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

"Theo Markettos" wrote in message
...
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


You've almost certainly got a dried out cap or a cold solder joint in the
power supply somewhere. A can of freeze spray can help you find that.

On Thu, 11 Oct 2007 19:54:11 GMT, James Sweet wrote:

"Theo Markettos" wrote in message
...
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

You've almost certainly got a dried out cap or a cold solder joint in the
power supply somewhere. A can of freeze spray can help you find that.

As well, on a machine that old (I have an HP LJ III - circa 1990), age
may have played a factor in one or more of the board-to-board connectors.
You could try carefully unplugging and re-plugging each one a couple of
times.

While clearing out "papers and stuff" several years ago, I came across
the cancelled check for that printer: $1,872.50 -- and *that* was in
1990 dollars! I taped the check to the lid of the printer.

HTH
Jonesy
--
Marvin L Jones | jonz | W3DHJ | linux
38.24N 104.55W | @ config.com | Jonesy | OS/2
*** Killfiling google posts: http://jonz.net/ng.htm

"Theo Markettos" wrote in message
...
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.
snip


Have a look he-
http://www.fixyourownprinter.com/search/forums?q=hp+4l

Colin

Allodoxaphobia wrote:
On Thu, 11 Oct 2007 19:54:11 GMT, James Sweet wrote:

You've almost certainly got a dried out cap or a cold solder joint in the
power supply somewhere. A can of freeze spray can help you find that.

Thanks, I'll take the PSU apart and have a look. I just tried it again and
it's still working though

I'll ask around to see if anyone locally has another one going for scrap I
can swap bits from - might save trying to measure the ESR of all the caps.

As well, on a machine that old (I have an HP LJ III - circa 1990), age
may have played a factor in one or more of the board-to-board connectors.
You could try carefully unplugging and re-plugging each one a couple of
times.

That's a possibility, but I didn't do anything to the machine (move it
around or anything) between switching it on when it didn't work and
switching it on when it did. Of course that means nothing with intermittent
faults - and maybe me taking it apart has fixed the problem, or perhaps it
was thermal.

While clearing out "papers and stuff" several years ago, I came across
the cancelled check for that printer: $1,872.50 -- and *that* was in
1990 dollars! I taped the check to the lid of the printer.

For me laser printers are deflationary:

1996: Panasonic KX-P4450: 25 pounds, sold it for 40
2004: Apple Laserwriter Select 360: 10 pounds
2007: HP Laserjet 4: 0 pounds

:-)

Theo

"Theo Markettos" wrote in message
...
Allodoxaphobia wrote:
On Thu, 11 Oct 2007 19:54:11 GMT, James Sweet wrote:

You've almost certainly got a dried out cap or a cold solder joint in
the
power supply somewhere. A can of freeze spray can help you find that.


I'll ask around to see if anyone locally has another one going for scrap I
can swap bits from - might save trying to measure the ESR of all the caps.


Save yourself the hassle and just replace the $10 worth of electrolytic caps
in the PS... it really doesn't take much longer to do 10 than 1 once you've
got it all apart on your bench, the iron's heated up, and you're ready to
go.

Dry solder joints can be affected by heat/cold in addition to physical
movements. I haven't run into soldering problems on HP printers,
particularly older ones but it's possible... HP had very good QC whilst they
dominated the laser printer market.

Dave S.

P.S. I've got a similar printer, circa 1993 HP laserjet 4L.

Dave wrote:
Save yourself the hassle and just replace the $10 worth of electrolytic
caps in the PS... it really doesn't take much longer to do 10 than 1 once
you've got it all apart on your bench, the iron's heated up, and you're
ready to go.

Thanks, I think I'll do that when I've had a chance to buy the caps (there
are some odd values like 180uF 400V and 1200uF 35V). There's some singeing
of the PSU PCB on the primary side but all the joints look OK... but I can't
measure the resistors near the singeing until I get my multimeter sorted
(see Fluke 87 thread).

Dry solder joints can be affected by heat/cold in addition to physical
movements. I haven't run into soldering problems on HP printers,
particularly older ones but it's possible... HP had very good QC whilst
they dominated the laser printer market.

The build quality is pretty good, especially with the service manual saying
exactly how to remove each part.

Theo

Theo Markettos wrote:
Thanks, I think I'll do that when I've had a chance to buy the caps (there
are some odd values like 180uF 400V and 1200uF 35V). There's some singeing
of the PSU PCB on the primary side but all the joints look OK... but I can't
measure the resistors near the singeing until I get my multimeter sorted
(see Fluke 87 thread).

I've now replaced all the caps and it's on soak test - seems to be working
fine (this post hereby constitutes my 'tempting fate' test). There wasn't
anything obviously wrong with any of them - I ought to make myself an ESR
meter sometime to test this sort of thing.

One thing I wondered - there were two 1200uF 35V 105degC electrolytics in
parallel after the coil acting as smoothing of the +5V output. Since they
were in parallel and that value is a bit tricky to get (I've not heard of
electrolytics in E12 values before) I replaced both with a single 3300uF 35V
105degC which fits (just about) in the same space. But I wondered if there
was a thermal reason for this - would having two caps in parallel cause them
to heat up less and so last longer?

Theo

I've now replaced all the caps and it's on soak test - seems to be working
fine (this post hereby constitutes my 'tempting fate' test). There wasn't
anything obviously wrong with any of them - I ought to make myself an ESR
meter sometime to test this sort of thing.


There usually isn't anything obviously wrong, they have to get REALLY bad
before they leak or bulge and most circuits will stop working long before
then. An ESR meter is second only to a DMM for repairing modern stuff.