Rubidium LPRO-101 fix - Page 1
Forum section: electronics / repair

author: Mrt12
author_rank: Regular Contributor
timestamp: August 24, 2020, 09:20:06 pm
content: Hi colleaguesI got a LPRO-101 which locks, but it has a somewhat unstable frequency. It has a frequency stability of around 3mHz at 100sec. which is worse than specified.I measured the lamp voltage, which is 6.2 Volts. Is this a sign that my Rb lamp is close to its end of life? can I investigate somewhere in the unit to find the cause of this unstable frequency?I halve already seen the LPRO-101 repair guide. Apparently, the lamp voltage should be between 6 and 9 Volts, so my unit is at the lower limit. How critical is this?

author: thinkfat
author_rank: Unknown
timestamp: August 25, 2020, 11:35:01 am
content: Hi Tobias,total lamp voltage decreasing could/would result in lower signal to noise on the detected signal and in consequence more noise on the servo signal that controls the XO.Two potential causes come to mind:- something obstructs the lamp light, maybe metal film on the inside of the bulb, in that case, remove the bulb and gently hit it with a hot-air gun. That should make the film disappear.- lamp temperature too low, in that case, check the temperature of the lamp housing, should be 100°C.How does the waveform on the servo test pin look like? Peak-to-peak amplitude is interesting.What is the visual impression of the lamp light?

author: Mrt12
author_rank: Regular Contributor
timestamp: August 25, 2020, 06:08:24 pm
content: Hi,many thanks for your hints!Well, if I remove the bulb and heat it with the hot air gun - how hot should it become, is there a minimum/maximum? is it maybe worth cleaning the lamp with isopropanol?(Actually, how does that lamp work? I read somewhere it is a gas discharge lamp, but with 6 Volts? seems a bit low, isn't it?).At this moment I cannot yet measure the lamp housing temperature. But I ordered some thermocouple sensors for my multi meter.Which one is the servo test pin? that one with the fancy waveform which then drives the SRD?the visual impression of the light: it is not so bright, and I think it is more like purple-blue (it should be red-ish, right?).

author: thinkfat
author_rank: Unknown
timestamp: August 25, 2020, 08:11:12 pm
content: Quote from: Mrt12 on August 25, 2020, 06:08:24 pmCleaning the bulb from the outside will not hurt, but not help either. The metal film forms inside. I have no exact temperature to give, but I think something like 200°C should be enough. The metal film would be visible with a magnifying glass.The lamp is excited with RF. Basically you pump it with RF and heat it, then it will light up.The "lamp voltage" is not what lights the lamp. It's the voltage from the photocell inside the microwave cavity, behind the Rb chamber. The photocell is what detects the absorption dip when the Rb gas in the chamber is excited at just the right frequency.Check the LPRO repair guide for the servo test pin. There's a sample waveform given, too. Check the peak-to-peak amplitude.

author: Mrt12
author_rank: Regular Contributor
timestamp: August 25, 2020, 08:16:38 pm
content: Ah yes I checked the repair guid and looked ad the servo signal. I couldn't adjust its amplitude - rotating the pot did not change the amplitude very much.However, I removed the lamp from its housing and looked at it - it didn't look like there was a metal film inside, but the bulb looks somewhat yellow-ish. I cleaned it and I also saw that the little window at the front of the lamp housing was a bit dirty and cleaned it.Then when I tried to install the lamp back in its housing, I was not sure how far it should protrude into the housing, so I protruded it as much as possible. And the cool thing is now I get almost 7 Volts at the lamp monitor pin! yay!Can I destroy the lamp if I heat it too much with the hot air gun?

author: thinkfat
author_rank: Unknown
timestamp: August 25, 2020, 08:30:52 pm
content: Quote from: Mrt12 on August 25, 2020, 08:16:38 pmThere's not a lot inside this bulb. Some noble gas, Argon, if I'm not mistaken, and some Rubidium of course. If you don't damage the lamp mechanically, probably nothing to fear. Take it easy, though. Better safe than sorry.Can you post a screenshot of the waveform at the servo test pin?