OT laser printer question

Tony Duell ard at p850ug1.demon.co.uk
Fri Mar 10 19:52:17 CST 2006

> Re:
> **********
> > If its the physical letters on the paper, if they are lumpy or if they 
> > are smearing if touched then your High Voltage power supply board is 
> > starting to fail.
> Intuitively, if the toner smears when touched, I'd assume it wasn't being 
> fused properly, and would investigate the fuser and its control system 
> first.
> **********
> That is correct.  If the toner smears when touched, the problem is in the
> fuser, and it has nothing to do with the HV power supply, drum or anything

Well, it's in the fuser _or the fuser control circuit_.

Cetainly on the CX and SX printers, if the fuser is totally dead (e.g. a 
burnt-out lamp), the control system will shut the printer down and give 
an erorr.

The fuser temperature is measured by a thermistor in contact with the 
upper roller, at least on the CX and SX models. It feeds into an ADC 
inputer ona microcontrolelr (IIRC a 7811), one output of which controls 
the fuser. That output drives a fairly complex protection circuit that 
then drives the triac in series with the fuser lamp.

If there as some problem with the thermistor or the resistors around that 
ADC input, the controller migth think the fuser was up to temperature 
when it was in fact too cold. That could cause smearable toner.

Ah yes, the protection circuit. Basically, the fuser is only turned on if 
there's a pulse output from the microcontrolelr (I don;t think it's any 
kind of PWM, though). Stuck high or stuck low will make the fuser turn 
off (I think on the CX, stuck high will deliberatly burn out a plug-in 
resistor on the prtrection PCB, casuing a relay to turn off and the fuser 
to shut down). On the SX, there's a circuit ther turns off a protection 
relay if it detects a current in the fuser circuit when the triac should 
be off. And there's an independant thermoswitch on the fuser that opens 
the lamp circuit if the thing gets too hot.

I've had electronic problems too. In particualr the control triac can 
fail. And the protection circuit can have interesting faults. One of the 
worst I've had to trace was in my own SX. The protection relay would drop 
out after a few seconds of power-on. I removeed the AC input module 
(which contaisn the fuser drive and protection circuitry) and powered up 
the protection circuit from a 24V bench supply. The darn thing still 
dropped out after a few seconds. Yes, the shutdown capacitor was 
charging, but all possible components it could charge via tested OK. 
Heck, even removing them didn't stop it charging. You guessed it, leakage 
across the PCB, due to gunge from a defective electrolytic...

> else.  In early laser printer (up to at least the Canon EX engine in the
> Laserjet 4 and, I think, 5), fusing was done with heated rollers.  The upper
> fusing roller was an aluminum roller with a silicone (non-stick) coating,
> and inside the hollow roller was a high-power tubular halogen lamp (500 to
> 900 watts).  The lamp was line-operated, thus different for 110v and 220v
> printers, and the cause of failure was almost always that the lamp had
> simply burned out (although lamp control problems are possible, but not
> common -- the lamp does not burn continuously, it's cycled based on the
> roller temperature).  The lamp can be replaced ($30-ish, typically) by a
> technician who knows what they are doing, but it's more common to replace

I'm not a technician, I probably don't know what I am doing, but I've 
replaced fuser lamps with no problems. The only 'gotcha' is that it's a 
quartz halogen lamp, handle it by the wires or the ceramic end caps only, 
don't touch the 'glass'.

> the entire fuser assembly ($100+ in most cases).

Now would I waste money like that?

> Later models (those that have near-zero warm-up) use "thin film fusing",
> which is more sophisticated, much more power efficient, and also more
> complex and difficult to repair unless you go for the swap of the entire
> fusing sub-assembly.

What's the difficulty (having never had the chance to take one apart)?


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