[time-nuts] Considerations When Using The SR620

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 2 23:39:40 UTC 2012


On 12/02/2012 11:36 PM, Volker Esper wrote:
> Hi,
> I was the one who wrote those nasty things about a fantastic instrument.
> I bought such a counter a few weeks ago. When I first peeked into it
> (because the fan was running at top speed after a few minutes) I
> noticed, that the arrangement of the components was anything but optimal.
> Examples: The osc is only one or two inches away from the linear voltage
> regulators. These are "cooled" by a 1mm iron sheet. The iron sheet has
> some cooling slots where the air can flow through, but these slots are
> miles away from the regulators. The sheet gets so hot that you won't
> touch it voluntarily. If at least the air could flow directly along the
> regulators - but they sit in a corner and the air flows diagonally
> through the case, avoiding contact with the regulators.

I am also not very impressed with the location of the linear regulators.

This forms a nice interesting relationship between line voltage and room 
temperature. As line voltage shifts, the rectified voltage varies, and 
then the voltage that the linear regulator needs to "burn off" varies, 
and thus their heat contribution varies with the line voltage. Now, the 
ambient temperature then controls how easy we get rid of this excess 
heat. The way the airflow goes, this cooling makes sure it couples very 
well with the electronics, so fan speed amongst other things depend on 
ambient temperature... and line voltage. All this assist to give us an 
"interesting" temperature dependence.

In contrast, I was impressed by how the HP10811A was located in a 
turbolence free corner of the HP5070A/B assembly. While itself also not 
particularly well designed in heat context, at least the heat of the 
linear regulators was supposed to mostly affect an external 
self-convection stream. Forces convection (fan) to stabilize the 
temperature of the heat sink would be a good thing there.


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