Re: VIC-II DRAM refresh

From: Francesco Messineo <>
Date: Fri, 14 Oct 2016 11:38:36 +0200
Message-ID: <>
On Fri, Oct 14, 2016 at 11:26 AM, Gerrit Heitsch
<> 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 didn't keep track of that (unfortunately, but I started when these
things didn't really matter).
If I have to think about recent (from year 2000 to now for example)
repairs, I've never happened to find a C64 with more than 2 DRAM chips
bad out of 8, so the batches are
reliable enough. If we see that 7 chips out of 8 works fine after so
many years, what we conclude?

> And removing chips that I know are bad or that I don't care about (cheap
> TTLs) is easy enough. Cut off all pins at the IC, then desolder all pins,
> clean the holes, add a socket.

yes, sometimes I use this method too, but a desoldering station is
almost as fast.

>> 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 :)

>> Most of the CIAs in my collection would "boot" fine in a C64 though,
>> they have one or two bad bits in the ports.
> I have 2 CIAs that when used together in the same system will fail the diag
> test. But if combined with other CIAs will show no problems.

indeed, at least 4 or more of the ones I have fails a diag test on
some systems and not on other ones. I've been always wondered why.
I've never found a possible explanation so far.


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Received on 2016-10-14 10:00:58

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