[time-nuts] Questions about HP 5370B

[John Miles]

> I hate to disagree with John who knows a heck of lot more than I ever
> will, but in this case it will protect the oven from cooking up if the
> control circuit fails with the heater full on, which can happen.

That might be plausible if the thermal fuse were anywhere near the oven
mass.  However, it's not -- it's on the outer PCB, separated from the oven
mass by a thick block of insulation.  The fuse won't open up until the oven
is already well into the China Syndrome stage.  If they were concerned about
protecting the oven, why wasn't the fuse installed on the inner PCB?

Also, the heating elements are the transistors themselves, with two
connected in series.  If one of them shorts out, the other will take over
its job.  If they both short out, or are driven full-on by a control
failure, the power supply overcurrent protection will presumably save the
day.

> I did have a 5370B with a 10811 that had a bad thermistor in it as well
> as an open fuse.  I'd guess that's why -hp- fitted it.

Earlier, someone (Rick K.?) mentioned that the fuse had to do with a
regulatory requirement to suppress fires that would lead to toxic vapor
emissions.  10811 fires are pretty far down my list of fears, so I don't
feel too bad about bypassing the fuse...

> John - Santa Clara designed the 5370A/B such that the fan sucks
> external air
> into the box. The box is designed so that a portion of this air
> loops around
> and then vents back to the outside through the three holes in the
> external
> heatsink. This helps cool the external heatsink, as the venting
> air blows by
> the three center fins of that heatsink assembly.  Having said
> that, if you
> end up running your particular 5370B in the long-term without a fan (or
> without a top cover), let us know how you come out with it.  :-)

Hmm.  Not saying you're wrong, but why wouldn't this be mentioned in the
service manual?  The only warnings I've seen are the usual ones about not
blocking the fan outlet during installation.  Maybe it was only an issue
with the earlier 5370A, with its additional digital hardware?

I haven't run a 5370B without a cover for days at a time, but it has
certainly run open for an hour or two during calibration.

-- john, KE5FX