Author Topic: YABE: "Yet Another Broken Efratom"  (Read 479 times)

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Online thinkfat

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YABE: "Yet Another Broken Efratom"
« on: October 26, 2019, 01:05:40 pm »
Hi, got another broken one, because, being able to repair one doesn't mean a thing, right?

Not sure what it is, the label just says "PART NO: 100334-004", I guess that's a customer part number. Some photos attached.

Didn't yet fire it up, don't know where to connect the power supply yet.

Any idea which of the many Efratom models this actually is?

EDIT: It's an Efratom FRS.
« Last Edit: October 26, 2019, 01:24:22 pm by thinkfat »
 

Online thinkfat

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Re: YABE: "Yet Another Broken Efratom"
« Reply #1 on: October 27, 2019, 08:52:31 am »
So, heater works, power consumption is nominal 1.7A at cold start, drops to around 400mA when hot.

Lamp voltage is 7V, this is too low. Should be above 9V. Might need to check the lamp.

Frequency tripler works, SRD checks out OK. Adjusted voltages to the nominal 3.38V (R25/R27, R30/R22, C30/L8)

Some spurs on the servo test point, might need to adjust the exciter frequency.

The servo test pin shows the characteristic pattern of an unlocked sweep, when the frequency is near the hyperfine resonance, but the amplitude is _way_ to low. it should be ranging between +10V and 0V, but it's only +-1V around 5V.
Possible failure modes:
* worn-out lamp -> we already know the lamp voltage is low
* defective error signal preamp
* detuned SRD bias -> I've already adjusted the voltages but the SRD might have aged. This was the problem with the LPRO I repaired.

The easiest thing to check is the error signal preamp. If that checks out good, I'll try disconnecting the servo from the XO and applying the tuning voltage manually. If done carefully, I can hit the hyperfine resonance long enough to tune the SRD bias back into a functional range.

If nothing goes, I'll disassemble the thing and have a look at the lamp itself.




 

Offline capt bullshot

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Re: YABE: "Yet Another Broken Efratom"
« Reply #2 on: October 27, 2019, 09:45:18 am »
Same as mine (has the same number on its enclosure):

http://wunderkis.de/efratom-frs/

Check the pots controlling the RF leveling loop
Edit: just seen you did that already
« Last Edit: October 27, 2019, 09:51:40 am by capt bullshot »
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Offline edpalmer42

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Re: YABE: "Yet Another Broken Efratom"
« Reply #3 on: October 27, 2019, 05:30:55 pm »
You said that the heater 'works'.  Have you checked the temperature of all the ovens?  I have one very old Rb standard where one of the oven thermistors drifted badly and reduced the temperature of the cavity by 20C.  This caused no second harmonic and no lock.

An obvious problem (not in this case - I'm playing to the audience) is that the output frequency is sweeping correctly, but the quartz oscillator has drifted far enough that the sweep doesn't hit the correct frequency.  I saw that once - really easy fix!

Ed


 

Online thinkfat

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Re: YABE: "Yet Another Broken Efratom"
« Reply #4 on: October 27, 2019, 09:35:42 pm »
You said that the heater 'works'.  Have you checked the temperature of all the ovens?  I have one very old Rb standard where one of the oven thermistors drifted badly and reduced the temperature of the cavity by 20C.  This caused no second harmonic and no lock.

An obvious problem (not in this case - I'm playing to the audience) is that the output frequency is sweeping correctly, but the quartz oscillator has drifted far enough that the sweep doesn't hit the correct frequency.  I saw that once - really easy fix!

Ed

Hi Ed,

thanks for joining. I didn't yet check the temperature in detail. It looks like I'll have to disassemble the electronics completely to get to the physics package, so I'm looking into checking the easy stuff first. Once I've ruled out that, I'll go deeper into the rabbit hole.
 

Offline edpalmer42

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Re: YABE: "Yet Another Broken Efratom"
« Reply #5 on: October 27, 2019, 09:51:44 pm »
Is that a hole I see in the middle of the oscillator board?  On the Efratom FRK, there's a hole that you can poke a temperature probe through to check the temperature of the Rb cell in the cavity.  Be careful, remember that the cell is glass!

The manual for the FRK has more diagnostic/adjustment/repair info than the FRS manual.  It might be helpful to grab a copy.

Ed
 

Online thinkfat

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Re: YABE: "Yet Another Broken Efratom"
« Reply #6 on: October 28, 2019, 08:06:42 am »
Is that a hole I see in the middle of the oscillator board?  On the Efratom FRK, there's a hole that you can poke a temperature probe through to check the temperature of the Rb cell in the cavity.  Be careful, remember that the cell is glass!

The manual for the FRK has more diagnostic/adjustment/repair info than the FRS manual.  It might be helpful to grab a copy.

Ed

That's the wrong end ;) The center hole goes to the lamp assembly and there's a sheet of thermal insulation foam preventing me poking a probe into it. The other hole goes to the trimmer cap for adjusting the frequency of the exciter oscillator.

The FRK manual is pure gold! I downloaded a copy when repairing the LPRO, it has a _lot_ of troubleshooting information and full schematics. The FRS manual also has full schematics and drawings for the part locations, that is also very good.

At the moment I'm scratching my head about the AGC circuitry on the multiplier board. The interaction between the two pots for setting the SRD bias and the AGC setpoint is really quite fiddly. Adjusting both to "nominal" values doesn't really do anything. I think I'll go for the basics now and check the physics package, heater temperature and especially the condition of the lamp.
 

Offline 5065AGuru

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Re: YABE: "Yet Another Broken Efratom"
« Reply #7 on: October 29, 2019, 04:57:48 am »
Hi,

Your unit is a version of the FRS-C. Manuals at http://ftb.ko4bb.com/getsimple/index.php?id=manuals&dir=02_GPS_Timing/Efratom

Nice details of lamp assy. problems and repairs.

Cheers, 

Corby
 


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