Hp Laserjet 4 Repair

[vasonline]

Hi, eevblog members

I trash picked a HP laserjet 4.
- Unit is dead. No power on.
- Opened smps.

Measured 400V filter cap = result Ok.
Used esr meter, and found cap C109 (near optocoupler?) open (you can see the bottom of the cap doesn't look good either)

Will try another cap out of my parts bin

Below is a picture.

[SeanB]

Check the others are not bulged, and check the board for solder joint problems as well, especially on the power devices. Those are about the only faults other than dead fusers and worn mechanical parts that stop these printers.

[vasonline]

Hi sean,

Thanks for the reply
I did a power up with the first cap replaced: 5V = OK
24V rail, around 21V.  Measured at TB1. unut shuts down after 10 seconds. But that's already better then before :-)

Meanwhile I found c207 also bad = esr 29 Ohm.
Did not replace that yet. Will do

[SeanB]

You might just want to replace all the secondary caps, as these printers are now pretty elderly and even the Nichicons HP used will be very close to dead now.

[Whuffo]

Pop the fuser assembly out (back door) and give it a close inspection - any damage here will require a replacement. It's a service part and they don't last forever - and they have a significant price. There are two power supply boards in this printer; the DC supply (where you're working now) and the AC supply. You'll also want to inspect the formatter board (that's the big one on the side of the printer); it's got a lot of electrolytics on it, too. HP doesn't supply components; these boards are FRU items. It's a great printer, but repairs like this can be a little pricey.

[vasonline]

Hi,

After replacing C207, the power supply appears mostly working :-)
(there could still be a cap on it's way out there - thx Sean!)
I was greeted with this picture on the VFD Display:

It says printer open, but that's Okay, since it is still half disassembled
Thanks Whuffo for the tips!

[Stonent]

The LJ4 was always my favorite.  If you take the sides off it's a big steel chassis inside. Starting with the LJ5 it just seems to have more plastic than metal inside.

Plus the LJ4 takes standard Fast Page 72 Pin SIMMs that you can salvage from any older Pentium (Pre-MMX) or 486. HP started using proprietary memory modules in either the LJ5 or the LJ4000, I forget which.

If you have access to an air hose, just blast it and get all the dust, fuzz and talcum powder (used on paper to make it not stick) out of it. That will help with the burning smell as well that these old printers seem to have.

[vasonline]

Hi,

I assembled the printer today. And it indeed worked!

I was greeted with "00 Ready" on the display.
Printed a test page: it has 103300 pages on its counter.
I think the tone cartdrige is a bit low on toner, because text kept getting lighter and lighter (and so on...)
...and the rest of the page got a bit grey.....

- I blew out the fan. (indeed quite a bit of dust came out)
- That ram looks worth upgrading. I have a few sticks lying around that would fit.
Found a website that explains what needs to be done.
(I threw out many more 72 simm's, some time ago. I tought I never needed them again ) Still have some of the even older 30 pin SIMMs

That's it for today.