[time-nuts] HP and other equipment failure

Robert Atkinson robert8rpi at yahoo.co.uk
Sat Jun 15 16:47:01 EDT 2013

Hi Perry,
While I agree with everything else you say, you CAN have too much filter capacitance. At least where dc rectifier / filter (smoothing) circuits are concerned. Increasing C causes increased ripple current and inrush current and can overstress rectifiers and transformers. Not actually a problem in most cases, but if the rectifier or transformer is already marginal, blindly slapping in a much (>2x) larger capacitor can cause troubles.

Robert G8RPI.

 From: Perry Sandeen <sandeenpa at yahoo.com>
To: "time-nuts at febo.com" <time-nuts at febo.com> 
Sent: Friday, 14 June 2013, 5:48
Subject: [time-nuts] HP and other equipment failure

There has been an on and off discussion of
equipment failure so I’d thought I’d add my experience.
First I’ve been repairing HP equipment since 1976
before many of you were born.
I now have over 16 pieces of HP test equipment and
several units now need repair.
In my experience, the vast amount of failures are
electrolytic caps with some aggravated by heat.
Someone floated the notion of not repairing HP
equipment but cannibalizing it for parts.
Please bear with me on my long story.
After WWII there were all sorts of surplus stores
selling everything in the mid-fifties.  I
even remember an add in popular Mechanics magazine for a Norden bombsite for
$29.95.  Much of my allowance was spent
on mysterious wonders like a IFF receiver.  Hams reveled in B-29 prop pitch motors for rotating beam antennas.  Since it was 28 volt stuff it was far, far
cheaper than commercial equivalents.
Then it all gradually disappeared.  Now people want $75 or more for a cruddy
ARC-5 receiver.
 Now this is
how it applies to us today.
If one peruses the Ebay adds for HP test equipment
one frequently sees a statement like *removed from a place that went out of
business* or something similar.
True, the equipment we are buying is 20 years old
or older.  But it is going away never to
return.  I saw an old Ebay invoice from
12 years ago where I won a working HP 3586B for $50.  The shipping cost me more!  Now a non-functional unit sells for $400.
These prices are only going to continue to rise as
the supply continues to diminish.
But this equipment is repairable unlike the
questionable test equipment from China.  Doing preventative maintenance on this equipment is not optional if you
want it to continue working.  All electrolytic
caps should be replaced, except for tantalums.  That will be more on a case by case business.
This is equipment you can repair.  This is not very true for the newer stuff.
On the HP 3586B for example, there are a dozen or
so of TVA atoms.  When I do mine I expect
it will then lock below 500 KHz as it is specified.  The HP 5370B needs far more cooling than
provided.  I have even given thought to
adding additional resistances to the pass transistor collectors on the outside
by the heat sink.  I found on my two that
the mother board was scorched from overheating by rectifier diodes.  This will have to wait until after we have
moved.  I will also add EFC to the 10811 oscillator.
(Why that feature was omitted can be answered by Ric).  
There are two long standing truths about
electronic equipment.  One you can’t have
too much filter capacitance.  Two, you
can’t cool too much.  (Please spare me
the  liquid nitrogen or submarine battery


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