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NeXT 400dpi laser printer restoration

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
Spent some other time on this desperate case. The signal FSRD on J208.2 is sent by the DC board and goes briefly up, but soon after the safety circuit kicks in and cuts the current. I have thoroughly inspected almost everything and everything looks fine. Now, according to the service manual, the current is cut in case of shorted triac Q101 (Page 2-26 “Protection against short-circuited Q101“). I tested it and to me it seems OK, too. But maybe it’s indeed NOK. So, next week I will try to replace both the triac and the solid state relay, as I could easily source both.
 

powerlot

Experienced Member
Joined
Nov 20, 2021
Messages
452
Location
Europe
I came to the same conclusion, my Q101 triac tested fine too. So far your printer is exhibiting exactly the same behaviour as mine. I didn't pursue further troubleshooting of that circuit... that would be a sneaky fault if those parts have gone bad despite testing good
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
I’d be surprised if the triac was faulty, but I’m running out of ideas. As it costs little money, I will give it a try.
My understanding is that it’s either the heater circuit that for some reason is draining too much current triggering the safety circuit, or it’s the DC board that senses something wrong and switches off the printer. In such a case, it could indeed be whatever. What keeps my hopes alive is that the printer did work fine, then one day did no more, but without any catastrophic event in between. So, I still hope that the damage is indeed very localized and confined to some aging-prone kind of component. To be continued…
 

njroadfan

Veteran Member
Joined
Jan 21, 2011
Messages
1,628
The contemporary Canon SX engine was notorious for power supply failures. I wouldn't be surprised if this engine suffered the same problems.
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
The contemporary Canon SX engine was notorious for power supply failures. I wouldn't be surprised if this engine suffered the same problems.
That's a very useful comment, thanks. By quickly searching about the SX issue, a faulty triac seems to be a likely issue behind the PSU failure.
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
So, bad news: I replaced the triac and the solid state relay and unfortunately nothing has changed. There were some good hints online about Canon SX failures due to the power supply, but it seems as though here we have a different issue.
In the small clip linked here, you can see what happens (or does not happen) when I power up the printer. In the same channel you can find a short video showing the correct start up (same printer, just before this nasty fault occurred).
Any additional idea is welcome.Screenshot 2024-01-27 at 17.15.04.png
 

njroadfan

Veteran Member
Joined
Jan 21, 2011
Messages
1,628
FWIW, the Laserjet IIP/Personal LaserWriter NT based on the Canon LX had power supply problems too. Maybe these all fail in different ways? I always suspected heat was a problem with these.
 

Al Kossow

Documentation Wizard
Joined
Sep 1, 2006
Messages
3,454
Location
Silicon Valley
Has a Canon SX engine service manual shown up anywhere? CX is easy to find, I've been looking for an SX manual for years now.
 

powerlot

Experienced Member
Joined
Nov 20, 2021
Messages
452
Location
Europe
Looks very similar, don't know if the whole unit would be a drop-in replacement. Worst case it could be a source for broken lamps if it uses the same one
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
I confirm it looks alike, almost identical. The upper steel plate with the slit for the wiper is lined with some kind of cloth in the unit I have, the only visible difference to me.
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
Another chapter of this endless saga.
Took some time for going through the AC unit. I tested all I could possibly test and to me everything seems to be in working order.
Interesting to note: as 24 V are fed into the unit via J151-1, the fan automatically starts at low speed (there’s a voltage regulator which limits the voltage to approx 11 V).
As you send some 3.5 V to the fan control pin J151-4, the diode Q158 shorts the voltage regulator (zener diodes) and the fan can spin at full speed under some 23 V (short video here).
In my case, the fan now is not spinning at all, means that there’s no voltage supplied by the DC power supply.
On to this one…
(No voltage, also means no possibility to trigger the relay and to feed current into the heater.)
 

Ruud

Veteran Member
Joined
Nov 30, 2009
Messages
1,322
Location
Heerlen, NL
I don't have any interest in printers except as using them as a printer. But I admire it that you want to repair one and I hope to read one day: "IT WORKS !!!"

So good luck!
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
Thanks for your words of encouragement! Totally agree, printers are quite boring stuff. This one is really catching me, though, as it did work until all of the sudden it just went on strike. I am now engaged in this fight, sometimes I feel like those gamblers who have lost everything and they keep on sinking money because they have the false illusion they can recover it. At least I keep on learning something, power supplies are still "understandable" (at least, to an extent...) even for someone fundamentally illiterate about electric engineering like myself (I have studied a lot of physics and electromagnetism back at the time, but very little practical applications).
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
So, I guess I have a Mosfet which is not OK, it didn't pass DMM testing.
Incidentally, it seems to be the most thermally loaded component of them all. So, I will try to replace it.
Additionally, on the daughter board there are two diodes (D20 and D18) next to a former leaking capacitor (C47) which have heavily corroded leads and need to be replaced (one has literally a deep crack in the lead, almost severing it).
However, there's no indication on the case, or maybe there was but then it got wiped out by the leaked stuff.
Would you have any suggestion?

IMG_3893.jpeg
IMG_3886.jpeg
 

andy

Experienced Member
Joined
Oct 22, 2008
Messages
237
Location
Austin, TX
When testing a MOSFET out of circuit, always momentarily short all three pins together to make sure the gate is discharged. It's easy for the gate to pick up enough voltage to turn on the MOSFET making it look like it's shorted.
 

Paolo B

Member
Joined
Apr 22, 2023
Messages
48
Location
Japan
The mosfet is a 2SK727. After consulting the datasheet, it seems to be OK, I was just not using the correct voltage at the gate.
 
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