Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old March 12th 14, 01:56 PM posted to sci.electronics.repair
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Default Dusty power switch in HP LaserJet 4

Hello all!

Short version: Cleaning dust out of a printer power switch makes it
work better.

Long version:

Last night, as I was preparing to render unto Caesar the things that
are his, I hit the power switch on my HP LaserJet 4 and got... nothing.
No LEDs flashing, nothing spinning up, no smoke, no sounds - just dead.

Power cycling, reseating the power cord, a different power cord, a
different outlet - nothing. My hunch was something like a line fuse
having blown, so I started to take it apart. I extracted the low-
voltage power supply, which has the IEC input socket and the main power
switch. It provides +5 V and +24 V (I think) to the printer logic, and
switched line-voltage AC (I think) to the fuser and high-voltage power

Taking the lid off, it has an input circuit breaker, not a fuse, and the
breaker wasn't popped. There wasn't any obvious mayhem on the board.
There was a small wad of dust lying on top of a couple of 22K resistors
tied into the primary of the main transformer; removing said dust
revealed a darkened circuit board in that area. None of the resistors
in that area were open and the diodes in the area acted diode-y to a

There was also small amount of oily goop on the board around the base of
something like a 33 uF, 16 V capacitor at the low-voltage output. The
capacitor can didn't look bulged, and I cleaned up the goop with
alcohol, but since I didn't prove that it was broken, I decided not to
fix that further.

I started doing continuity checks from the IEC inlet forwards. I didn't
notice any difference with the main power switch open or closed, which I
thought was odd. I couldn't get continuity from either line pin in the
inlet to one of the power switch pins I could get at. I took the
circuit board out of the bottom half of the metal case, which exposed
all four pins on the line switch. I figured it was a DPST switch, but
I couldn't get continuity where I should have been able to.

Time to break out the iron and solder sucker. All four pins unsoldered,
and the switch carefully (like, carefully: the circuit board wants to
bend if there is still a little extra solder on the pin) pried out of
its hole in the heatsink... still no continuity between terminals that
really should have it. This switch is mechanically weird; the actuator
is a goofy shape and is operated by a pushrod from the front panel, so
I figured it was unobtanium. As a short-term fix, I was prepared to add
jumper wires where the switch used to be, and just let the line plug
deal with the inrush, but I decided took a closer look at the switch.
(Learn By Destroying, (tm) Jeff L.)

I popped the weird rocker out of the switch body with a tiny
screwdriver. It came out with two attached springs. Two stamped metal
contacts fell out... and so did a wad of dust.

The dust was similar to what was on the circuit board... not oily or
greasy; not animal hair; just a slightly lumpy and coherent wad of dust.
The switch body and rocker didn't look melted; each fixed contact had a
small pit in the center of the button, and so did each movable contact.
The switch didn't look like it had ever had grease in it.

Thinking I had nothing to lose, I swabbed off all the contacts with
alcohol, figured out how it went back together, and reassembled the
switch. The multimeter liked the cleaned switch much better; 0.1 or
0.2 ohms across either contact when closed. (Shorting the probe tips
is 0.1 ohms.) I pondered rigging up a higher-current test of the
switch, but then I decided just to put it back together.

I got it all back together enough to run the printer. I didn't put all
the printer covers back on - partly for easier smoke detection, and
partly because if you completely reassemble something before testing
your repair, your repair will not work.

Plug in, switch on... and action! Things seemed to be coming up as
normal. When the display got to "WARMING UP", I figured the fuser lamp
was probably on, and I hit the switch to see if it could open up a few
amps: no drama, the printer just powered off. Hit it again: power comes
back up. Off and on a couple of times: OK.

With a watchful eye for smoke, I started a print job. Ran fine. 150
or so pages later and it's still cranking along.

I'm not sure why the dust picked last night to settle in that spot. A
couple of weeks ago, I *did* haul the printer to another location to do
some printing there. When bringing it back, I slipped on some ice while
carrying it; I managed to end up between the printer and the ground, but
it would have been tilted at some odd angles. I brought it back home
and set it up and I'm almost certain I turned it on when I did that.

Perhaps the IRS is getting really creative in encouraging people to
e-file. I didn't *notice* any dust delivery drones lately, though.

I'm still not 100% sure if I trust the switch... it seems a little weird
to do sub-component-level repair on something that has RICH, CHUNKY
VOLTS going through it. I might order a (used) low voltage power supply
for future replacement.

Anyway, that's the story. The firmware date codes on this thing are
from late 1993. With a little luck, maybe it'll run for another 20
years. I wonder if that would be enough to outlast HP itself...

Matt Roberds

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Old March 12th 14, 02:26 PM posted to sci.electronics.repair
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Default Dusty power switch in HP LaserJet 4

My HP 4M was purchased in 1992. About a year ago, worrying that the power
switch might fail within my lifetime, I dug out a Zenith Christmas-lights RC
remote control. The power switch is now permanently on, and I don't have to
fumble for it.

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Old March 12th 14, 07:08 PM posted to sci.electronics.repair
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Posts: 378
Default Dusty power switch in HP LaserJet 4

On Wed, 12 Mar 2014, William Sommerwerck wrote:

My HP 4M was purchased in 1992. About a year ago, worrying that the power
switch might fail within my lifetime, I dug out a Zenith Christmas-lights RC
remote control. The power switch is now permanently on, and I don't have to
fumble for it.

My HP 4P is only about 10 years old, but then I bought it at a Rotary Club
"garage sale" that year for $15. It had a low page count, something under
10,000 pages. The cover for the RAM expansion area was missing, unless I
lost it on the way home, and despite the low page count, the cartridge was
a generic replacement, that didn't last that long. I don't know why
someone got rid of the printer in the first place, likely some business,
but I assume the cartridge was replaced because it was being given away.

It's a toss up, I don't use it that often, so I'm not sure I'm worried
about the switch. On the other hand, it is something I turn on and off,
precisely because I don't use it much.


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