate the 4264 chip, but then it failed to boot at all, just made a continuous clicking sound from the speaker. Again that doesn't exactly surprise me since I don't understand memory or how it works, but, any ideas on what I should do to make it recognize the RAM properly?

David_M
February 16th, 2017, 05:10 AM
You cant just short an address line to ground. Its and input which means there is a address multiplexer IC elsewhere on the board that with its output shorted to ground.
That address line you want to short is also needed to refresh all the memory cells, even without the issue of killing another IC tying that input to ground would prevent 75% of the cells being refreshed.

I'd swap out all the RAM and do the mod, if you still have the error then look at data bus buffers and address multiplexers. The reason for swapping them all is that most of the RAM chips pins are wired in common. (ie all RAM's have there address lines tied to each other and the data in and out pins are shared between banks etc). So one faulty chip can and often does effect the operation all the rest.


Edit: I wouldn't wait too long to do the recap, at least one of rifa caps looks ready to blow anytime.

After zooming in on the photo you posted, I'd only replace that rifa cap. Maybe the 100uf 200v mains filter caps, it's hard to tell but one looks a little curved on top. If its just an optical illusion in the photo then I'd not bother replacing them.


I attempted to replace the bad chip with one of the ones I just got, the TMM41256CP-12. However, it still did the same thing (RAM error) - which doesn't surprise me since it's not the same capacity RAM chip, but I don't have any idea what it should do. I tried shorting address pin 8, the highest, to ground, to emulate the 4264 chip, but then it failed to boot at all, just made a continuous clicking sound from the speaker. Again that doesn't exactly surprise me since I don't understand memory or how it works, but, any ideas on what I should do to make it recognize the RAM properly?

Caleb Hansberry
February 16th, 2017, 05:20 AM
Hmm I see... kind of, RAM does confuse me. But doing the mod is a whole another can of worms, I don't readily know how to do that stuff either. I could just replace the chips with 4264s then to avoid the issue?

In related news, I found that shorting A8 to Vcc shorted 5v to ground, since in the datasheet for the 4264 is says "NC" on pin 8, but according to my multimeter it's actually connected to 5v on the mac motherboard, so that explains why it wouldn't boot with it shorted out like that, I'm glad it didn't explode. Also, I accidentally replaced a good RAM chip last night, so it's no surprise it still has the same symptoms... thats pretty funny.

arrow_runner
February 16th, 2017, 01:18 PM
Alright, so really there's no reason to not use 41256, right?

Your 128k will have a lot more value if it stays at a 128k.

David_M
February 16th, 2017, 06:26 PM
Switch mode supplies have short circuit protection and shutdown for a short time before attempting a restart. So not a huge danger of anything exploding.

Because A8 is tied high you can probably get away with using 41256 in the circuit unmodified. If it was left floating you have the potential to get one of four different memory locations at random depending on the level at A8 at the time.
What it will mean is only every fourth memory location of the 256k will be getting accessed and refreshed.

Because A8 is tied high, I suggest replacing all the RAM with the new chips and once it is working introduce the old chips one at a time to identify the faulty ones. Then to maintain the machines value as an unmodified board, purchase a few 4164's to replace just the faulty ones.


Hmm I see... kind of, RAM does confuse me. But doing the mod is a whole another can of worms, I don't readily know how to do that stuff either. I could just replace the chips with 4264s then to avoid the issue?

In related news, I found that shorting A8 to Vcc shorted 5v to ground, since in the datasheet for the 4264 is says "NC" on pin 8, but according to my multimeter it's actually connected to 5v on the mac motherboard, so that explains why it wouldn't boot with it shorted out like that, I'm glad it didn't explode. Also, I accidentally replaced a good RAM chip last night, so it's no surprise it still has the same symptoms... thats pretty funny.

Caleb Hansberry
February 16th, 2017, 07:45 PM
Ah, I see, thats very good.

That point is too, I'll see if I can just use them unmodified.

About that last point - I don't wonder which ones are faulty, its F11 and F12, the Mac tells me when it boots. Also, the internet states the Apple chips are prone to bitrot - if this is true, there would be no point in not replacing any, as they'll eventually all be faulty.

Caleb Hansberry
February 24th, 2017, 06:33 PM
Alright, it's all fixed now! Here's a picture:
http://i.imgur.com/uSMAaLs.jpg

Here's a pic of the motherboard:
http://i.imgur.com/wVYRMcu.jpg

I replaced five RAM chips. Two were bad, but then I was clumsy and desoldered the wrong one twice, and then it said one more was bad. So, right now it has lots of original chips and 128k. I'm not sure it won't fail again because of the old chips, but I plan to sell it so for now it's good. I also cleaned and lubed the FDD which was failing to boot. It was a lot of fun! Thanks to Sark on the IRC channel!

As for the caps, I hope to let that be someone else's issue - this model doesn't have many caps and they're not actually failed at this point.