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: [email protected] [mailto:[email protected]] 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
<[email protected]> 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<[email protected]> 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<[email protected]>
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
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
