I had a LaserJet series II with exactly the same symptoms: 50 SERVICE and that very clicking sound. I fixed that printer last week after quite a bit of troubleshooting.
First: the clicking sound comes from a mechanical relay in the PSU, which controls, together with an SCR solid-state relay, the sequence in which power is applied to the heating lamp in the fuser. The sound itself is not a problem, it is a symptom. You probably have a bad thermistor in the fuser assembly, or the signal path from the thermistor to the control board is corrupted.
If you are good with electronic troubleshooting, try the following:
(From: Mark Wolfe (email@example.com).)
Just to speed things up a little, override the printer open switch, and remove the plastic cover from the right end of the fuser to get at the power connector for the bulb. Turn the thing on, and read this connector, should see about 115 VAC, if you don't, check Q101 in the Power Supply. The problem you had sounded like a rarity, as it's usually the halogen light in the fuser, or Q101 in the power supply. Yours was the sensors going back to control Q101. Anyway, if you ever have one of the power supplies opened, it looks as if HP intended this triac to fail with it's whimpy heat sink. Hope this helps.
I had the same thing happen to me on a IID, check the connections on the fuser. I swapped fusers with my III, and both printers worked, swapped them back and both printers worked. The power connector on the right end of the fuser seemed a bit loose, could've had a bad connection. Anyway, IID is still going, and this was in august when I did this.
(From: Chris Holmes (firstname.lastname@example.org).)
I dont know if Error code 50 is the same as Service 50 on the HP Laserjet II but if it is you are in luck.
I once had this and spent about an hour and a half stripping cleaning and reassembling it, and it worked! When I got back to civilisation I checked the manual. The official HP action for Service 50 is - switch off for 10 Mins!
Still i'm sure the clean did it good.
(From: Frank Reid (email@example.com))
A fuser-heating fault in HP Laserjet II or III (or the Apple equivalent) causes a capacitor on the dc controller board to charge. Discharge takes 10 to 30 minutes. If you repair the problem and restart the machine before the capacitor discharges, the error indication remains.
(From: Stuart Elflett (stuarte@OntheNet.com.au).)
Does the 50 error now stay on constantly, e.g. if you turn the machine on after a 30 minute wait, does it still say 50 error without doing its self test?? If so, I'd head towards the D.C. Controller. If the bulb in the fuser comes straight on, and the delivery rollers don't turn a little before hand, I'd be looking at the AC Supply. Is your fan spinning?? A stalled fan can often result in a 50 error. Is the fuser heating up?? If you're technically minded, you could remove the fuser and check the bulb for continuity.
Parts are commonly available almost world wide now.... you could check my links page for some sites that provide parts.... there's plenty more out there that aren't on my list..... try a web search for Laser Printer Parts....... They're not too expensive, as long as you get exchange parts where possible!! A manual would be a good purchase if you intend doing the work yourself, however they are still relatively expensive compared to parts!!
(From: David Gardner (firstname.lastname@example.org).)
Error 50 is called a fuser error, which it is, but I have found that it can also be caused by a bad High Voltage power supply or by a bad cable that runs from the power supply to the fuser.