Re: 7712 info

On 6/19/20 11:23 AM, Clockmeister wrote:
> 
> 
> On 19/06/2020 4:00 am, Francesco Messineo wrote:
>> On Thu, Jun 18, 2020 at 9:49 PM Gerrit Heitsch
>> <gerrit_at_laosinh.s.bawue.de> wrote:
>>> Yes, but often, if you have a dead C64 with a lot of 77xx chips on it
>>> it's one of those that is the problem (unless it's the PLA, but that is
>>> also known as '7700-xx' or '8700-xx').
>> I've repaired dozens of C64 :)
>> Once I've found a C64 that had almost all 77xx logic chips and one by
>> one I had to change them all to revive it.
>> I don't desolder parts before making sure they are really bad usually.
>> I've never said they're reliable, but they aren't either all failed by
>> now for sure.
>>
>> What is really "disturbing" in the MOS reliability issue is that it
>> appears to  depend on the part number too.
>> Either some parts were made in different farms, so the less reliable
>> farms were producing the less reliable part numbers
>> or it was heat-related maybe (like the hotter parts failing more 
>> often)...
>> I tend to believe it was a "farm" issue.
>>
>> Frank
>>
> 
> Sorry Frank my previous replies were meant for the list not just you by 
> email, apologies for that.
> 
> C64 DRAM has a high failure rate but that's true of DRAM generally in 
> all my vintage equipment these days it seems (yes some brands fare much 
> worse like mT).  I rarely had to replace was the 77xx logic on any 
> Commodore computers though, I would say about the same rate as for other 
> systems that used 74XX logic.

I had a lot more dead 7708 (74LS257), 7709 (74LS258) and 7707 (7406) 
than their 74xx equivalent.


> It is however my opinion that power supply ripple, not only over voltage 
> is quite capable of damaging IC's in the long term including DRAM and C= 
> supplies were notoriously rubbish back in the day and haven't gotten any 
> better with age.

A 78S05 (as used in the C= supplies) doesn't produce much ripple though, 
at least as long as the large capacitor in front of it is still within 
spec. You get a lot more from any switching regulator.

  Gerrit