I'm not sure what the original resistance of the device was but about 1-2 k is what works empirically feeding the next stage which I haven't attempted to measure the input resistance of.
This device is about 9mm in diameter - I'd guess simply a tube with the bulb at one end and the whatever at the other. It has a symbol for a bulb and a resistor to denote the ends, so I assumed it was an LDR. It's quite big enough to allow fabricating a replacement mechanically easy.
I'll do just that.
The ORP 12 photoresistors from memory, were tens of K in the dark, a few K in room lighting and a few hudred ohms in bright light.
Plenty of variation to ensure that you get a distortion-free output.
Its the amplityude that is teh problem as even slight variations in bulbs can give a siignificant variation in output.
You will have to re-calibrate the amplitude - but thats usually one adjustment somewhere, and can be done with a good DVM.
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