[time-nuts] HP5065A Rubidium questions: some progress

[David Scott Coburn]

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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