[R-1051] Frequency standard module

[jan at skirrow.org]

Hi Cecil ....

I did post to this thread when it first came up, but have only just got 
around to trying to do something with my flaky standards.

The biggest issue is that what's inside the freq std seems to bear little 
relation to the schematic I have. My radios these came in (or were removed 
from) are plain R1051s.

The freq stds are marked 2N6625-078-4718. The parts layout on the box does 
agree with what's inside. The manual I have is is NAVALEX 0967-LP-970-9010.

I have removed the assy that contains the xtal osc and related circuits. It 
connects to the main electronics with a small coax and a single power lead. 
So it can be operated separately.

The oscillators both start at about 5.16MHz, about 4-5 volts p-p output. 
The ovens heat and do shut off after some time. The temp seems low - maybe 
145F. After awhile the output starts to droop and ultimately dies. One of 
them does pop down to 5.0000 before (sometimes) dying. Turning the power 
off and then on brings them back for a bit.

The circuit is seems to be marginal, with the temp possibly being a 
problem. Trouble is, I can't find any trimmer (my schematic calls it R15) 
in the temp sensing bridge. I've tried to measure the temp by leaving the 
foam insulating sleeve on the osc can until the temp seems to have 
stabilized, then slip it back and use a contact thermometer on the case at 
the bottom end of the heating coil.

There is a small transformer (?) inside the heated can that is adjustable 
thru a hole in the can. Is this a coarse freq adjustment? Needless to say 
it's not on my schematic!

I should be able to find the bridge by hunting around, but thought it would 
be useful to have the right schematic and/or see if you had any other 
suggestions.

Jan Skirrow (3 - R1051s and 1 R-1051D)


At 02:48 PM 5/15/03, you wrote:
>Greetings group,
>
>            I have been out of town for nearly a week on business and am
>catching up on the posts.
>
>I have not experienced any failures of the Crystal in the Frequency
>Standard.  I do find the 5 Mhz osc dead quite often though.  I find
>components in the oven that fail because of the temperature they are exposed
>to all the time.  Why they put all that in the oven is a mystery.  The items
>that seem to fail most often are the two electrolytic caps.  It's not
>unusual to find them shorted.  I have started replacing them as a matter of
>course when doing a repair/restoration.  The next highest failure item is
>the temperature sensor that is embedded in the base of the oven.  It usually
>goes open and the oven fails to work.  It is a special part and quite
>expensive....around $10 for something that looks like an 1/8th watt
>resistor.  But don't try a 3900 ohm resistor...it won't work.  Been there
>done that!
>
>The Standard is very repairable with most of the problems being on the 5 Mhz
>osc\oven control card.  I have found some that are damaged because it
>appears the oven has run away and gone as far as melting the solder on the
>back side of the board.
>
>The oven does need to be set to 85C\185F for the stability to be to spec's.
>Too low or high and you fall off that curve that Bob mentioned.  It takes a
>while to get it adjusted....also you should only set the frequency after the
>oven is calibrated and the cabinet has been closed up and temperature
>stabilized over night.  You have to slide it open make a quick adjustment,
>close it back up tightly and check it again in a half hour or so.  Once set
>it usually won't drift more than a half cycle in 24 hours if all is well!
>
>The price you pay for that stability is the half hour to one hour necessary
>for the radio to get on frequency when powering it up cold.  I don't know
>any way to get around that unless the radio is modified to power the oven
>continuously.
>
>Mark I have found the R1051 plain to be a little harder to sell than the
>"B".  It is every bit as good a radio...but everyone wants a "B".  I prefer
>the plain modules because they can be repaired.  Some of the IC's in the "B"
>synthesizer are non-existent!  If one goes bad you scrap the module....
>
>I would guess a complete, unrestored, untested Plain to be worth around $150
>with decent cosmetics....( lack of front panel and chassis corrosion, broken
>knobs etc....)
>
>It's not unusual to have to put 40 or 50 hours into one to get it up to
>spec.  Not to mention parts.... Once that is done you can usually get up to
>around $500 if the cosmetics are pleasing.  The "B" usually will support
>$100-$150 more and that is supplying new audio and power connectors and rack
>mount hardware.
>
>Cecil Acuff
>The R1051 Shop