FE-5680A Rubidium Standart Repair

[chick0n]

Hi

So I got this broken FE-5680A (PLL didnt lock). The PLL was moving around 50.255 MHz, searching for lock. (Its the old "Analog" Rev.B Version with the 3 stacked PCBs. Not the new Xilinx FPGA/CPLD Rubbish)

The housing was full of smoke traces. I found, the Isolation of the Rb-Lamp Coil was burnt. So I wound a new Coil and replaced it. I also gave the Rb-Lamp the Hot-Air treatment.

Then I tried to tweak the C245 Quartz-Trimmer a bit. But still no Lock (Luck).


Then the "Abuse" began. 

I completely disassembled the Physics-Package, like Mike did:

I checked the SRD-Diode with a pickup coil, and 4GHz Spectrum-Analyzer. I could pickup harmonics at 3.4GHz, so the SRD should be working.

I removed the 2 Rubidium Cells,  and gave them Hot-Air treatment too.

Then I inspected the Photodetector, and it was a bit dirty/fogged. So I cleaned it with Isopropanol. (I think that was the whole problem).

Then I put everything back together.

Somewhere in the process, I destroyed the Temperature-Sensor of the Rb-Lamp housing, so I had to replace it with a 2k-pot for now.

After tweaking C245, and the 2k-Pot a bit, the friggin thing locked. 

Pin3 went from 4.2Volts to 100mV, and I can measure stable 50.255MHz.


The automatic Temperature Control of the Lamp-Housing seems to be very Important, so I need a replacement for the temp-sensor.

Could anyone who owns a FE-5680, measure the Resistance of the sensor? (Detaching the Sensor-Wire from the PCB, and measure Ohms in cold condition, then apply Power to the FE-5680 for ~30sec and measure Hot condition, would be optimal)



Thanks!!

[jdragoset]

Got two FE 5650A - 58 mod.
One shows lock, when locked.
The other does not, even though locked, as verified by the frequency counter.
That sure is one cute lavender/pink glow from Rubidium lamp when locked
Every unit should have a light pipe to the front panel, at least you would know the RF lamp circuit is working.
Looking at the color shift, it seems they use a mix, with Argon gas to start that vaporizes the Rubidium.
It does appear the used market price of these have doubled since this pand****

[chick0n]

Ok..my Ghetto Rubidium is working now, somehow.

I replaced the broken Temp Sensor with an NTC (10K @25°) and a 10K-Pot in series.
I tweaked it, that it keeps the Lamp housing at ~100°C.

It is locking now.

I programmed the DDS to 10MHz, and measured it against a good Rubidium, and it is off ~670milli Hertz.

The frequency is also wandering up and down a bit, altough it shows that its locked.

Is this normal behaviour?

[dietert1]

You can expect better stability with the base plate mounted to some kind of heatsink and with the cover on. Also you may need to attach the NTC with glue.
We have two of those units and under lab conditions (stable room temperature and stable power supply) they are good for 1E -11.

Regards, Dieter

[chick0n]

I measured without the DDS PCB, and it bounces back and forth between 50.2550566MHz and 50.2550580MHz. I dont know if this is within spec.

I think the problem is on the Crystal-Oscillator PCB, its very sensitive against Temperature fluctuation.

When I measure my 2 LPRO-101 against each other, they are Rock-Solid.