[time-nuts] LPRO-101 internals

iovane at inwind.it iovane at inwind.it
Mon Aug 18 05:56:33 EDT 2008

Sometimes trying to fix a faulty unit is simply a challenge.

Some more tips.

The LPRO with cover removed cannot work in the presence of artificial light coming from AC powered sources!! It locks randomly to the light ripples.

My definition of the normal color of the rubidium light would be pink-violet, and the abnormal would be red-violet. The definition of colors may be subjective, but the color should not vary so much after power up.

The crystal looks like a transistor (TO5) and has a heatsink. It is located in the corner opposite to the lamp. The other similar component on the board is actually a transistor.

Your OCXO Volt is good.

I think you have a faulty resistor.

Without disassembling the unit, try measuring the voltage at the base of Q703 (1P SMD) located near the lamp assy. At power up it should read some 11 V, and after warm-up it should read some 5 V. If the voltage doesn't drop down, then R721 (61.9K smd, bottom of pcb) might be interrupted: measured onboard it should read some 14K if good, 18K if interrupted. For replacement you could use a 56K and a 5.6K in series.

> Some more info on the flakey LPRO:  I put the cover back on the LPRO that seemed to have magically started working.  After a while it went UNLOCK.  Took the cover off,  LOCK.  Seems like mayby a temperature problem after all (or perhaps the cover is affecting the tuning of some component).  When looking at the lamp light through the side port of the physics package the color appears a lightish violet.  When looking at the lamp base cover (there is a white plastic insert there) the color is more a pinkish red.
> I measured the LAMP and OXCO control voltages.  OCXO is around 7.3V.  LAMP is around 5.6V  When warming up (even for some time after LOCK is first indicated)  the LAMP voltage is unstable...  it varies from around 4.3 to 5.7V (usually varying around 0.1V between successive meter readings)
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