Printer and Photocopier Troubleshooting and Repair Collection : Notes on HP LaserJet III error code 50  
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Notes on HP LaserJet III error code 50

(From: Tony Duell (ard@p850ug1.demon.co.uk).)

[Error 50, SX Engine]

Remove the fuser (4 screws hidden down holes - it helps to remove the plastic cover from the left hand side (1 screw).) At the right hand side there's a 2 pin plug that goes into the AC block. Test for continuity between the 2 pins on the fuser. If it's open, then it's either the fuser lamp or the overheating-protection thermal switch that's open. It's pretty easy to trace the connections and test them separately if you can figure out how to dismantle the fuser (I can talk you through it).

If the fuser is OK, then remove the outer plastic casing, and the AC block under the upper fan [at the right rear of the printer. Remove the two screws securing the black plastic ozone filter housing, pull that housing straight up and off. Three vertical screws secure the base, plus one horizontal one on the outside rear. Then pull the AC block straight up. The fuser must be removed prior to removing the AC block].

Most of the time the thing that fails is the Triac, Q101 on the lower board in the AC block. This means the fuser can't heat up.

Here are a few notes on what goes on in the AC block (totally unofficial, BTW).

Lower board:

R101, L101, C101, C102, C103 form a mains filter. The output of that goes to CB1 (circuit breaker) with VZ1 connected across the output of that lot as a spike suppressor. The output of that goes to the 2-pin plug (J104) and thence to the DC power supply. Watch out for J104 if you run an AC block out of the printer as it carries live mains.

The fuser control circuit on the lower board isn't complex either. The live side of the mains (output of CB1) goes to T101 (current sense transformer) primary. The other side of the primary goes to the contacts of RL101 (fuser protection relay). There's a snubber SQ101 across the relay contacts. The other side of the relay contacts goes to one pin of J103 and thence to the fuser lamp. The other pin of J103 (and thus the other side of the fuser lamp) is returned to the neutral side of the mains via the triac Q101. This is triggered by SSR101. Oh, there are a couple of resistors (R102 and R103) in the triac gate circuit, and suppression components L102 and SQ102.

There's a 10 pin connector between the lower board and the upper board. The pinout seems to be:

 Pins 1,2    T101 current sense transformer secondary
 Pins 3,4    RL101 relay coil
 Pins 5,6    Solid State relay (SSR101) input (6 +ve)
 Pin 7       Fan control
 Pin 8       Fuser drive from the DC controller
 Pin 9       Ground
 Pin 10      +24 V (input)

The last 4 pins are connected to the 4 pins of J101 which is connected to the DC controller. Shouldn't be hard to trace those.

Upper board:

This virtually never fails, so I'll simplify the description by missing out resistors, etc. You can trace the signal flow from this description anyway.

The fan control is trivial - the fan control input drives Q159 with the fan as the collector load.

The fuser control signal is AC coupled (C157) to Q157. The output of that is rectified/smoothed (D157/C156) and drives Q156. The collector load of Q156 is SSR101 on the lower board.

There's also a protection circuit, which works as follows. Normally Q153 is on which turns on Q152 and Q151. This turns off Q155 which has the protection relay as its collector load.

If the fuser control circuit fails so that Q156 stays on too long (which would cause the fuser to overheat), then C153 is discharged by D155 and the collector of Q156. This turns off Q153, Q152, Q151 and turns on Q155, energizing the protection relay and turning off the fuser.

If the fuser takes too much current then the output of the sense transformer T101 becomes large enough to trip the comparator IC151 (LM393). This turns on Q154 which discharges C153. The rest of the circuit operates as above.

(From: Jon Martin (jon.martin@unisys.com).)

There is a partial Schematic of the AC power supply at: Fuser Controller RG1-1438.

The fuser thermistor comes in on pin 4 or J206. There's an 18k resistor (R237) to +5 V, and a 4.3K resistor to ground via SW205 (which is open when the base cover is on the printer -- the idea is to shut the fuser down if you power up the printer when dismantled). Then there's a 3.3K resistor in series (R747). And then a 4.7 uF cap (C210) to ground to smooth the signal out a bit. Finally we get the AN0 input on the main microcontroller IC218.

The fuser drive signal comes out on port PD2. This is buffered by IC219e ('04) and IC211e ('07) before going to pin 2 of J208 and then to the AC block.

There is one more bit of circuitry to consider -- the overheat shutdown. If the fuser overheats (or the machine things it has), then a signal comes out on port PD3. This turns on Q210 (2SC1815) which charges C211 (100 uF) via R749 (330 ohms). There's a 4.7M resistor (R748) in parallel with C211, which gives a time constant of many minutes. That's why to have to leave the printer off if the fuser seems to have overheated. Anyway, the voltage across C211 is monitored by the AN3 input on the microcontroller

So, what to check? C210 is a favorite, it goes leaky. If that's not it, check (with the printer assembled and therefore SW205 open -- you have to solder wires onto the points so you can look at the signals) the following

  1. The voltage across C210 (should be around 2 V I think). If wrong, check the fuser thermistor and connections, the R's and C's between it and the microcontroller, etc

  2. The voltage across C210. If it's more than 0 V, then switch off, discharge C210 (short the leads together), try again. If it rises again, find out if Q210 is being turned on (Check the base voltage -- it's an emitter follower). If so, then either the microcontroller is faulty, or there's a problem you should have spotted in (1) :-). Check Q210, the R's and C's, etc.

  3. Any pulses at the input to IC219e (pin 11 of IC219). If not, then again, suspect the microcontroller if you've got this far. If you do have a signal there, check it again at the output (pin 10) of IC219e, then the input of IC211e (pin 11) and the output of IC211e (pin 10 -- this should be a 24 V level signal, not TTL). If you get a signal at the output of IC211e, then make sure it gets to the AC block. If it does, then the fuser should work.

If the microcontroller is faulty then as far as I know, the only place to get a replacement is off another DC controller board.

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