[time-nuts] HP5065A Rubidium questions: some progress

Tue Feb 21 22:11:10 EST 2017

Great news and what a deal a dead 5065 lives.
I don't think its that critical and not sure how you would test it. But do
get a good stable high temp. As I recall the rn70 series was pretty good.
For the moment you can reassemble it and see if the rest works the resistor
should hold up for a 30 or so minute run. Don't push it if its a plain old
cheapy carbon.
Best of luck, though I think you have had plenty so far.
Regards
Paul
WB8TSL

On Tue, Feb 21, 2017 at 7:56 PM, David Scott Coburn <scotttt at optonline.net>
wrote:

> I installed a 10 ohm 1/2 watt carbon resistor and the Rb lamp came right
> on!  Woo-hoo!
>
>
> I will get a better quality, high temperature resistor to replace this
> one.  (RN70?)
>
>
> How critical is the resistance value?  10 ohm?  12?  Is there a way to
> test for an optimum value?
>
>
> Scott
>
>
> On Tue, Feb 21, 2017 at 10:01 AM, paul swed wrote:
>
> Totally agree with the try a 12 ohm resistor. Nice simple and low risk.
>> If thats actually the issue then you do need to obtain a high quality
>> resistor that can live in a hot environment. Anything will work for a test
>> run.
>> Regards
>> Paul
>> WB8TSL
>>
>> On Tue, Feb 21, 2017 at 2:22 AM, timeok  wrote:
>>
>>
>>>    Hi Scott,
>>>    the emitter resistor have to be in the range 9 to 15 Ohm. Probably the
>>> high temperature and current have burned it out.
>>>    Replace it with a 12 Ohm 1/2 W resistor, It will work.
>>>    Luciano
>>>    www.timeok.it
>>>
>>>
>>>    From "time-nuts" time-nuts-bounces at febo.com
>>>    To time-nuts at febo.com
>>>    Cc
>>>    Date Mon, 20 Feb 2017 19:25:37 -0500 (EST)
>>>    Subject [time-nuts] HP5065A Rubidium questions: some progress
>>>    Hi All,
>>>
>>>
>>>    Thank you all for the very helpful suggestions!
>>>
>>>
>>>    I will try to reply to you all in this one post.
>>>
>>>
>>>    I opened up the A12 assembly and was happy to find that the lamp,
>>>    windows, and reflector all appear to be in good physical condition.
>>>     There is some very light corrosion on the aluminum reflector, all of
>>>    the windows are clear, and the photodiode is very visible at the
>>> bottom
>>>    of the oven.
>>>
>>>
>>>    The Rb bulb looks very slightly darkened, maybe like a pair of very
>>>    lightly polarized sunglasses.
>>>
>>>
>>>    I measured the resistance of the 20 VDC supply line to the lamp
>>>    oscillator and it is 3350 ohms, just as Corby said and as I see it
>>>    should be from the schematic.
>>>
>>>
>>>    I measured the current to the lamp oscillator and it is only about 13
>>>    mA.  Paul mentioned that if it was about 15 mA it was probably not
>>>    working.  And, Poul-Henning mentioned that it should draw about 3W
>>> when
>>>    working, which at 20 VDC would be 150 mA.
>>>
>>>
>>>    I powered up the lamp oscillator with it on the bench and I do not see
>>>    any light from the bulb.  I am assuming that it should be visible, but
>>>    I'm not sure!  What is the spectrum from these lamps like? Visible,
>>>    UV???
>>>
>>>
>>>    I measured the resistance of the small coil wound around the bulb, it
>>> is
>>>    about 0.3 to 0.4 ohms, which seems reasonable.
>>>
>>>
>>>    (It looks like someone has been in this assembly before.  There is
>>>    solder flux on the transistor leads, the 20 VDC connection, and the
>>> bulb
>>>    coil leads.)
>>>
>>>
>>>    One odd bit is that the resistor in the transistor emitter leg
>>> measures
>>>    about 350 ohms (this is R3 in my schematic).  The schematic says it
>>>    should be 10 ohms.  This resistor is on solder posts, so it looks
>>> like a
>>>    'selected at assembly' item.  Corby: it this the 12 ohm resistor you
>>>    mentioned?
>>>
>>>
>>>    The voltage across this resistor when the power is on is about 1.6 V.
>>>
>>>
>>>    The base voltage is about 2.2 V (this is across CR1 also) and shows
>>>    "flat-lined" on my scope with the board powered.
>>>
>>>
>>>    So, I'm betting that the collector of the transistor is cooked. (The
>>>    base-emitter drop of ~0.6 V probably means the base and emitter paths
>>>    are still OK.)
>>>
>>>
>>>    But, it is also possible that the R3=350 ohms is way out of line??
>>>
>>>
>>>    Comments welcomed.
>>>
>>>
>>>    Corby: the serial number for the unit is 0968A00302.  It has a
>>> 105-6012
>>>    quartz oscillator, and a mechanical PP clock.  The A12 unit is a
>>> series
>>>    1220, and is green in color.  (The OCXO and the bands around A12 are
>>>    blue.)  The A12 unit has a warranty expiration of 30 April 1978. It
>>> has
>>>    had its "Mag Filed" and "Thumbwheel" blacked out with marker and new
>>>    values written in.
>>>
>>>
>>>    Cheers,
>>>
>>>
>>>    Scott
>>>    _______________________________________________
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