Remex Tape Reader - Pre-power up advice?

[Jay Jaeger]

On 4/18/2017 3:17 AM, Christian Corti via cctech wrote:
> On Mon, 17 Apr 2017, Rod Smallwood wrote:
>> There are what appear to be 1976 date codes on some caps.
>>
>> If its that old then replace all and any electrolytic capacitors plus
>> any paper based caps.
>>
>> If they aint bad now they soon will be.
> 
> *shaking head*
> 
> Sorry, this is just a plain dumb answer. If they are good now, they
> probably will be good in 10 years, too. We never change any caps just
> because of their age.
> 
> I suggest: check for electrical safety, then plug it in and try it;
> after all, it's "just" a tape reader with a simple PSU, not a 50s era
> mainframe.
> It will just work, I guess. If there should be a problem with those "big
> caps", you'll see it. But it's much faster and easier to test them
> beforehand (i.e. short or no short) than to foolishly replace everything.
> 
> Christian
> 

While I also do not typically replace capacitors outright, I don't think
that the answer is "dumb".  It just comes from a different perspective -
typically from those who for one reason or another wish to maximize
reliability and don't care to deal with failures down the road, and for
whom preserving original components is not a priority.

But I also I don't think that just plugging the unit in and turning it
on (aka a "smoke test") right off the bat is necessarily prudent.  A
shorted input capacitor can easily take rectifiers with it (a capacitor
that needed reforming did that to a rectifier bridge on my PDP-12 at one
point), and a shorted output capacitor can take regulator components
with it.  Also, a shorted capacitor can generate enough steam to
explode, if it doesn't have a pressure relief plug, and that can be
messy, regardless.

So what I typically do is locate the larger capacitors in the supply,
and re-form them (I think so far I have only run into a couple that
needed replacement due to an unacceptably high ESR).  This spots shorted
units along the way, and one only need disconnect on lead to the
capacitor to accomplish the work.  Also, it allows one to bring the
capacitor up to its rated voltage, rather than just the in-circuit voltage.

JRJ