[time-nuts] HP10811 Oscillator Thermal Fuse

Richard (Rick) Karlquist richard at karlquist.com
Thu May 11 12:51:34 EDT 2017

On 5/11/2017 4:25 AM, Bob kb8tq wrote:

> Ultimately it all came to no good. The energy conservation rules simply took over. Shutting
> down everything during off hours became the way a lot of outfits did things. Don’t turn off all

A couple of comments on this, probably no surprise to many
time nuts.  All or most HP instruments with 10811's keep the
oven energized even if the instrument is turned "off".
Many classic HP instruments used power transformers for
power supplies.  The secondaries were regulated by a
combination of switches and linear regulators.  The
power module was mounted on the rear panel, along
with the transformer, to help get rid of the heat without
heating up the box too much.  The dilemma was that you
had to choose between four bad alternatives:

1.  Put the power switch on the rear panel.  This is
inconvenient for the user, especially if the unit is

2.  Put the power switch on the front panel.  This
entails running wiring with all the safety approvals
up to the front panel and back to the rear panel.
I don't know of this ever being done at HP.

3.  Have a button on the front panel that actuates
a long plastic shaft the goes to the real switch at
the back.  Works, but quite a hassle.

4.  What was usually done:  leave the power transformer
primary connected all the time and switch the secondaries.

The HP transformer shop would often scrimp on the amount
of iron in the transformer, which would increase the
core loss.  Their idea of good engineering practice was
to allow the loss to heat the transformer up 10's of
degrees.  Note that core loss is independent of how
much power you are drawing from the secondaries, if any.

So in the overall scheme of things, the 10811 is small
potatoes.  Once it is warmed up, it doesn't draw much

Rick N6RK

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