[time-nuts] Lamp oven repair info for HP 5065A

[Chuck Harris]

The main oven on my 5065A was made with a bifilar winding of enamel
covered nichrome wire wound directly on the aluminum oven.  To cancel
the magnetic field, the far end of the coil was shorted, making the
heater a hairpin loop....  No twist that I recall...  The enamel was
compromised somewhere along the winding, causing it to put the oven
into thermal runaway, thus toasting the lamp assembly.

(this was my first exposure to how stupid HP could be with some
  of the designs they made...)

The runaway oven got hot enough to reflow the solder, and cause
several parts to fall out of the lamp's circuit board... the board
was turned into just plain old fiber glass matting, as all of the
epoxy... or whatever it was, turned to charcoal.

When I did this stuff, I was fresh out of EE school, the web didn't
yet even exist as an idea, and I knew nothing about how rubidium
standards worked.... I knew they were special, so I called up HP,
and ordered a $300 manual... but I digress...

My first fix was to disassemble the oven, and wrap it with a solenoid
of teflon insulated heating wire.  It worked nicely, except that it
made a magnetic field, and shifted the frequency with temperature..

<Whack!>... DOH!

Next, I found some thin coaxially shielded nichrome wire meant for
I know not what... maybe electric blankets?  And put a crimp on
connector at the far end to make a coaxial hairpin loop.  I tightly
wound that around the oven, and used "great stuff" urethane foam
to replace the oven's insulation.

It worked quite well, until the next thing went wrong, and I put the
reference on my to be fixed someday shelf where it has lived ever
since.

Back then, I didn't know anything about capacitor plague, and carbon
composition resistor drift, and all the usual failure modes of HP
equipment... I ought to take it off of the shelf and give it a go
once again... should be easy... for an engineer that now has 35 years
more experience... Right?

-Chuck Harris

cdelect at juno.com wrote:
> Hi, Just thought I'd share some info on repairing a defective HP 5065A
> lamp oven.
> These ovens can fail either shorted to the oven cylinder
> or have interwinding shorts.
> I have repaired three optical units so far with this failure.
> The original winding was insulated twisted pair wound
> directly onto the Aluminum oven cylinder.
> I used teflon insulated heater wire, Pelican P2332ADVFEP.009BL.
> It is approx. 5 Ohms/ft. and I use a  50 Ohm  length doubled and then
> twisted tightly.
> This is then wound onto the oven cylinder which is first covered with a
> single layer of kapton
> tape.
> The original thermistor is replaced with a DigiKey 615-1007-ND.
> The thermistor MUST be surrounded by heat sink compound.
> I also install a digiKey TO220 100 degree thermal cutout switch for
> future
> protection. It is mounted to the top of the lamp assy. using one of
> the 3 original mounting screws. The thermistor and heater wires are
> brought out tie wrapped along the original cable and then soldered
> to the appropriate pins on the db9 connector.
>
> Cheers,
>
> Corby