[R-390] Ovens and Synchronization

Bob Camp ham at cq.nu
Sat Aug 7 18:20:08 EDT 2004 

Hi

The crystal ovens in the R-390 and the R1051 are different beasts. The 
1051 oven standard is designed like a modern OCXO. The crystal inside 
it is cut so it is very stable at the oven operating point. The oven in 
the 1051 is designed so it is very temperature stable.

Unfortunately none of this is true of the crystal oven in the R390 
series of radios. From talking to the guy who did the design of that 
part of the radio it was known to be a stop gap solution at the time. 
The main reason for it's inclusion was a requirement for modest 
stability at temperatures well below freezing.

The crystal oven in the R390 swings in temperature over something like 
a 10 degree C range in normal operation. A fairly normal room should be 
stable within a degree or two for quite a while. Since the oven cycle 
time is in tens of minutes the oven is definitely a poor choice for 
basic temperature stability.

The crystals in the R390 are not specifically cut for oven operation. 
This is a good thing since we don't turn the ovens on. In order to 
optimize them for an oven environment you cut the crystal so it's upper 
turn temperature is approximately at the oven temperature. Since there 
is a tolerance on the cutting process it is hard to do anything more 
than get them close to the oven temperature.  When you cut the crystals 
this way a side effect is to make their performance at room temperature 
worse than a normal crystal.

The oven on the PTO is kind of the same thing. It does not cycle over 
quite as wide a range as the crystal oven, but it does cycle further 
than your room probably does. The issue with the PTO is coil form 
shrink. The hotter you run the PTO the faster the master coil shrinks. 
Eventually the coil shrinks enough that you can no longer get the PTO 
on frequency with the correction adjustment. There is an article in 
Electric Radio back a few years that goes into all this stuff. Strange 
but true I worked for the guy who is quoted in the article. At the time 
I worked for him he would just barely admit working on the PTO's ....

Finally, when you turn on the ovens on the R-390 you almost double the 
power in the radio. The oven in the R1051 is not as big a power hog as 
the ovens in the R390. When the heat in the radio goes up by that much 
the whole radio will get hotter. Unless you run some kind of fans to 
cool everything the result will be parts that run hot and wear out 
faster. Of course the fans would cool off the ovens which draws more 
power which heats up stuff more ... nasty cycle.

One note - The ovens we are talking about are the ones on the crystal 
oscillator deck and inside the PTO. The plug in oven that has the 
calibrator crystal in it does not get turned off by the "oven on/off" 
switch. The plug in oven does do a good job, it does not pull a lot of 
power and it should be left running.

One of the neat things you can do with a bunch of R390's (like say you 
just picked up a dozen of them ...) is to sort the crystals in the 
crystal oscillator deck. The closer you get to a matched set of 
crystals the less you will have to adjust the radio each time you 
change bands. I still think  they should have put tuning caps on the 
crystals. The reason they didn't : " Well we didn't do it because it 
wasn't in the spec". Darn Ft. Monmouth.

	Take Care!

		Bob Camp
		KB8TQ



On Aug 7, 2004, at 5:14 PM, Cecil Acuff wrote:

> Hey guys,
>
>         Lets talk about the crystal ovens in the 390A.  I seem to 
> remember
> it is common practice to not use them anymore for the sake of 
> longevity.
>
> My R-1051 experience is that one can't do without the oven in the 
> Frequency
> Standard because without the oven the stability the 1051 is known for 
> goes
> down the toilet.  The crystal in the oven was designed to work at a 
> specific
> temp to be stabile.  There is a nice flat spot in the curve up around 
> 85C
>
> Is this not the same with the crystal deck, calibrator xtals and maybe 
> to a
> lesser degree the PTO in the 390A?
>
> Looking in the Y2K manual it seems recommended to fire them up a few 
> hours
> before an alignment but then go back to the practice of turning them 
> off for
> our day to day ops....seems that would negate the alignment.  Wouldn't 
> one
> be better off doing the alignments in the state one would normally 
> operate
> the radio?
>
> I am at the point in my R-390A ownership that I am starting to get 
> technical
> with the 390A now that time presents that opportunity.  As a result I 
> guess
> I'll be asking some of the typical beginner questions.
>
> Coming to a post near you:  How does one go about putting a radio back
> together where everything is totally out of mechanical and electrical 
> sync!
>
> I pulled the whole IF deck apart for complete cleaning....4 years ago. 
>  No
> problem on getting everything back together just need some starting 
> point
> for getting everything re-synchronized.  It was a nasty blue 
> striper...won't
> know it once it's complete....it cleaned up very nicely.  The gear 
> train is
> separated from the IF deck chassis and all that is separated from the 
> xtal
> deck.
>
> Gear train works like a swiss watch....love that Mobil 1
>
> Thanks in advance!
>
> Cecil Acuff
>
>
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