Re: IR Printer 82240A repair Message #5 Posted by Tony Duell on 19 June 2004, 7:51 a.m., in response to message #1 by Miki Mihajlovic
The printhead on this printer consists of 8 (IIRC) tiny heater elements. They are driven by 7 sections of a ULN2803 octal driver IC (the other section drives the motor) together with a discrete transistor. Drive to the ULN2803 comes from one port of the 8048 (?) microcontroller. The user intensity control is part of an RC network that the 8048 'measures' and is used to determine the pulse width sent to the driver (longer == darker of course).
I'd start by carefully unplugging the printhead tapewire from its connector on the main PCB. Then check for low-ish resistance between the common connection (IIRC thickest track in the tapewire) and each of the others in turn. If it's open, then the printhead or tapewire is defective :-(
Then look for drive pulses at the inputs to the ULN2803 while printing. If there's nothing there, suspect the 8048 (custom programmed, you have to get one from another 82240A), or the intensity control and related components.
If you've got drive to the ULN2803 and nothing at the outputs, then that's dead.
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