[time-nuts] Efratom FRK - no 10Mhz oscillator

Don Lewis dlewis6767 at austin.rr.com
Wed Dec 21 17:10:18 EST 2011


Thanks, Ed, all good ideas.  Thanks for the tips.

Today, I unhooked the 10 MHz crystal from the oscillator circuit and tried
to drive the crystal at low-power with my signal generator and look for
resonance my scope.  There was no resonant-peak at 10 MHz ...so either the
crystal is bad or perhaps not connected inside the oven.

I took the cover off the little crystal oven, ...it is really clean inside,
...there is the gold-plated crystal and then a lot of grey RTV around the
crystal socket and covering the temp-sensor. 

I think I will try to dig out all that RTV and see if hopefully a wire to
the crystal is broken.

The crystal oven is not connected to ground (or at least it is currently not
grounded.)



-Don








-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf of Ed Palmer
Sent: Wednesday, December 21, 2011 12:12 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Efratom FRK - no 10 MHz oscillator

Hi Don,

I'm assuming that you've checked for obvious things like bad solder 
joints.  After 20 or more years in that heat, anything is possible.  
Luckily, the oscillator board doesn't have too many connections to 
remove when you pull the board.  I use a 20X magnifier to really get 
close.  I've often found problems that you can't see with less 
magnification.

In one message you mentioned that touching the crystal oven with an 
external 10 MHz signal starts the oscillator.  Isn't the oven supposed 
to be grounded?

I agree with other posters that if you have a bad crystal, you'll have 
to find a replacement that's made to be used in an oven.  However, any 
old 10 MHz crystal will be useful to diagnose the problem.  It might be 
possible to sacrifice another OCXO and transplant the crystal into the 
FRK.  Your best bet would be an OCXO with an AT crystal.  I wouldn't try 
using an SC crystal.  The circuitry would be significantly different.  
The oven would have to be retuned to match the new crystal, but that's 
just a resistor change.

If you're up for a bigger challenge, you could try interfacing an 
external OCXO to the Rubidium loop.  Rubidiums are typically marketed 
for low drift applications rather than low Allan Deviation or phase 
noise.  I've wondered whether a high quality SC-crystal OCXO would make 
a significant improvement in a Rubidium's short-term performance.  I've 
never had a Rubidium with a dead oscillator to experiment on.

Ed


On 12/21/2011 8:04 AM, Don Lewis wrote:
> Thanks, Pete.
>
> Good tips.
>
> I ordered 10 crystals this morning, ...they are dirt cheap.  Will try and
> test one as you suggested.
>
> If it is the 'crystal', a bad crystal, .......what then?
>
> Do I try to place one of these cheap ones in the oven, ...or do I try to
> just run it outside the oven or find a high-temperature crystal???
>
> The crystal oven/controller seems fine, ...hot enough to hurt my fingers.
>
> Thanks, ...-Don
>
>
> ------------------------------------------
>
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
> Behalf Of Peter Bell
> Sent: Wednesday, December 21, 2011 5:28 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Efratom FRK - no 10Mhz oscillator
>
> Yeah, that was why I suggested just sticking a random 10MHz xtal
> across the pins on the board (with the existing xtal wiring
> disconnected) - if the rest of the circuit is good, then the
> oscillator should start up with no problems - if it doesn't, then
> suspect a fault somewhere else.  I just pulled out the manual, and I
> would start by lifting the connection on E7, and connecting a 10MHz
> xtal in series with a 10pF cap between the E7 pin and ground.  This
> will bypass both the xtal and the frequency adjustment circuit.  If
> that doesn't work, then the oscillator circuit is the problem - if it
> does work, then remove the 10 pF cap and connect the xtal directly
> between E7 and E8 - if it still works, it's almost certainly a bad
> xtal.  If it worked with the 10 pF cap but not with the E8 connection,
> then check the control voltage (E9: 0.5V-17V) - if that looks good,
> then suspect the frequency control circuit.  If it's bad, then try
> hooking it to +5V to verify the oscillator and then troubleshoot the
> servo board.
>
> Regards,
>
> Pete
>
>
> On Wed, Dec 21, 2011 at 5:00 AM, Azelio Boriani
> <azelio.boriani at screen.it>  wrote:
>> 10MHz crystals used in OCXO are designed for the temperature of the oven
so it is not possible to find an off-the-shelf suitable crystal. Better find
a crystal coming from an OCXO.
>>
>> On Tue, Dec 20, 2011 at 7:10 PM, Peter Bell<bell.peter at gmail.com>  wrote:
>>> Hi, Don
>>>
>>> It's been quite a while since I worked on a FRK, but from what I
>>> remember the only signals going to that oscillator board are power and
>>> the control voltage. It might be worth checking the control voltage to
>>> see if it's at a plausible value (nominal is 5V, spec range is 1V-16V,
>>> IIRC) - if you have a spare xtal around, just try connecting it to the
>>> pins on the PCB and see if it starts up.
>>>
>>> I'm not aware of the xtals being failure prone - I have seen one FRK
>>> with a bad one, but having seen the condition of the system it was
>>> removed from, that was clearly impact damage.
>>>
>>> Regards,
>>>
>>> Pete
>>>
>>>
>>> On Wed, Dec 21, 2011 at 1:29 AM, Don Lewis<dlewis6767 at austin.rr.com>
>>> wrote:
>>>> A little help, pls, from the more experienced.  This is my first time
into one of these .I have listened and learned from you' all for some time,
though.  It can be intimidating in this close-packed FRK.
>>>>
>>>> I am trying to bring up an Efratom FRK  I have had for a while.  The 10
MHz oscillator does not start upon power on.
>>>>
>>>> Any ideas where to look?
>>>>
>>>> I have done the following:
>>>>
>>>> *       I have it opened-up and applied 24vDC and the current starts up
initially at 1.45 amp and slowly goes down to ~.6- .8 amps.
>>>> *       Since I have no 10 MHz output, .so I looked at the oscillator
board and nothing out from it.
>>>> *       The 17 volt dc supply-rail is good, and the VCO input signal to
the varacter control-circuitry is constantly ramping up and down slowly
between ~1volt and ~15 volts.
>>>> *       All the transistors in the oscillator circuit seem ok with a
VOM,
>>>> ..and when I purposely inject a 10Mhz signal into the collector of the
'oscillator' transistor (Q3), .I do get a clean 10Mhz out of the unit (of
course it won't lock), .and I haven't even been into the rest of the unit.
>>>> *       I think the crystal is bad, .I took the cover off the crystal
oven, it 'looks' ok, . and the crystal heater in the oven is heating and
regulating.
>>>>
>>>> Any ideas?  .does the 10 MHz crystal in these have a history of
failing?  Can I get a replacement  crystal for it easily?
>>>>
>>>> How can I test this crystal for pass/fail?
>>>>
>>>> Thanks, .just my first test of the waters as a fledging time-nut.
>>>>
>>>> Don Lewis
>>>>
>>>> Austin, TX

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