[vcf-midatlantic] Reforming capacitors

Herb Johnson hjohnson at retrotechnology.info
Thu Dec 12 14:03:25 EST 2019

Bill S. says: "The only issue was that all of the [Processor Tech SOL]
unregulated S-100 supplies were out of spec.  This turns out to be a 
design issue, with a specific engineering note to address it (adding a 
bucking transformer) which I applied." - end quote

Bill, for those not in-the-know: a "bucking transformer" reduces AC 
voltage by turning a transformer into an autoformer; by connecting the 
secondary in reverse and in series with the primary, to reduce the 
output voltage. One can also use a reducing transformer of course. 
That's too brief but some Web searching will explain further.

Point being: many vintage computers of the 70's which use dem big 
transformers, expect line voltage to be 110 volts. Modern AC line 
voltage may be 120 volts; mine is about 122. I've had Heath/Zenith H89's 
blow up their regulator tantalums because the  unregulated DC to their 
little DC regulators, exceeded the voltage ratings! Again, review H89 
schematics for details.

And of course, S-100 computers with non-switching power supplies and 
only a few S-100 cards to power, often produce excessively high 
unregulated DC voltages to the S-100 card DC regulators. As in IMSAI.

I too have had to either a reducing transformer, rig a 12V AC 
transformer to "buck", and/or upgrade tantalum caps to higher voltage 
ratings (or replace same when they shorted). Or, just run the thing off 
a Variac.

This was a good discussion. I will capture it for future reference, and 
possibly ask the participants privately if I can include their notes on 
my Web site; where I have similar information.

Herb "pop!" Johnson

Herbert R. Johnson, New Jersey in the USA
http://www.retrotechnology.com OR .net
preserve, recover, restore 1970's computing
email: hjohnson AT retrotechnology DOT com
or try later herbjohnson AT retrotechnology DOT info

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