Temperature Controller help needed

I have an older Blue M oven that I am having trouble regulating the temperature. The style of controller is called Power-O-Matic 60. It looks like a
split transformer with a pivot on the bottom that closes the top of the transformer end when the temp dial is closed. The weird thing is that the thermocouple Blue M does not even connect directly to the transformer unless it is thru the housing. The 240 single phase powers the heating elements diretly and then thru a large transformer that has several 77V and 100V center taps. These center taps connect to another transformer connected to this variable dial for the temp control. When the controller dial is in the lowest position I get no volts and when it is wide open I get the 100V from the center taps. However there is not much precise control. The voltage tops out at too low of a temperature.
What can I do to repair this without buying a new controller? Can I get some variable AC voltage source and test it? No Actually I would need some type of resistor decade box to control the feedback voltage since the dial control is only generating resistance I assume. The oven is rated at 19 amps 240V. Any Ideas on how to further bench test it? Some of the symptoms are not enough heat - I need 105*C and sometime have to set it for +200*. Any help would be appreciated.
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It sounds like what you're calling the thermocouple is actually a over- temperature system; a tube full of gas that expands when heated. When the oven gets too hot, the gas forces a switch open and the switch drops out your heater contactor. The over-temperature safety switch contacts are in series with the coil on your heater contactor.
There should be a set screw that changes the spring tension that acts against the pressure of the gas. The higher the spring tension, the hotter the oven needs to get before the gas opens the over-temperature switch.
The older Blue M ovens I've worked on did not a feedback system, like a PID. You set the dial/rheostat and you have to use a thermometer inside to verify the temperature.
If your oven is not heating up, there are several things that need to be checked.
Check your heating elements. Disconnect each element, ohm them out and megger them to ground. The heater resistance is based on the wattage; power = [volt * volt] / resistance
Your oven should have an air recirculation fan, either mounted on the top or in the back of the oven. With the door open and the oven heating, you should be able to feel air coming out of the vent.
With the oven cold, put a clamp on amp meter on L1/T1 or L2/T2 and crank it up. It should draw close to the nameplate 18 amps.
With the oven dead, pull the wires on the rheostat and ohm it out. It should have a high resistance with the dial set low and should decrease linearly as the dial is turned up.
Also, check your over-temperature safety circuit. If the bulb/tube assembly is bad, the safety switch can be opening and closing below the safety set point.
Without a print or the oven model, I'm kinda guessing.
http://www.tenney.com/blue-m/bluem.asp
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Thanks for your comments. Today I took the thermocoupler apart. It appeared to be sticking. I switched out the the overtemperature thermocoupler and it appears to be working fine for now. It is holding the temperature nicely. The thermocoupler did have a spring in it but the wire was wound around a shaft and that shaft was connected to a pushrod. When we put a torch on the thermocouple we could see it move ever so slightly.
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gtslabs wrote:

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