[time-nuts] MTI 260-0624-D OCXO

timenut at metachaos.net timenut at metachaos.net
Fri Mar 11 23:19:50 EST 2016


> Hi

> Anything electrical (duty cycle/ harmonics / output level / etc) 
> is designed in on a modern part done in high volume. 

> The only candidates for a pot on top of the oven are:

> Inner oven temperature
> Outer oven temperature
> EFC

> You have already ruled out EFC (it would be a coarse set so
> visible on a normal counter). 

> That leaves inner oven and outer oven. 

> Why inner oven - to adjust for the data from your temperature run. The parts
> are at best coarse set based on crystal temperature. 

> Why outer oven - same as above or to set for inner / outer separation
> and clearance after the thermal stabilization in the sealed can. 

> If there is no apparent change in frequency (and from the location of
> the pot) that suggests the outer oven. Regardless of which oven is being set, the
> adjustment is likely a “move it 1.25 turns" sort of thing.  You then 
> re-run the temperature test to see if it is in spec yet. 

> Bob


Bob,

In a separate communication, Angus has stated...

> On the two that I tested, it appeared to adjust the temperature of
> both of the ovens.

> One was a 5 MHz 260-0545-B. The outer oven was at 78.2 degC and at
> least around that point the temp changed at about 1 degC per turn. The
> EFC adjustment on this one was not quite enough to bring the frequency
> to exactly 5 MHz, but the oven temp did look to be close to correct.

> The other was a 16.384 MHz 260-0546-G which was unused old stock, but
> had overall poor stability. It turned out that the oven temp on it was
> 1-3/8 turns from the turnover point, but when or how that happened I
> don't know.

> It would be interesting to hear from somebody who knows how this type
> of thing is actually adjusted in production.

I finally got my 260 (sort of) repaired and have done some measurements.

Generally, I can confirm what Angus was saying, to the limits of my ability to
measure. One of the things that I can measure is the source voltage. I have a
hacked together 12v supply. When hooked up to the 260, it initially reads just
over 9v. As it warms up, it eventually tops out just under 10v. There appears
to be roughly four different points, around 9.1v, 9.35v, 9.8v and 10v. The 260
starts at around 9.1v, quickly moves up to 9.35v and after a while to 9.8v. It
will then sometimes move briefly up to 10v and back down to 9.8v.

I interpret this as different heating stages - probably both ovens on, one
oven on, some sort of maintenance heating point and no heaters on.

What I find as I adjust the pot (after letting the 260 warm up) is that
turning the pot counter clockwise the supply voltage will go up to 10v and
turning it clockwise it will drop to 9.35v or 9.1v. So, from that it would
appear that you are right in that the temperature set points are being
adjusted by the pot. Whether only one is adjusted or both are changed as Angus
says isn't clear.

I also noticed that the 260 consistently has an adjustment range of +/-2e-7,
regardless of where the frequency is set, with 0v applied to the adjustment
pin giving a frequency 4Hz higher than when the 5.65v reference voltage is
applied to the adjustment pin. It will adjust further if the reference pin is
hooked up to the supply voltage.

So, I decided to get as much information as I could. I turned the pot
clockwise until it "ticked" once each turn. That dropped the temperature to
the minimum. I then turned it counter clockwise 1/2 turn at a time and
recorded each temperature / frequency reading. I did this until it "ticked"
once each turn. That took nearly three days. To obtain the temperature, I
used Kapton tape to tape a temperature sensor for my multimeter to the casing
of the outer oven. This is not necessarily the hottest spot, nor does it
actually measure the temperature in either the inner or outer oven. Further,
the multimeter's precision is only around 1 degree C. The accuracy is unknown,
but it should be repeatable and roughly linear. I used my (uncalibrated)
2465BCT to measure the frequency. This is limited to 7 digits without a
calibrated frequency standard and is probably slightly off. Still, that gave
me a good bit of data. Here is the raw data.

   60    5.000001
   61    5.000002
   61    5.000003
   62    5.000003
   62    5.000004
   63    5.000005
   63    5.000006
   64    5.000006
   65    5.000007
   65    5.000008
   66    5.000008
   66    5.000009
   67    5.000009
   67    5.000010
   68    5.000010
   69    5.000010
   70    5.000011
   71    5.000011
   72    5.000012
   73    5.000012
   74    5.000012
   75    5.000012
   76    5.000012
   76    5.000013
   77    5.000013
   78    5.000012
   79    5.000012
   80    5.000012
   81    5.000012

Here is a rough graph. It shows the expected SC temperature / frequency type
of curve. However, what is interesting is the range over which adjusting the
temperature affects the frequency. This is a total of 12Hz! Also, it should be
noted that the outer case was not in place, so the outer oven is exposed to
ambient temperature. That probably limited the maximum reachable temperature.
I could noticeably change the both the temperature and frequency by simply
blowing on the 260.


   5.000013                                                                  *   *
   5.000012                                                  *   *   *   *   *       *   *   *   *
   5.000011                                          *   *
   5.000010                              *   *   *
   5.000009                          *   *
   5.000008                      *   *
   5.000007                      *
   5.000006              *   *
   5.000005              *
   5.000004          *
   5.000003      *   *
   5.000002      *
   5.000001  *

            60  61  62  63  64  65  66  67  68  69  70  71  72  73  74  75  76  77  78  79  80  81

The vendor has refunded my money for the 260 (the one that rattles), so as
soon as I get a chance I will try a new opening technique and see if it is
possible to repair it. Having learning a lot about the construction from the
first one, I think it is at least a possibility.

I will let everyone know what happens - with pictures - once I have done that.
That may take me until next week. I am attending the Charlotte hamfest
Saturday.


Mike











More information about the time-nuts mailing list