NeXT 400dpi laser printer restoration

[Paolo B]

Hey all, my parts arrived. I replaced the thermoswitch, pickup roller and expeller gear. Unfortunately it still behaves the same as before the thermoswitch replacement (the lamp is still not glowing it seems), but there is continuity now at the J-112: Around 5-6 Ohms before I turned it on.

No idea if it's supposed to be like that, it would be great if someone with a working printer could confirm a similar reading.

I'll go back to the service manual once more to find some clues

I suspect we're now experiencing the same issue. All checks are so far OK, but the printer doesn't start.

Actually, what is happening is that the safety relais RL101 closes and opens again just a brief instant later (what it takes to the capacitor C156 to charge up, I assume).

According to the documentation, it should be the safety circuit that is cutting off the current for preventing damage to the heater.

The triac Q101 is tested good. C156 is brand new and Q154 is also tested good.

There seems to be a short somewhere, which is draining an instant excess of current upon powering up, triggering the safety circuit.

Point is where...

By the way, at J112 I read 4 Ohm, without dismantling anything else.

 

[powerlot]

I'll bring the printer back from the basement in the next few days. It'd be great to have someone who has one of these in working condition, but I assume very few have survived by now. Those parts Q101, C156 and Q154 were good in my case as well.

I thought of applying line power to the lamp connector to see if it turns on and triggers the thermoswitch, but I'm not sure if the lamp can be powered like that and I don't want to ruin it.

 

[Paolo B]

As far as I could get from the pictures of the disassembled heater you have shared, it seems to me (just speculating, though) that the thermoswitch is installed in parallel to the heater. In the printer service manual the schematics are reading series, but could be it's just a conceptual representation.
Default state is open, all current through the lamp for heating.
When the temperature raises above the safety level, it triggers the switch, which closes and shorts the lamp. The excess of current is then detected by the safety circuit and the relais immediately goes open, cutting the current.
Normally, the thermoswitch should then automatically reset (open again) after cooling down. Maybe in my case it didn't because it's faulty and it keeps on shorting the lamp.
As a matter of fact, I clearly remember I let the machine and the printer on for some time, so it could indeed be that it overheated and I just didn't realise it was already off when I turned the Cube off... Just a possibility.
I need to disassemble the whole fuser unit for checking all the components, but due to upcoming festive season I won't have any time to pour on this project for a while... I count on restarting at the beginning of the next year.

 

[Paolo B]

Umm... seems like my conjecture is definitely wrong. In the HP Laserjet II service manual, they clearly state that the configuration is in series and therefore the thermoswitch default state is closed.

 

[Paolo B]

As for the thermistor, it is indicated some 1.1 kOhm resistance at room temperature.

 

[Paolo B]

So, I confirm I have the following readings:

3.5 Ohm for the fuser at J112
1.5 MOhm for the thermistor at room temp (some 1k higher ?)

Upon a cold start, the printer gets briefly powered up, the fan spins for a fraction of a revolution, but immediately soon after the safety relay is triggered open.
Upon a hot restart, however, there's no action at all.

I would speculate that all the power supply chain is operating fine and either the thermistor or the thermoswitch are behaving erratically, triggering the protection system.

 

[powerlot]

I heated my thermistor with hot air and resistance appeared to go down (in post #63), but I couldn't tell if it's working properly or causes wrong behaviour with the protection system. I think you're on the right track, something is misbehaving, triggering this protection.

 

[andy]

You can always try substituting a fixed resistor for the thermistor to see if the fuser lamp will turn on. Choose something close to the expected room temperature resistance of the thermistor. If it turns on, you should replace the thermistor with the correct part.

 

[powerlot]

@Paolo B did you ever hear back from Rob Blessin regarding the missing pages? https://www.nextcomputers.org/forums/index.php?topic=4791.msg30957#msg30957
If he has spare boards I would certainly buy some but I would like to pinpoint the issue first.

 

[Paolo B]

did you ever hear back from Rob Blessin regarding the missing pages?

No, but I understand he’s having a lot of personal issues and I don‘t want to insist.
I should soon be able to get back working on the printer, hopefully for making some progress.

 

[Paolo B]

Yup, definitely a problem with the fuser circuit. Doesn't get hot and this stays around 4.5-5 Volts

Today I checked the fuser unit, only focus on the thermistor and on the thermoswitch, I didn't touch the lamp, as I'm sure it is OK.
The thermoswitch is labelled Elmwood 2450RC. According to the documentation I could find online, it should be closed as default state. Close it is and close it remains.

The thermistor looks OK, at room temperature the resistance is around 1.3-1.4M Ohm, in line with the specs.
However, I must confess it's still not fully clear to me how the whole circuit should work.

I tried to measure the voltage between pin 4 and 5 of J851 during start up phase. As the start up signal is delivered, the voltage goes up and then the safety circuit kicks in.
In which conditions did you measure 4.5-5 V?

 

[Paolo B]

Today I have checked the thermistor. Resistance goes down as temperature increases, it goes back to nominal when it cools down.
So, all in all the fuser unit seems to be OK.
At this point I would say the issue is on the DC board side, which is bad news...

 

[powerlot]

In which conditions did you measure 4.5-5 V?

I soldered some jumper wires, put everything together and measured the voltages while turning on the printer, computer etc.

 

[Paolo B]

soldered some jumper wires

I think I will now have to do the same with mine.
It’s very frustrating to proceed in the dark, but I am still positive.
In my case, the safety circuit kicks in a brief moment after the printer powers up. The cooling fan can spin for a few revs before the current is cut off.
How about in your case: does the fan spin?
Page 2-26 gives some hints, but without the missing pages of the manual with the schematics it’s almost impossible to proceed…

 

[powerlot]

The cooling fan can spin for a few revs before the current is cut off

Since I didn't have any point of reference how it should behave, are you saying it shouldn't do that? Because that's the behaviour of my printer as well.
I hope the schematics can be obtained somewhere, I was stuck with the same problem before putting it aside

 

[Paolo B]

My printer worked perfectly until - all of a sudden - it started (mis)behaving like that.
The fan should be always on as documented in the linked movie.
Printer Start Up Movie
The fan is shut down - along with the rest of the printer - by the safety circuit kicking in and triggering the relay.
I think we need to track down what exactly is in turn triggering the safety circuit, the service manual indicates some possibilities…

 

[andy]

At this point, I would be tempted to bypass the safety relay. While there is some risk, it might give you some clues. Just be sure to keep an eye on the fuser temperature since it could overheat quickly.

 

[Paolo B]

Maybe the forced shut down is not related to the heater operations, but it’s triggered by the DC control board as a consequence of a failure somewhere else.
In my case, it must be something that suddenly blew off or similar, as again the printer worked faultlessly until… it did no more.
Visual inspection of the DC board is OK, fuser module should be OK.
I will re-check the high voltage unit, may there’s something there…

 

[powerlot]

Some HP LaserJet service manuals mention that any abnormal DC voltages or currents trigger a shutdown, this could possibly happening here. I'll bring everything back in running order so I can measure other voltages/currents in the next few days or weeks. I haven't looked at my HV unit at all, this is a good idea to check there as well.

 

[andy]

Since there were leaking capacitors, it's possible it's related to that. There could have been a damaged trace, or via that later went open. I've also seen components like ICs and resistors damaged by capacitor juice. Inadequate cleaning could have left leakage paths that later became a problem.