NeXT Laser Printer not working (connection established but doesn't print)

[kamiru]

Hi,

I recently bought a NeXT Cube with the original 400 dpi laser printer and it has been really exciting so far. The printer verifiably worked days before I picked it up as the seller printed test pages on this exact Cube and cable, and I have them (toner is crisp and fully fused).

Somehow though, the printer now refused to print anything. After troubleshooting for two days without result, I've decided to take it to the forums   

Setup:
- NeXTcube (1990)
- NeXTSTEP 3.3 (fresh install)
- NeXT 400 dpi Laser Printer (N2000, European unit)

Symptoms:

Any print job: printer powers up (fan/motor spin briefly), no paper feed, then fails. Terminal usually shows "np0: serial command parity error"
Sometimes it shows"np0: ioctl error; fatal + Cannot get device status: I/O error"
Printer→Cube status works flawlessly: cover-open, no-paper, cassette-type alerts arrive instantly and correctly, never corrupted. nppower on/off also works (audible). So the link is NOT fully dead — specifically the Cube→printer command exchanges fail.
The fuser never heats. Stone cold after 3+ minutes powered, tested properly: fully reassembled printer (all covers incl. rear door), toner cartridge + paper in, and respecting the 10-minute-off heater lockout from the service manual before testing.
TEST PRINT button on the DC controller: no reaction (consistent with engine never reaching READY, since RDY requires fuser at temperature).
Interesting history: even when the printer worked at the seller's, jobs sometimes hung and he'd routinely fix it with pause+resume in the print queue — so the link was already marginal before transport.


Already done/ruled out: fresh OS install (no change); cable ends inspected/cleaned/reseated many times; wiggle tests while tailing /usr/adm/messages (no correlation — errors drift on their own between "modes"); J112 and J851 reseated; CB101 breaker checked (springy, doesn't feel tripped); voltage selector verified; full reassembly; interlock conditions met. No multimeter measurements yet (being fixed this week — I know J112 fuser-side should read ~3.5–4 Ω per powerlot/Paolo B, thermistor 1.0–1.6 MΩ).

My working theory (heavily informed by a VCFed thread): either two independent faults (transport-killed lamp/thermoswitch + flaky command path), or one common root — aged AC module / LVPS electrolytics producing dirty rails that both corrupt serial command sampling AND starve the heater control chain (à la Paolo B's under-specced optocoupler resistor / C152+C156 story).

Questions:


Has anyone traced np0: serial command parity error to a specific cause? (cable conductor vs interface PCB vs supply rails?)

Is a simultaneous dead-heater + corrupt-command-link failure a known signature of the AC module / LVPS caps?

Any other ideas?

Thanks a lot in advance!
If any more information or photos are needed, then that's absolutely doable.

[ZombiePhysicist]

No idea. I could swear we had a thread on fixing a laser printer, and that might be of help