[R-1051] Frequency standard adventures ...

[Jan Skirrow]

Hi Dennis ...

The potting material was a green - very hard - coat. The break was well 
away from it. I tried lacquer thinner and acetone to soften it - the 
acetone touched it, but is was going to be a long job. I was about to 
get my Dremel tool out to grind it away when I discovered the broken 
lead. The oven does run hot, and I'd think failure of any of the 
components would be elevated greatly given that these things operated 
24/7. I have a number of new PTC resistors from mil surplus. None are 
the correct part or value, but if I have one fail I do have some 
possibility of getting the thing working again with a somewhat changed 
bridge.

Cheers

Jan

Dennis McLaughlin wrote:
> Jan,
>
> I had the same problem.  The transistor lead corroded at the 
> transistor just where the sleaving ended.  Did yours have a blob of 
> putty over the transistor, PTC resistor, and terminal block?  Mine 
> did.  I was thinking that something in the putty was corrosive over 
> time and operating at high temperature.  The transistor operates at a 
> high case temperature (approx. 70 deg C).  The junction will be even 
> higher.  Max junction temp for the transistor is 200 deg C. Type 
> 2N4238.  60Vceo, 6W Pd, 3A Ic.  Gain is 30 to 150 @ 500mA.  This 
> transistor takes a beating.
>
> I installed an all plastic TO-220 style out of an old computer power 
> supply on the base plate and ran wires up to the bottom of the oven. 
> Refilled the bottom of the oven with spray insulating foam (Great 
> Stuff).  Worked for a while, then the PTC resistor failed.  I haven't 
> found a replacement for the PTC resistor.  Tried a NTC thermistor. 
> Inverted the feedback, etc.  But no success.
>
> The later designed ovens used in the G and H models moved the 
> transistor  from the bottom of the oven to a bracket mounted on a 
> additional circuit board.  The heat transfers into the cover through 
> metal fingers on the bracket. Transistor type 2N3441.
>
> The three spare frequency standards that I bought from Murphy's 
> Surplus had either bad PTC resistors, bad transistor in the bottom of 
> the oven,  or cooked circuit board in the oven.  The trim pot that 
> sets the temperature of the oven must also fail.  Wiper opens up?  
> Drives the open into overtemperature?
>
> Dennis McLaughlin
>
> Jan Skirrow wrote:
>> 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
>>
>
> _______________________________________________
> R-1051 mailing list
> R-1051 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-1051
>
>