[R-1051] Frequency standard adventures ...

[Jan Skirrow]

Interesting. And true! It seems pretty obvious that you'd not try to 
drop 28V (the input to the regulator) to the 27V shown on the schematic. 
Anyway, the zener ref on the reg module is a 1N3026 - 18V at 1W.  I 
first ran the oven without the regulator module - just put 27V directly 
on the oven module. It ran very hot, and the frequency overshot in the 
opposite direction from 5MHz from its cold freq. So I figured something 
was wrong and checked the reg module. I ran the directly applied voltage 
down to 18, and the osc was _much_ happier.

At least one of the freq std modules I have includes a very different 
regulator module - and I think the DC amp on it is mounted outside the 
oven box - but I've not really looked at it closely enuff to state more 
than my suspicions.

Jan

Steve Hobensack wrote:
> 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
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