[time-nuts] HP 5065A rubidium lifetime

[Chuck Harris]

Hi Joe,

The original winding was wound with parallel (non twisted) bifilar
wires.  The two wires at the entrance to the winding went to the oven
circuitry, the two wires at the exit to the winding were shorted, thus
forming the "hairpin".  There were hundreds of turns of wire tightly
wound in a single layer.  I recall that the wire was about #36 AWG.

I replaced the winding with some coaxial heating wire that I found
someplace.  It has been something like 25 years since I did this.

As I recall, HP wound some Kapton tape on the aluminum oven assembly,
and then wound the nichrome wire over the tape.  The whole assembly fit
loosely in the mumetal can that encloses the entire physics package.
They expanded some kind of foam in the space between the assembly

I knew the oven was shorted, and I had little to lose, so I just pressed
the assembly out of the can.  This exposed the oven winding.

When I foamed the oven, I used a couple of wooden wedges to hold the
aluminum oven centered in the shield.

I seem to recall that there were two mumetal cans, with foam between
them.  One had a tuning solenoid winding, and the other had a heater
winding.

All of this is pretty foggy...

-Chuck Harris

J. L. Trantham wrote:
> Chuck,
> 
> Can you provide any other information about this repair?
> 
> How much disassembly of the RVFR, what kind of prep of the lamp casing, how
> much wire, where did you find the wire, did you 'bend' the wire into a
> 'hairpin' or did you wind two wires then short one end and feed the power
> from the other end, what kind of prep for the outside cylinder, what kind of
> 'jig' to hold everything in place while applying the urethane foam, etc.,
> etc.
> 
> I have two of these and one arrived dead after which I 'killed' it some more
> with a total melt down.  If this should arise again, a repair guide might be
> very helpful.
> 
> Joe
> 
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Chuck Harris
> Sent: Saturday, March 20, 2010 7:35 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] HP 5065A rubidium lifetime
> 
> 
> I can't say for certain.
> 
> The heating element is a single layer of formvar coated nichrome wire....
> about #36 gage.  To avoid magnetic fields, they wound it bifilar, and
> shorted the far end of the bifilar wires... forming a "hairpin" loop.
> 
> I found a short about 1 inch into the 5 inch winding, and the oven driver
> transistor was open circuited.  The 1 inch that wasn't shorted was
> surrounding the area of the lamp assembly.
> 
> I recall that the oven fuse was ok.
> 
> -Chuck Harris
> 
> John Miles wrote:
>> What was the root cause of the oven failure in your case?
>>
>> -- john, KE5FX
>>
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com 
>>> [mailto:time-nuts-bounces at febo.com]On
>>> Behalf Of Chuck Harris
>>> Sent: Friday, March 19, 2010 9:58 PM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] HP 5065A rubidium lifetime
>>>
>>>
>>> When my 5065A had its oven failure, it got so hot that it melted all 
>>> of the solder joints on the lamp board.  I resoldered the joints,
>>> rewound the oven
>>> winding, and foamed the unit with some spray can urethane (Great
>>> Stuff), and
>>> had it working again for a couple of years.  Then something else failed.
>>>
>>> I'd sure like to fix it, but Scott McGrath, representing himself as 
>>> an employee of Harvard University, borrowed my manual more than a 
>>> year ago, and refuses to return it.  Oh well!
>>>
>>> -Chuck Harris
> 
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