Electronics > Repair

Hp Laserjet 4 Repair

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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.

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.

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 :)

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.

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.


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