[R-1051] Oven Bridge Resistor

[Steve Hobensack]

Dennis, 
 
Thanks for that information. I downloaded the catalogue but didn't see it. I might have to call a salesman and inquire. I hope there
isn't a minimum order $ or a special fabrication.  
 
I was working on another oven that uses a true thermistor. Its cold resistance is about 40k, I quickly measured it hot and it was around
4k. The bridge circuitry is different than on most R1051 oven boards, and there is no temperature adjustment pot. I removed the plastic foam at the bottom with a  drimmel wheel and found the brown mud. It had eaten away one wire from the transistor with green corrosion like one would find in an old battery compartment. I was able to do a lot of scraping with a jewelers screwdriver. The mud would chip away pretty easy. The transistor is equivalent to an ECG184, flat TO-126 package. It must must have been a newer freq standard module because of the newer style transistor. I was able to get this module working again.

If I cannot obtain the correct resistor, I could add more resistance to the oven heating element and let it run wide open. I would have 
to experiment with the right resistance to add, so the ball park temperature would be near 80C. If room temperature is always around 70F, It should work reasonably well. Of course, it would be a kluged up job, but its fun to experiment.
 
I think there is a misprint on the schematic. It shows Q6 as a NPN. Q6 can be either a 2n2906 or a 2n2907, both are about the same
thing. However, my ECG book shows it as a PNP. That throws a kink in my logic as I am trying to follow the thoery of the bridge and
how it regulates. I wonder why the 4th resistor of the bridge is on the bottom of the oven instead of on the circuit board, as it is still
within the 85C heat?  

 
(snip)
> Steve,
> 
> It is a positive temperature coefficient resistor. From my notes,
> 
> PRC
> PT094
> 3900 +/- 1%
> TC.45%
> 
> Made by
> PRECISION RESISTOR CO., INC.
> 10601 75th Street North, Largo, FL 33777-1421 U.S.A.
> Tel: 727-541-5771 Fax: 727-546-9515
> E-mail: prc at precisionresistor.com
> Web Site: http://www.precisionresistor.com
> 
> Most thermistors are a negative temperature coefficient. I tried to get 
> a 10k ohm one to work by reversing the bridge output to the amplifier, 
> but had no luck.
> 
> Also check the transistor in the bottom of the can. I have some that 
> the gold plated leads of the transistor have corroded off at the body of 
> the transistor. Something in that putty like stuff might of done it 
> over the years of operating at 75C.
> 
> Dennis McLaughlin
> 
> 
> 
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