[time-nuts] 5 MHz Racal MA259 / / Sulzer Crystal Oscillator

Bob Camp lists at rtty.us
Thu Oct 25 08:25:15 EDT 2012


Hi

I have a GR rack mount OCXO of the same vintage. It took a couple of months
to settle to a reasonable aging rate. 

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Adrian
Sent: Wednesday, October 24, 2012 8:45 PM
To: Time-Nuts
Subject: [time-nuts] 5 MHz Racal MA259 / / Sulzer Crystal Oscillator

Hello,

recently I couldn't resist to acquire one of these Racal MA259 units 
with a SA 500 crystal oscillator which was designed by Sulzer.
What a beautiful piece of ancient high precision laboratory gear!

Well, it wasn't claimed to still meet the specifications, but it was at 
least in a working condition.
First I was surprised about the high frequency drift that was about 50 
times as high as specified, that is, a few 1E-8's rather than the 
impatiently awaited 5E-10 per day.
I could however observe the drift slowly getting down, and, after 
reading the manual, I was less concerned as it pointed out that the 
specified accuracy can be expected to be met after a full month of 
continuous operation. That was before I realized that the inner oven 
wasn't heating at all, and its temperature being low by a few degrees.

So, I gotta have a look inside that huge cylinder with the large dewar 
flask inside. Unfortunately, the toroid transformer of the temperature 
bridge had developed a short beween the primary and secondary windings. 
I could burn the short out, but there is now some noise visible on the 
meter needle as well as, much slower of course, on the frequency 
stability plot. There might still be some conductivity left between the 
windings, or something else is noisy.

Next I noticed that the meter reading of the oscillator amplitude was 
quite a bit off. I was already suspecting those infamous carbon 
composite resistors might have drifted as usual. So, I decided to 
replace every resistor in the oscillator, the buffer / amplitude control 
stage and the inner oven. Indeed, several of the resistors that set the 
oscillator amplitude were drifted by up to 100 percent and were even 
noisy on the ohmmeter. Probably not that great for a ultra high 
precision circuit... Well, ahould I have replaced the capacitors, too??

Now the amplitude is right, and the drift is starting to settle again. 
For the time being, I want to see how it stabilizes. Then come the next 
project to take care of the inner oven noise, and to fix the 100 kHz 
divider that has stopped working.

Is there any information about the Sulzer desiagn available? Anyone who 
repaired one of these oscillators?

Adrian



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