PDP 11 gear finally moved

Tothwolf tothwolf at concentric.net
Wed Jul 22 10:17:43 CDT 2015

On Wed, 22 Jul 2015, tony duell wrote:

> I think he did answer it. If the unit is operating correctly then the 
> capacitors must be sufficiently good at that time for that unit.
> Now, whether they will go on working is something that is very hard to 
> tell. But that applies to every other component in the unit. An IC might 
> work find now and suffer bond-out wire failure later on the same day.

Going purely from the historical data, failures of most semiconductors are 
/far/ less frequent than an aluminum electrolytic capacitor (except for a 
handful of certain TTL logic, which has been discussed previously here on 
classiccmp and elsewhere).

>> Just like the NiCd and SLA batteries I mentioned, aluminum electrolytic 
>> capacitors by their very electrochemical nature degrade as they age and 
>> as they are used. You cannot claim that a 20-30 year aluminum 
>> electrolytic
> Semiconductors also degrade both with time and use. I would think a 
> 30-year-old 3 terminal regulator IC was also beyond its design life. So 
> do you replace those 'anyway'? The damage done if one those fails is 
> likely to greatly exceed the damage done if a capacitor fails.

Honestly, I've not seen all that many failed 3-terminal voltage regulators 
in the field. I've seen some which failed due to insufficient cooling or a 
loose heatsink, or with a hole blown in them after being hit with 24V AC, 
but not any that have failed short under normal use.

> Do you replace all EPROMs in case they develop bit-rot (They are most 
> likely way beyond their design life by now)?

I think like most of us on this list, anything I have which is mission 
critical gets backed up and can easily be reprogrammed if it fails. Most 
of the problems I've seen with bit-rot of EPROMs have been cases where 
their quartz windows were not originally covered. I use opaque foil 
stickers (I can see if I can find the part number for the stickers I use 
if anyone needs it) on any UV erasable memories I program and I haven't 
had issues with corrupt data.

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