Re: VIC-II DRAM refresh

From: Gerrit Heitsch <>
Date: Fri, 14 Oct 2016 11:26:57 +0200
Message-ID: <>
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.

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.

>> 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.
> 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.

> 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.


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Received on 2016-10-14 10:00:43

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