HP4M Plus printer problem - LED display is blank

[Muse]

I had a sense that something was wrong starting a few weeks ago. This printer is little used, I turn it on for a few minutes once or twice a week.

Now, I've had an HP4M printer since 1995 or so, but this HP4M+ I actually found set out on the sidewalk 5 years ago. I loaded it up in the back basket of my bicycle and brought it home. It was dirty, obviously hadn't been used for a while, I cleaned it up and got it working fine.

LED display now is completely blank. A couple of weeks ago the LED display started acting a bit weird. Instead of the usual message when it was ready:

[ 00 READY ]

the LED said:

[ 000READY ]

Otherwise it seemed OK, but as I say, this printer gets little use and probably turned it on only 3-4 times until last week, when I turned it on, it made the usual noises during the self test, and everything looked and sounded as normal except that the LED display was blank. I printed my stuff OK from my PC (i.e. it WAS READY). I did the same today, LED is still blank but it prints without issue.

So, what is likely the problem? Thanks for help!

PS My old HP4M from 1995 has an issue (I think failing PSU), so I'd like to fix the HP4M Plus if reasonably possible.

 

[dave_the_nerd]

LCD is burned out or has loose wiring?

 

[Muse]

LCD is burned out or has loose wiring?

Well, in this thing I think it's LED technology, not LCD.

How can I deal with it? I'm hoping I can do something myself rather than bring it to HP or someone else. I'm DIY when I can be.

As long as it works, I don't need the display, but it won't be long before the machine is flashing a message that I do need to see!!! It happens all the time.

The manual has one suggestion for this, being to press all the buttons. It literally says that one of them may have gotten stuck. I pressed all the buttons repeatedly to no avail.

I've posted here before about these printers and gotten knowledgeable responses, from folks with experience with this once very popular work horse business printer.

 

[LTC8K6]

Loose/dirty display connectors?

 

[dave_the_nerd]

Well, in this thing I think it's LED technology, not LCD.

How can I deal with it? I'm hoping I can do something myself rather than bring it to HP or someone else. I'm DIY when I can be.

LED/LCD doesn't really matter. Take it apart and check all the wires, replace the __D if you need to. *shrug*

It isn't going to be easy to find the part, although I suppose you could swap it in from your other printer to see if it's a repair you can do.

Otherwise, 20 years is a long time for a printer.

 

[Muse]

LED/LCD doesn't really matter. Take it apart and check all the wires, replace the __D if you need to. *shrug*

Replace the __D ... what do you mean? What's the __D?

It isn't going to be easy to find the part, although I suppose you could swap it in from your other printer to see if it's a repair you can do.

Otherwise, 20 years is a long time for a printer.

Maybe I could swap out the part from the HP4M to the HP4M+. Don't know if I have the service manual(s), if it's available I probably have it!

Well, 20 years is a long time but the duty cycle on these is probably 20x what's actually on these machines. I think they are both somewhere like 20k prints, very modest for these machines, that often go over a million prints.

 

[dave_the_nerd]

Replace the __D ... what do you mean? What's the __D?

The LED. Or LCD. Whichever it is.

Maybe I could swap out the part from the HP4M to the HP4M+. Don't know if I have the service manual(s), if it's available I probably have it!

It's worth a try.

Well, 20 years is a long time but the duty cycle on these is probably 20x what's actually on these machines. I think they are both somewhere like 20k prints, very modest for these machines, that often go over a million prints.

Duty cycle is one thing, age is the other. The parts age and eventually degrade/break regardless of whether they are used or not. Lubrication dries out, plastic breaks down, etc.

 

[Muse]

The service manual suggests replacing HP4m+ formatter C2038-60004 (if I'm reading it right), and if that fails, replacing the Control Panel. Looks like I can get the formatter off Ebay (used) for around $25 shipped.

The combined service manual for those Laserjet printers indicate a different formatter board for the 4 and 4+ printers, so I can't expect to be able to swap them.

Before ordering the formatter board I'm going to do some other things, follow the flow chart here:

http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c02577172

I'm ordering the paper version of the service manual for $20, it's a combined and will suffice for both my printers. The graphics in the PDF are very fuzzy on my 23" display..., they were evidently converted to very low resolution for inclusion in the service manual PDF.

 

[Muse]

Replacing the formatter PCA didn't fix the problem. I then ordered a used control panel and installed it today and the display works, so that was the issue. The replacement had one of the 3 tabs on the plastic housing broken off, but I put it in anyway rather than switch out the overlays. It seems sturdy enough. Now I have some extra parts:

Control panel plastic housing including buttons
Formatter PCA and metal housing
Jetdirect card (one was included with the formatter PCA/housing)

I also have the paper service manual. I like having that but the graphics are as insanely low resolution as that of the PDF. I can't believe HP allowed such crap to happen. Most of the photos look as if being viewed by someone who is virtually blind.

 

[Muse]

Looking at the failed control panel yesterday I noticed something. There are two capacitors on top of the PCA, C1 and C2 they are labeled. C1, the smaller of the two, had some carbon around the leads and one of the two wire leads had burned through where it goes through the board. I figure some high amperage must have passed through that wire for some reason and melted it. What would do that? Would that be a failure of the capacitor or was it something else in the electronics that supplied the excessive current/voltage combination that melted that wire?

I don't see anything in the service manual regarding that capacitor, the service manual isn't granular concerning wiring, schematics, etc (absolutely none is shown). It treats the control panel as a unit and if there's a problem they expect the technician to replace the failed component, not think in terms of repairing it. However, I'm going to investigate replacing that capacitor. It's very small. I'd have to remove the circuit board, which I actually did yesterday in an effort to determine how to swap the plastic housing, inasmuch as the one that came with my replacement (used) control panel had one of its three snugging tabs broken off.

I encountered a page online that details what you do to fix reduced brightness of the control panel display. How to fix HP Laserjet 4 Plus LED display dim In doing that, I'll again be removing the circuit board from the control panel assembly. I may have two working control panels when I'm done with this stuff. I am going to bring the control panel with the supposed bad capacitor to my long-time local independent electronics supply store and see what they say about that capacitor and my likelihood of fixing the component by replacing said capacitor... in a day or two.

Edit: Actually it says here that:

A capacitor can fail shorted which is probably the worst thing that could happen. This is one of the reasons why it is important to replace capacitors which are failing early. A shorted capacitor would probably damage the motherboard to an extent that it would be impossible to easily repair requiring components to be taken from a donor board and used as replacements. It is possible for a capacitor to fail shorted also with no visible signs.

So seems that replacing that capacitor and expecting that control panel to work is evidently a very iffy proposition. It's a printer control panel, not a motherboard, but I can't expect that it didn't suffer some damage other than that capacitor's lead burning out. I'll see what the folks at the store say. They deal with this kind of thing day in day out.

 

[Muse]

Replacing the control panel with a used one fixed the problem, but the replacement is very dim. But it is legible if you look close, and you usually don't need to...