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Repairing the HP LaserJet Series II power supply

My HP LaserJet Series II
In May 2009, after 20 years of loyal service, my HP LaserJet Series II (in fact, a LaserJet III disguised as a LaserJet Series II) showed the Error #50 service message. The Internet reveals a lot of useful information about this error. It is displayed if the fuser does not reach its temperature after a given time interval. There can be a number of reasons, the most obvious being a burnt out fuser lamp. I also learned that the cause can be a failure in the AC power supply.

If the error appears, the DC power board memorizes it with a charged capacitor, which takes 10 to 15 minutes to discharge. For repeated tests, it is therefore vital to turn off power for like 20 minutes to be sure that the self test is run completely when the printer is cycled the next time.

If the error remains, it is most likely that the fuser itself (that’s the green felt-covered rectangular box which can be seen when the printer latch is open) is burned. It can be tested with an ohm-meter, the procedure is well documented on the net, and I don’t have to repeat it here. In my case, however, if the printer was turned on 20 minutes after the error occurred, it completed its self test. If turned off overnight, the error #50 would reappear, and disappear upon the next cycle. This behaviour is typical for a capacitor which has lost part of its capacity.

A little research on the net revealed that C154, a 22uF capacitor with 25V nominal voltage, is known to cause problems. There are other parts in the AC board which are likely to fail, in particular the triac, but since its failure would cause a permanent error, I decided to give the capacitor a shot. Besides, I wouldn’t have had a replacement triac readily available.

Removing the AC power module
The following disassembly instructions are strictly valid for the LaserJet II, the model III requires a slightly different procedure. Of course, the usual caveats apply: You do things on your own risk, and if you hurt yourself, your printer or both in the process, I assume no responsibility.

The AC board is located at the back of the printer, to the right, where the AC plug is located. First remove the 8 screws of the main body cover, then gently turn the cover over from the back to the front. You may want to unplug the display/panel, although that is not absolutely necessary. I believe the LaserJet III cover parts must be removed individually, although I do not know for sure.

Next the ozone filter cover must be removed. There are two black screws on top of it which need to be loosened.

Now the fuser assembly can be removed. There are 4 screws, one of which has a star washer. Be sure to remember where this one belongs. It is important for proper grounding.

The AC power assembly is not accessible, four screws hold it to the base plate, one of which has again a star washer. So far, so good. The procedure until here is well documented in the service manual, which can also be found on the net. The repair itself, however, is not, since the LaserJet II and III are designed for a module-swapping repair approach.

Opening the AC power module
First we need to remove the fan, which is mounted with three black screws to the module. Unplug the red/black two wire cable first.

Next we have to remove the metal cover which is held in place with three silver screws. Underneath a printed circuit board can be seen, which is held in place with two black screws. This board contains the famous C154 capacitor, which can be seen on the picture (the green cylinder).

This capacitor is surprisingly small for a 1989 22uF 25V unit. The only suitable capacitor I had was a 40V unit, considerably larger than HPs original capacitor. There is enough place to mount it, though. I had some difficulties removing the old capacitor, desoldering wire is useful.

While at it, also have a look at the lower PCB with the triac, mounted on a piece of tin to disperse heat. I also visually inspected R102 and R103, but could not find anything suspicious.

The unsoldering/soldering part really took most of the time of the operation. Reassembly went fast. After powering on, the printer ran through the self test with no problem. I hope the power supply is good for another 20 years now!

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