Macintosh 128k newbie

First, you need to establish if this is a 128K with original RAM, or if it has been upgraded to 512K of memory.

Looking at the memory ICs (16 of them), you'll see either 4164 or 41256 printed somewhere on the ICs.

Depending on which, will determine which you need to buy.

These memory ICs are no longer manufactured (AFAIK), so anywhere you can get them (eBay for starters) will suffice. Search for "4164" or "41256" RAM.

Ensure that they are 150nano Seconds or faster. IE, 150ns 120ns 100ns etc. For example, A 150ns chip will be marked with a "-15" after the 4164.

Removing the chip from the motherboard is the hard part. A trick can be to use side cutters to cut the pins of the chip at the top "shoulder", then remove each pin one by one. Using an IC socket can save the same pain later.

Have fun!

bombo said:

Thanks! I will crack her open and see what I find. Will let you know tomorrow.

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Looking at that error code, the offending chip should be at F6. (Circled in red).

MacTech68, you are a wealth of knowledge and always very helpful!

bombo said:

Ok so I have a few chips with different numbers (looks like some may have been replaced already)
at F6 I have MCM6665BP20/FQD8416 <---this one is also located at F7 & F11

and all the others are HM4864P-3

I believe they are both 64k chips. So does not look like it was upgraded.

Can someone confirm?

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Confirmed. They're 64Kx1 so your board is still 128K.

Interestingly, the HM4864P-3 which is Hitachi, is a 200nano second RAM.

The replacement MCM6665BP20 (Motorola) is also 200nano second. We used to use 150ns, but 200ns should be fine, but no slower.

I would be careful since it is pretty rare for the RAM to fail (though it does). For two to fail at the same location may indicate poor replacement of the chip at F6.

Pics of the solder pads underneath F6 and a view from the top would be handy.

Are the replacements socketed?

Looks like there were just different parts in the hopper when the got stuffed in the board at manufacturing... they look wave soldered to me.

There's no removing and reseating that chip - it's soldered to the board. You'd need to desolder it, install a socket, and then fit a new chip. You probably want to socket all the RAM as long as you're in there. Definitely not a beginner task, unless you're already familiar with the tools involved.

bombo said:

Sorry by reseating I meant desoldering and resoldering back on.
But by the looks of it connections all seem good.

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My guess - it's not the connections, it's the chip guts. Re-flowing the solder won't help.

bombo said:

So you suggest putting in a socket over just getting a new chip and soldering on?

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Yes. Desolder a couple of chips and you'll understand why you don't ever want to do that on the same chip again. Besides, it isolates the new chip from the heat from those of us that are a little more ham-handed with our soldering irons.

David Schmidt said:

Looks like there were just different parts in the hopper when the got stuffed in the board at manufacturing... they look wave soldered to me.

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Whilst it's VERY unusual to have a mix of RAM chips from initial build (in fact, it's the first I've seen on a Mac128), those pics certainly look like factory originals.

David Schmidt said:

My guess - it's not the connections, it's the chip guts. Re-flowing the solder won't help.

Desolder a couple of chips and you'll understand why you don't ever want to do that on the same chip again. Besides, it isolates the new chip from the heat from those of us that are a little more ham-handed with our soldering irons.

Click to expand...

Yup, though I wouldn't bother doing ALL of them, unless you're a sucker for punishment.

I forgot to say, excellent pics! Very helpful.

Additionally, removing the chip isn't easy if you haven't done one before. As I suggested above, unless you have specialized tools, cutting the shoulders of the chip's pins and removing each pin individually is easiest.

You'll then need to remove the solder from each thru-hole pad, BUT you'll probably have difficulty with pins 8 and 16 (circled in pic below).

My suggestion would be to clear all OTHER holes of solder, then place your socket and heat up pin 8 and push the socket into that hole, then swap to pin 16, heat that pin and push it thru. Keep alternating until the socket is flush with the circuit board, then solder the other pins.