[time-nuts] Fw: HP 105 powersupply problems

Robert Atkinson robert.atkinson at genetix.com
Tue Apr 4 03:23:23 EDT 2006

Hi all,

Using a Variac over a long period of time is great with conventional
(Transformer-rectifier-capacitor) linear PSU's, but be careful with more
modern equipment with switchmode PSU's. They can draw excessive current
during undervoltage conditions.
Still use your Variac, but ramp it up a little quicker, maybe 30s to a
minute. Watch out for auto supply voltage (115/230) switch designs too,
it's best not to cross the switching boundary too slowly as they may not
switch back from voltage doubling to full-wave rectification resulting
in very excessive internal voltages.
If you don't have a Variac, try a low wattage (40W) light bulb in series
with the supply to limit the current.

Robert G8RPI.

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Poul-Henning Kamp
Sent: 04 April 2006 06:28
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Fw: HP 105 powersupply problems

In message <002901c6578c$9d928120$0e8fae43 at hsd1.in.comcast.net>,
"Richard and M
arcia Putz" writes:

>I had similar problems with my HP105, not bad transistors, check
>out the filter capacitors. Electrolytics in any equipment past 15-20
>years of age could be suspect. I'd look there first.

In general when powering up equipment that has been turned of for
years:  Use a variac and ram the voltage up slowly over a couple of

Electrolytics can loose their polarisation over time and if exposed
to full voltage they are likely to splatter all over the place, but
if you give them a slow start they will most often recover their
polarisation and work fine.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by

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