Re: VIC-II DRAM refresh

From: Gerrit Heitsch <>
Date: Fri, 14 Oct 2016 10:41:04 +0200
Message-ID: <>
On 10/14/2016 10:17 AM, Francesco Messineo wrote:
> On Fri, Oct 14, 2016 at 9:54 AM, Gerrit Heitsch
>> 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.

Yes, you can... But I also do repairs for people who don't know how to 
use a soldering iron and I had it happen more than once that I changed 
the offending DRAM and a few weeks later the next one died. Just 
replacing all MT4264 is cheaper than mailing the system back and forth 

You can use 41256 as replacement for 4164 in a C64 BTW, you only need to 
add a connection between Pin 1 and Pin 16 of the DRAM. 41256 are easier 
to get as NOS and are cheap too (less than a Euro a piece). They use 
8Bit refresh so there is no problem in the C64.

> 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).

Another kind of chip that doesn't age well are the MOS TTL replacement 
they made when LS-TTL were in short supply. Especially the MOS 7708 
(74LS257) are known to fail now.

> 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.

NEC, Hitachi and Fujitsu are quite good and when they fail they tend to 
go into meltdown (chip gets very hot) so they are easy to find.

> 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.

Well, if you have a board with a dead 7708 and the other one has the 
same datecode, better think about replacing that one as well.


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Received on 2016-10-14 09:00:39

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