[R-1051] Frequency standard adventures ...

[Jan Skirrow]

I have several 1051s, and several different versions of the internal 
frequency standard module. One extra module was in my junkbox, as the 
oven was kaput altho the oscillator itself worked but way off freq. I 
have a need for an oven for another project, and decided to see if I 
could fix this one - I know the ovens have been a source of problems in 
the 1051s, and if the oven isn't working, you can be pretty sure the 
oscillator will be well off frequency.

As everyone likely knows, the oven element is in the collector circuit 
of a DC amplifier. The DC amplifier is driven by a circuit that contains 
a resistor bridge to set current to the heater coil. One of the bridge 
resistors is a sensistor bonded to the metal oven shell and responds to 
temperature. Another arm of the bridge has a trimmer to set the 
operating temperature of the oven. Quite a nice little circuit. It will 
take awhile to reach its intended temp, but holds well once it does and 
the std is remarkably stable.

I think Cecil Acuff once told me that a defective sensistor was often 
the problem with the oven. But mine tested fine. The DC amplifier 
transistor (in this version anyway) is in a TO-5 metal case, and is 
press fitted to the oven shell so that it is heat-sinked. The transistor 
tested open. So I decided to remove it (no small task as it is cemented 
in place) and replace it with a similar transistor I had. But when I 
clipped the collector lead off the heater coil terminal, the wire fell 
apart (it was in a piece of clear plastic tubing.) It had corroded thru 
despite being gold plated but the break wasn't visually obvious. When I 
connected the remaining collector stub back to the heater, the assy 
worked perfectly and the oscillator was on-frequency once the oven was 
at temp.

So, the lesson in all this is to not assume that a freq std oven problem 
is the sensistor. The plastic sleeving on the collector lead was 
heat-discolored (this baby runs hot!) and the break wasn't noticeable 
until I had freed up one end.

Jan