Electronics > Repair

Help needed for repair of Efratom FRS-C rubidium oscillator

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ejd.pol:
Hello,

Recently I have acquired faulty Efratom FRS-C 2A8A4C Rubidium oscillator. It was the internal reference of a PM6685 frequency counter.
The supply of the frequency counter is toast, and maybe the main board is too. So I am in the process of seeing what can be saved.
The first and foremost thing to try to save is the reference. The serial number of the oscillator is 20702, date code 50/94.
 
Unfortunately, the coax output signal is not the expected 10 MHz sine wave. Instead, the output is a sine wave that hovers somewhere around 60 Hz.
Fortunately though, the device still seems to be able to lock: after a few minutes warming up (it does get warm), the "unlock" led goes off.
So this gives me some hope that not all is lost, and a repair is possible. (The unlock led is connected to pin 1 of the external connector: lock monitor)

So, I took off the top cover, to see what was underneath. A nice construction of four boards making up the sides, interconnected by a flex board on top.
The flex board seems to have a number of numbered test points, so I measured the DC values on these points, after warming up and unlock off.

2: 0.65 V
3: 6.38 V
4: 2.55 V
5: 23.22 V
6: 17.09 V
7: 0.00 V (I used this point as measurement reference for the other points)
8: 8.86 V
10: 5.92 V
11: 5.75 V
12: 0.00 V
13: 0.00 V
14: 2.51 V

While doing these measurements, i noticed that point 11 is very sensitive: by just touching it, I can unlock the device (led on).
And in that state, I can see on my scope that the output sine frequency changes, and even seems to cross 0 Hz.
The highest frequency that it produces during the sweep is about 160 Hz.

Perhaps the symptoms as described are known for the FRS-C type, and hopefully the repair is known as well.
Of course, more measurements can be taken (DMM or scope) if useful.
I have found some documentation on the FRS-C on the web, but the complexity involved is daunting.
I hope someone can give me some guidance on how best to proceed.

I have taken some pictures of the disassembly, so if there is any interest, I can post these as well.

Many thanks in advance, Evert-Jan

sanleontexas:

May I suggest that you relist this in the metrology section? There's a lot of talent there,
and likely that someone is familiar with this.

ejd.pol:
Sure, thanks for the good advice, much appreciated. I will open a new thread in Metrology.

Halcyon:
Don't "relist" posts in multiple parts of the forum.

If something needs to be moved, we can move it.

ejd.pol:
Hello, I am sorry to have overstepped the rules! Can you help in moving this thread to "Metrology"?

Many thanks in advance, Evert-Jan

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