[time-nuts] HP 5065A rubidium lifetime

[Bob Camp]

Hi

I think I can answer part of that, though I've never dissected a 5065.

To make the heater work right you need the proper resistance / foot heater wire. Cupron is a pretty typical material if you want to solder to it. Nicrome is fine if you are welding to it.  THe real trick here is to find somebody with a spool of the right stuff and then beg a few feet from them. 

Doing a hairpin and then twisting is much harder to do right than winding it tightly and then shorting the end of the pair. 

The easy way to make the twisted pair is to use an electric drill. Once you wind the stuff, both ends are scrap, but the part in the middle is quite good. You loose an inch on each end and get a few feet (or how ever much) out of the middle.

Since the maximum temperature the oven can go over ambient rises as the heater resistance goes down (good old P = E^2/R) you might put a heater on thats a slight bit higher in resistance than the original. that would be a problem when it gets to 0 (or -20) in you basement, but it would take the load off of the rest of the parts.

Bob


On Mar 20, 2010, at 8:30 PM, 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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