[R-1051] Frequency standard adventures ...

[Steve Hobensack]

I have worked on those also. One thing to look out for is the voltage 
regulator feeding the oven. My schematic labels the regulator output as 27 
volts. I think it is a misprint and should read 17 volts. It actually works 
at 27 volts but if something goes wrong in the temperature bridge circuit, 
things really cook inside the oven. A three terminal regulator type 7815 
works good here. I set the temperature using a mercury lab thermometer. Fair 
Radio used to sell the frequency standard modules at a reasonable price. I 
don't know if they still do.

I have a freq standard module that is so old , it is not adjusted with 
trimmers. It is adjusted with slugs in the rf transformers. I wonder if 
anyone out there has a skiz diagram of that type of freq standard module? 
Thanks
....Steve...N8YE


>From: Jan Skirrow <jan at skirrow.org>
>Reply-To: R-1051 Discussion Group <r-1051 at mailman.qth.net>
>To: R-1051 at mailman.qth.net
>Subject: [R-1051] Frequency standard adventures ...
>Date: Sun, 24 Sep 2006 10:52:18 -0700
>
>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
>_______________________________________________
>R-1051 mailing list
>R-1051 at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/r-1051