I'm trying to track down a rather odd problem with an 1993-vintage HP
Laserjet 4. On occasions the printer works fine, test prints and parallel
port prints work fine (apart from a minor paper jam, but that's why the
printer was free).

But particularly when running it from cold (been off for a few hours/days)
it does something odd. On powerup the fans come on, the fuser starts
warming up, everything behaves normally except the display doesn't work.
The control panel has about 8 buttons, 3 LEDs and a VFD. Normally the VFD
would say '05 SELF TEST' immediately on powerup and three LEDs come on, then
go off once self test mode has completed.

But from cold the VFD doesn't come on. What's odd is that the LEDs do light
up as normal, but at half brightness. If you hold down some buttons the
LEDs get dimmer, the more buttons the dimmer they get. Would that suggest
the LEDs are being powered parasitically through some pullup resistor? The
LEDs go out at the same point in the warmup sequence as they do when the
display is working, but the printer refuses to print from the computer. The
self test modes are accessed through the menus, which of course I can't see
with the display dead.

There are two boards to the printer: the DC controller and the formatter.
The formatter has the CPU, the DC controller is just a pair of ASICs to
control all the motors, switches etc. The display plugs into the formatter.
There's an 'engine test' button that bypasses the formatter, which works
fine (prints a page of vertical lines) whether the display is working or
not. So I think either the formatter is working but confused (which would
account for the lights going off at the right time) or it's not working at
all in this case and the lights-out is from the DC controller. The 10 pin
display cable is tracked off to an ASIC on the formatter, so no help there.
Needless to say the display is plugged in OK.

There's a good service manual, but unfortunately this fault isn't covered.
It says there are two power supply rails, +24V and +5V (plus a HV feed to
the fuser). I would measure them, except having taken the thing apart I
have to wait another day before it starts exhibiting the problem again.

So any suggestions to possible areas to look? I'd guess maybe something
like:

1) Power supply dropout: +5V is dropping out enough to fail to run the
formatter correctly
2) Power on reset isn't working
3) Some kind of earth loop? It's difficult to test because it'll start
working spontaneously anyway. But it still happens even when the parallel
cable is removed (parallel comes from JetDirect printerserver box connected
to ethernet and same mains socket as printer)

Might it be a dry cap somewhere? That might explain why it sometimes works
OK after it's been power cycled enough times. There are only a few tant
decouplers on the formatter board and none on the DC controller.

Cheers,
Theo