Hi Canide,
thanks for your reply . I already found and downloaded the HP service manual (publication C3990-90991, May 97). But unfortunately there is no detailed, component-level schematic. If yours does have it, would you please send it to me.
Today I tried to replace the triac which controls the heater (Mitsubishi BCR8PM) by an BTA06.700B (just because I have this one under hand). There is a clear improvement: the printing now only requires 2 attempts (instead of 4). So I am wondering if there is a trigger problem on the triac. It would be much easier to investigate with a schematic than just blind-trying .
Anyway I send to you my e-mail.

Best regards,
TTR

Hi TR78,

unfortunately I also own only the manual you just downloaded, so I think there´s no need to send it to you for a second time. I also don´t have the schematics for this printer, so I hope you can get rid of this strange error even without the detailed schematics of the board.

Best regards
Canide

From time to time I also encounter the failure described by Thuja (paper stops while exiting, the toner is well fused on beginning of the page, but not on the remainder). But there is no control leds on the Canon.
Anyway, if I find a remedy, I'll let you know.

Best regards

Hi,
finally I found the problem. It is related to transient failure of the pass-through connector of thermistor (fig 6-15, service manual). This kind of connector is convenient for maintenance, but increases the probability of contact failure. If it appears again, I will replace the connector with soldered connections.
Best regards