Re: VIC-II DRAM refresh

On Fri, Oct 14, 2016 at 9:54 AM, Gerrit Heitsch
<gerrit@laosinh.s.bawue.de> wrote:
> On 10/14/2016 09:39 AM, Francesco Messineo wrote:
>>
>> On Fri, Oct 14, 2016 at 9:34 AM, Gerrit Heitsch
>> <gerrit@laosinh.s.bawue.de> wrote:
>>
>>>
>>>
>>>
>>> Hm? They did use Micron DRAM, the dreaded MT4264-xx, in the C64 though.
>>> That
>>> must have been a very early revision of the VIC2.
>>
>>
>> what's wrong with the uT4264s (u as in greek mu)? I've seen them in
>> many C64 boards but I don't think they failed more often than other
>> brands.
>
>
> But they do... You usually get the 'Out of memory in 0' message when you
> turn on the C64 plus some garbage characters on the screen. Or you get a
> blank screen. If you get the garbage, you can easily figure out which one of
> the RAMs is bad. There should be a SPACE, $20 there. Look up the screencode
> for the garbage char and see which bit different.

I repaired way too many C64, so I know very well :)
This one for example:

https://www.youtube.com/watch?v=djsYNUNOk-A

And yes, this particular one has MT drams, but as I said, I've never
considered these MT to be less reliable than others, maybe they indeed
are however, I didn't really pay attention to the maker of the chips
when I substituted DRAMs. I'm just curious.
Also maybe the MTs where way more common than others? One should
really look on percentage of failed parts VS installed ones.

>
> If you have a C64 with all MT4264-xx on it and one goes bad, it's a good
> idea to replace all of them, it's very likely that more will go bad soon.

Maybe, or maybe not. I've never liked to change working parts.
The C64 in that particular video was used almost daily from 1984 to
1990, then once in a while
from 1990 to 1996 more or less. It will never see much use now, so why
would I change them all? In case of another failure, I can change the
failed chip(s) easily anyway.
I've a good collection of failed MOS chips, for some reasons I've
never kept bad chips from other brands, just discarded them (so I
sadly don't have a collection of failed DRAMs).

>
> I've had other DRAMs fail in the C64, but at least nowadays it's mostly the
> Microns when it's a bad RAM. And when I once bought half a kilo of mixed
> DRAM on ebay, most of the bad ones I foud when testing were Microns. Back
> then they worked, but it seems they don't age well.

I have hundreds of DRAM chips from recycled boards. I think no MTs
among them. Mostly
are japanese (NEC, Fujitsu). I've never found a bad chip among them,
but I test them only
when I need to install a chip somewhere.

it's a bit like the MOS logic chips, they fail more often than any
standard 74xx TTL chip, but I still have quite a few of them still
working, and I won't touch them as long as they work.
>
> As for the 'MT' or 'µT' on the Chip. To me it looks like a Mix between an
> 'M' and a 'µ'.

yes indeed, it's a mix. We better stick to MT as in the datasheet :)

Frank

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