Pls Help me understand and diagnose this oven control circuit

[dangerboy]

I have a Canadian made 1956 GE Speedster range model no. CJ405NA1W. I think it's pretty much the same as the US made J405 of that year except for maybe a few small differences here and there. One difference appears to be that the Canadian factory sourced its oven thermostat from a Canadian company, Oak Hart Mfg. Canada Ltd. and I think the US Factory probably used a US made thermostat/control.

The oven has two control knobs. One sets the baking temperature and the other is used to select Off, Bake, Time Bake or Broil.

This range has been in the family since 1973 and I am almost 100% certain this is the original thermostat for the range. The markings on the thermostat are as follows:

30A 115 - 230V AC Only
Cat. No. 891A1 15H M
Oak Hart Mfg, Can. Ltd.

On the back of the thermostat there are four screw terminals marked as follows:

1 | 2
L | L1

I have the thermostat out of the oven and the oven is 400 miles away at my cabin which I won't be going back to until next spring so I'm trying to figure a way to determine if the thermostat is still working using the oven I have here and a multimeter.

The reason for this is that I suspect maybe the thermostat is working and the problem of the element only heating to slightly warm is due to something else. I suspect the thermostat may be working because I took it apart and put the sensor into a working oven as it was heating up and measured the height or width of the bourdon tube piston thingy (technical term) to see if there was any change when the sensor was heated up and then cooled down. There was 3 or 4 mm of expansion going from freezer temperature to 400F so it looks to me like there's still gas or mercury in the bourdon tube and that part of the thermostat is intact and working.

Attached is the circuit diagram for the range. Can someone please help me understand how the thermostat is supposed to work here? I think I have it figured out but I'm not sure.

At room temperature, I have continuity across contacts L and 1 and L1 and 2 but no continuity from any of the terminals on the left side to any of the terminals on the right side because the elements are not in the circuit.

What's supposed to happen when the oven gets up to the set temperature? When I had the thermostat apart and pressed on the metal spring plate that sits behind the bourdon tube piston thingy, I got continuity between the right and left sides of the thermostat box. Based on that, I'm guessing what happens is that when the bourdon tube piston thingy expands far enough, contact is established between the 1-L side and the 2-L1 side and somehow that shuts the current to the element(s) off until the oven cools enough for continuity between the two sides to be broken and then current starts flowing through the elements again and the oven begins heating again and so on.

Is that how it works? If yes, why does the current stop going through the heating elements when continuity is established across the thermostat? Is it because a path of lower resistance has been created across the thermostat and the current goes that way instead? That seems plausible to me.

In that case, should I be able to test this thermostat by setting it at 350, sticking the sensor bulb from this thermostat into a working oven set at 400 and then checking to see if I suddenly get continuity between L and L1?

 

[dangerboy]

Okay, I did some more experimentation with the thermostat sensor bulb in the oven and answered my own question. I now see that (a) my thermostat works (yaaaay!) and what happens is that contact between L and 1 and 2 and L1 is broken when the temperature in the oven hits the set temperature. When the oven cools down enough, contact is re-established and current starts flowing again.

So the problem lies somewhere else with this oven. What could it be?

When you turn the oven control to Bake and set the temp control setting at a temperature, the bake element heats up but only to the point where it feels a bit hot to the touch but not hot enough to glow the element or heat the oven up. I think the same is true with the upper element when you set on Broil.

The oven switch?

I can also see that my thermostat, although working, is not that accurate. When my digital oven here is set at 400, the thermostat trips at about 325 with all the adjustment allowed in the knob angle used up. Attached is a picture of the thermostat and the back and front sides of the temperature control knob so you can see how the adjustment is done. When the oven is set at 300, the thermostat trips at about 225 - 230-ish.

I'm thinking I might have to get the dremel tool out and notch out a spot for the pointer to go in about 20 degrees further to the right (clockwise) from the last notch to and drill a new hole for the screw left (counterclockwise from the left most edge of the adjustment slot to get the thing properly calibrated. Anyone else have any other suggestions for how I could get this thermostat calibrated a little more accurately?

 

[stevet]

Calibration Adjuster...

Starting at the end of the journey and working backwards to the lack of full power...
That small screw with the slot and the lock nut in the picture. Does the screw contact the bellows that actuates the switch? If so, then that is what you use to calibrate when the thermostat cuts off. So don't go grinding anything. If the bellows expands and breaks the circuit, then you would want to screw down on the adjuster so the bellows will have to expand more to break the connections.

Calibration adjustments can be very touchy in that a small amount of turn can mean a difference in either a very few or a very many degrees. I suggest that you figure out why you are not heating the element fully which could be a loose or broken wire or a problem with the power line going to the oven. Then install the thermostat where it should be in the oven and then calibrate it to that oven with a known good thermometer or preferably, a digital thermometer located in the dead center of the oven to insure proper adjustment to the thermostat.

I guess since you won't be back there for a while, maybe set it up near to the sensor in your test oven and get a rough calibration that way. Especially if you are 75 degrees off.

I bet John L will see this and explain what could cause an element to half heat as he has so much experience with domestic ovens. It can also be a problem with the bake/broil selector or another switch in the circuit.My expertise is more on the commercial end where we are dealing with ovens with 10 kw elements in them. And very often, it turns out to be a bad wire, a burned off terminal or a problem with one of the lines supplying power to the oven.

Also, it may be possible to get the American thermostat and retrofit it into your oven once you get a good part number for it.

 

[diesirae7]

check the fuses

I had the same issue on a 1965 GE oven, was only getting warm, I replaced the theromostat, still had the issue, it was a blown fuse, it had two cartridge fuses and only one was bad, was only getting 110v instead of 220v.

 

[dangerboy]

> check the fuses

> I had the same issue on a 1965 GE oven, was only getting warm, I replaced the
> theromostat, still had the issue, it was a blown fuse, it had two cartridge fuses
> and only one was bad, was only getting 110v instead of 220v.

Of course! The elements only getting 110 instead of 220 would totally explain the partial heating of the elements. If you look at the circuit diagram that I supplied, the 15 and 20 amp fuses at the far right of the fuse box go to the oven controller. I'm not an electrician or an oven repair technician but it seems to me there's a good possibility that one of those two fuses has blown. Can someone who knows more about this please have a look at the circuit diagram and let me know if the elements only getting 110V due to a blown fuse is a possibility for this oven?

I don't know why I didn't check the fuses before I pulled the thermostat out. That was just plain and simple stupid. I broke my own rule of always doing the easiest things first before assuming the cause is something more complicated. #smack-myself-in-the-forehead-emoji

The oven is out from the wall and the back is off of it so I may be able to get the caretaker to check the fuses for me the next time he goes into the cabin to do a check. It would be such a relief to find out that it's just a blown fuse.

 

[dangerboy]

> That small screw with the slot and the lock nut in the picture. Does the screw
> contact the bellows that actuates the switch? If so, then that is what you use to
> calibrate when the thermostat cuts off. So don't go grinding anything. If the
> bellows expands and breaks the circuit, then you would want to screw down on the
> adjuster so the bellows will have to expand more to break the connections.

I will look into it. There is a similar set screw with lock nut on the bottom of the thermostat which you can see in the the attached picture. I'm wondering if one or both of them are involved in calibrating the temperature range (i.e. one sets the high trigger and the other sets the low trigger). It would be great if an experienced tech could chime in here. Anyone?

 

[dangerboy]

Attached is a photograph of the internals of the thermosat with the diastat taken out. The annotation shows where the diastat presses down. In this state there is continuity between the left two contacts and the right two contacts. When the diastat presses down far enough, the centre spring bends on the upper side of the fulcrum lifting the floating contacts on the lower side of the fulcrum causing them to separate from the fixed contacts below them breaking continuity between the upper and lower contacts on each side.

The adjustment screw on the top of the thermostat seems to limit how far the floating contacts can lift off the fixed contacts below them. The adjustment screw on the bottom seems to control how much compressive force is applied by the retaining spring at the bottom that keeps the whole contraption in place and under compression.

I'm having difficulty conceptualizing how adjusting either of the adjustment screws will affect the calibration of the thermostat and therefore don't know which screw to adjust to make the trigger temperature higher and which way to adjust that screw.

Feel free to make a suggestion if you understand how this system works. ;-)

 

[dangerboy]

This is the range. I can get my caretaker to look at the fuses but I can't remember where they are on this unit. The back is off the unit but they're not back there.

Do you have to lift/hinge the top off to get at them?