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

paul swed paulswedb at gmail.com
Sun Aug 2 22:21:29 EDT 2015

I suspect it actually is for you. As you say you now have a great deal of
insight that you did not before and as I have often read here. Its funny
how we have all learned about the nasty effects of age. Ahhh I mean
equipment that is.
So I would bet you could get it going. The RB elements in the 5065 seem to
last a long time its worth a shot.
Kind of like the HP 5060/5061 was worth a shot and I learned a lot. Can't
help that it actually works now. That was unexpected. Bythe waythat effort
sort of cured me of the I want a CS. :-)

On Sun, Aug 2, 2015 at 6:36 PM, Chuck Harris <cfharris at erols.com> wrote:

> 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
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