Electrical conundrum!

Well, it's a complete mystery to me as the little circuit board is in a plastic box with one wire
going to the relay thing and the only other connections are the two wires that go to the photocell
which is mounted on a rubber insulating material. I appreciate everyone's comments. It's
just frustrating.

According to the schematic, there is a white to the photocell and a blue coming off of it. I would still imagine there is something on the circuit board or the back of the circuit board that is making one of those two connections.

This of course is assuming that the schematic is your model and that I am reading it correctly. Maybe you could post a picture of the front and back of the circuit board.
 
While looking for a circuit diagram of the control unit, I found this interesting photo on the web.


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After some research, it looks like all the older heaters all used the same basic SCR circuit with the latch relay.
I still have not been able to find the diagram I am looking for. Knipco, Master, Reddy, and Desa all used the same
control unit. I can only guess there were law suits and thus the reason for so many names for the same
basic design. At the moment, it is 31 degrees below zero and the heater is a few miles away so it will be
a while until I can work on it again. It kinda makes one feel warm just looking at the photo.:grin: Meanwhile,
I'm still looking for the diagram of what is in the little box.

Actually, I am somewhat impressed by that photo with the what looks like airable tires.....hmmmmm... AllPro,
another one I have not heard of.
 
This is beginning to make more sense to me now. The last test I did when the heater was turning itself
off, I noticed the nichrome box resistor(on the little circuit board) was warm indicating that a fair amount
of current was passing through it. The photocell in darkness goes high in resistance and must cause the
SCR to become conductive causing current to flow through the resistor and through the bimetalic latch relay
and dumping the fan and ignition. According to the diagram below, the current to shut off the unit would have
to flow through the photocell from the white(hot wire) to the latch relay. If the photocell is failing or possibly
dirty, the heater would shut down. The part I was not understanding was that the photocell does not connect
directly to the circuit board. If I disconnect one lead of the photocell in a test, I would think the heater would
not shut down, probably indicating that I had not only a bad SCR but also a flaky photocell.
 

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It does not make sense because you are not looking at the whole circuit. just a piece.
No different that trying to figure out how a transister works with just 3 leads. It is those sneaky internal connections that get you every time.
The wite wire going to the PCB is the neutral, The black wire that goes to one of the terminals is the hot wire. Ill bet that the single wire connecting the board to the switch is also the hot wire. So you have a hot and a neutral connected to the PCB to power it. The blue wire is from the PCB to the photocell most likly supplying the hot and the other side of the photo cell is connected to neutral. ANother complete circuit. The red wire goes to the igniter and the fan to supply them with a hot when instructed to by the PCB and the other side of the igniter and fan connect back to neutral. It all makes sense as 3 seperate but complete circuits.
 
Thanks for the post Flyinfool. I will take along my VOM and a note pad the next time I look at it.
 
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