[R-1051] Oven Bridge Resistor

[paul swed]

Crazy thought run it from an external 5 Mhz ref and leave the oven off.
I run mine from a GPS referenced RB standard. I know crazy but its always
accurate and no oven issue.
They were actually pretty poor even when they operated correctly. I think 1
X 10-7.
Regards
Paul.

On Wed, Mar 28, 2012 at 9:53 AM, Steve Hobensack <stevehobensack at hotmail.com
> wrote:

>
> 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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