[R-1051] Frequency standard adventures ...

[Dennis McLaughlin]

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
>