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"I'm trying to repair a Canon PC20 copier, on which the main problem is a cold fuser.The fuser lamp has blown, but it looks as though it might have overheated before doing so : in two places the tube has expanded as though it got very hot while under compression from the spring endcaps. The 47R sacrificial resistor in the fuser lamp relay circuit (I think it's there to stop the control circuit holding the lamp on permanently) has also gone open circuit.
What would cause the lamp to overheat ? The thermistor (which was coated in toner) appears to work : it measures about 300K at room temperature and falls a few 10K with finger heat, so it's not open circuit (though it might be incorrect). I'd like to check out the rest of the copier before buying spare lamps etc. - can anyone tell me what the thermistor should measure (hot & cold) so I can test the control circuit and fool the copier into working?"
The thermistor may be working. It sounds like a bad triac or whatever is used to actually switch power. Get a 300K pot as a sub for the thermistor during testing.
(From: Adrian Godwin (agodwin@piresearch.co.uk).)
I had a go with this : about 4K fools the control board that the roller is hot, and allows the rest of the cycle to run. However, this _doesn't_ cause the heater power to switch off - in fact, only turning the resistance almost to zero has any effect, and at this point it appears that the controller attempts to burn out the 47R resistor.
This might make sense, except that as far as I can see there's no way that the control board COULD turn off the heater power without burning this resistor : the fuser lamp is powered through a relay which is excited by the main 24 V supply via this 47R resistor. There's no way that I've seen so far that the controller can turn off the lamp other than to short out the relay (which burns the protective resistor). So I wonder if there's no control - just a monitor for the copier control board and an overtemperature safety circuit that disables the fuser lamp (and simultaneously disables themain motor control triac). This surprises me, as I have a Canon SX-based printer where the lamp power cycles (though not using a relay, as it's quiet).
(From the editor).
There must be a triac or other mechanism for cycling power to the lamp - its temperature must be tightly controlled regardless of line voltage fluctuations and ambient temperature. (--- sam)