Re: VIC-II DRAM refresh

On 10/14/2016 11:38 AM, Francesco Messineo wrote:
> On Fri, Oct 14, 2016 at 11:26 AM, Gerrit Heitsch
> <gerrit@laosinh.s.bawue.de> wrote:
>> On 10/14/2016 11:15 AM, Francesco Messineo wrote:
>>
>>> What if you would find a Fujitsu DRAM chip bad among the 8 on a board?
>>> All with the same datecode?
>>
>>
>> I'd change the failed chip, so far only the MT4264 have shown that 'if one
>> fails, others will follow' pattern when it comes to RAMs.
>
> well, I'm really interested in numbers to be a believer :)
> I guess you have records of how many RAM chips have you seen and how
> many were bad.

I don't keep records that detailed, but I'm also active on forum64.de 
and for the last years if there was a question about a C64 and it was 
bad RAM, the wast majority were MT4264. As I said before, they don't 
seem to age very well.


> If we see that 7 chips out of 8 works fine after so
> many years, what we conclude?

Not much since the MT4264 have been working fine for years and only 
lately started failing and when they fail it's a consistant pattern of 
not the whole chip failing but a number of cells.



>>> yes, I've changed quite a few of them. On the other hand, my
>>> collection shows more 7707 failed than 7708. I still have 7708 and
>>> 7709 working, no 7707 working left that I'm aware of (maybe inside
>>> some 1541/SX-64, I didn't check very well on those).
>>
>>
>> The 7707 is the 7406 replacement. One of the drivers has to work against a
>> 180 Ohm pullup and most of the rest is directly connected to the outside
>> world. I'm not surprised that those fail easily. The original 7406 goes bad
>> often enough.
>
> I've had one failure on a regular 7406 that I can remember... pretty
> reliable against the 7707 failure records :)

I had a few 7406 where the driver that has the 180 Ohm Pullup went bad. 
Which means the 74LS257 don't come out of tristate and the CPU doesn't 
get to talk to RAM or VIC. They were all made by National Semiconductor.


  Gerrit



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