How bad are MK4096 chips... well let me tell you

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How bad are MK4096 chips... well let me tell you

My traditional NOS sources for MK4096 have dried up, so now I have to scrounge around for “pulls” for Apple-1/Mimeo ram.

Some MITS s100 ram cards have nice Mostek MK4096 dated 1976 and early 77. Pulling chips off two MITS ram cards, out of 64 chips total... I got about 18 good ones. Don’t worry I’ll replace the ram with nice reliable MK4027 type ram not appropriate for Apple-1 but plausible for MITS since they tended to use what ever was cheapest in Albuquerque the week they were built.

How scary is that 18 chips.... pretty bad failure rate. Sure it could be that these two cards at some point were stored badly or the MITS card just kills ram chips.

Anyone else have this kind of experience with MK4096 ram chips?
Cheers,
Corey

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I had good luck with a ND

I had good luck with a ND Nuclear Data RAM Board (board # ND-ORM 50-1089-00) with three banks of 16 Mostek MK4096N-11 with 7650 date code. All 48 tested ok.

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Uh oh, is Unicorn sold out? I

Uh oh, is Unicorn sold out? I've been procrastinating…

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I think unicorn supplies

I think unicorn supplies MK4027, which work fine in a Mimeo, MK4096 are more original.

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Interesting. I hope not. I

Interesting. I hope not. I just ordered some and they were listed explicitly as "MK4096". At $5.99 apiece, they're certainly priced like they're the real thing.

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Same for me, Unicorn sent me

Same for me, Unicorn sent me MK4096N-16 with date code 7726.

My experience is that you have to take some care of the cycle time. The original board uses MK4096N-11, which has a cycle time of 500 nsec, whereas the MK4096N-16 supports a slightly shorter cycle time of 450 nsec. I needed to adjust the 27K resistor at B2 down to 22K to get the MK4096N-16 reliably working. Of course also the real value of the 47pF capacitor goes into the calculation of the Apple-1 memory cycle.

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He must have gotten some

He must have gotten some mk4096. Cool. Last I heard he was sending MK4027.

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Ok. Update....

Ok. Update....

apparently MK4096-15 which is what MITS used on early 16k DRAM cards are not compatible with MK4096-9 or 4096-16.  They are for “hobby market” and use 15V not 12v.  I think they are actually MK4096-16 rejects which work at a higher voltage.

 

So so be careful if you are buying MK4096 chips that they aren’t the -15 version.   

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MK4096N-6 Datecode 1976

Hi, for my MIMEO project i sourced (new old stock?) Mostek MK4096N-6 chips locally. Reading through this post i am now unsure whether i can really use them. Can you please let me know your thoughts?

They are saying they still have >300 pieces on stock. If somebody else is looking for them just send me a PN...

 

thank you 

Denis

 

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About the /CAS timing

Ansgar wrote in post #6:

 

" My experience is that you have to take some care of the cycle time. The original board uses MK4096N-11, which has a cycle time of 500 nsec, whereas the MK4096N-16 supports a slightly shorter cycle time of 450 nsec. I needed to adjust the 27K resistor at B2 down to 22K to get the MK4096N-16 reliably working. Of course also the real value of the 47pF capacitor goes into the calculation of the Apple-1 memory cycle. "

 

According to my experience, the 480ns timing spec for the 74123 /CAS timing oneshot as seen in the Apple-1 manual is correct, but for faster DRAMs and slower 6502 it may be a bit early, so the 6502 may not have the write data ready yet when /CAS comes. I also found out that the main culprit for the /CAS timing deviating from the 480ns is neither the R nor the Mica C, its the 74123 itself. Mica caps typically have very tight tolerances around 2%. The carbon composition resistors always have been junk, and this is exacerbated by age. I found several with a resistance way too high for the 10% tolerance spec, and these were Allen Bradley ones made in the 1960s, still sealed in the original factory bag. I suspect they deteriorate over time, possibly developing internal cracks or grain boundary rot within the carbon composition mass. The only safe way to use them I have found is to measure them, reject the out-of-tolerance ones, then heat the survivors on one leg with the soldering iron for a few seconds (as in soldering) and let them cool down again to RT. Then measure them again and if the resistance has changed too much from what it was before the thermal torture, they also are suspect and need be discarded.  For builds where I don't need authentic looks I use modern metal film resistors for the timing, and of a value that has been determined with a 50K trim pot in the same build using the same 74123 (actually I use 5K fixed + 50K trimpot in series to avoid any possibility of damaging the 74123). I adjust the trim pot to get 480ns and then power down, pull the 74123, and measure the resistance between pins 16 and 15. The nearest higher value goes then replaces the trimpot, and all is fine. If I have to use the dreaded carbon composite resistors for authentic looks, I select the best fitting one (they have  tolerances wide enough to always find one that fits) but check it again after soldering it in and after its has cooled down.

 

The interesting find with this procedure was that the 74123 cause most of the deviations. In the past I ended up with R's between 22K and 25K to yield 480ns (the C being selected to be spot on), but the last tube of 74123 I bought, which were newer, needed R's of 20K, and very consistently so.

 

If you have a PROM based RAM test program, you can also determine the go/nogo limits for the resistor and then chose one that is centered in the middle. This does not work too well with a RAM based RAM test program, for obvious reasons.

 

 

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