22 Feb
2015
22 Feb
'15
9:14 a.m.
On Sat, 21 Feb 2015, Robert Jarratt wrote:
I have been desoldering and checking all the
electrolytics. I have set
each one up on my bench PSU with the current limiter set, and taking
them up to their rated voltage, or as near as I could because some are
rated 50V and my bench PSU only reaches 30V. None of them seemed to show
any leakage, but my bench PSU will not display currents below 10mA, I am
not sure if that is enough to show leakage that matters. I also tested
their capacitance values, a few are well above nominal value, but within
the tolerance shown on the printset. The worst one is +41% when the
range is +50-10%.
I started checking some of the tantalum capacitors. They all looked
fine, but I don't know what a sensible ESR value is for these. One of
them (C315) is rated 1uF 35V and has an ESR of 3.4. Is that high for a
tantalum?
One respondent asked for a picture, the printset is here,
http://manx.classiccmp.org/details.php/1,5422 and the relevant drawing
is on page 58.
Is C441 really 10uF 25V? That is what both the parts list and schematic
show, but even as old as a VT100 is, I wouldn't think a standard 10uF 25V
part would be in an 18mm diameter can.
I've had this discussion with several people recently, but why bother even
attempting to reform the capacitors on this particular board? If you are
going to desolder them anyway, why not just replace them and be done with
it? From the high resolution photo, the original axial mount parts at C439
and C437 have already been replaced and almost all of these parts are
cheap and very easy to obtain. The only exception I can see is C437, which
is a 75uF 6V part. Even with that one, you can still get 75uF axial mount
parts from Vishay in 25V and 50V which would probably fit the pad layout
just fine.
I dunno...maybe I just have a totally different way of rebuilding older
gear. I'd rather replace any 20-30 year old aluminum electrolytics
wholesale with known good modern parts (which as long as you stay away
from the knock-offs and counterfeits on eBay, are
likely to far outlast
the originals) and then move on to testing and
troubleshooting other stuff
and not have to later return to troubleshooting a power supply or
something else because of intermittent issues caused by old aluminum
electrolytics. This is definitely the way things are done in the arcade
and coin-op world (no point in troubleshooting faulty logic chips until
you clean up any power supply ripple), and also the way things are usually
done with vintage TVs and radios (vacuum tubes/valves), but I've seen
pushback from some people in the vintage computing community to wholesale
replacement of aluminum electrolytics which are long past their life
expectancy and I just don't get it.
In the vintage audio communities, there are of course a handful of
"purists" who would rather have 40+ year old leaky (as in voltage) dried
out paper caps in there audio gear than modern poly film parts, but why
the reluctance to changing out aluminum electrolytics in things like DC
power supplies? It's not like these parts wouldn't have been replaced
already had a vintage computer still been in a production environment.