[time-nuts] HP5065A Rubidium questions: some progress

David Scott Coburn scotttt at optonline.net
Mon Feb 20 19:25:37 EST 2017

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, 

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 

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.



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