Author Topic: Taking apart the HP3478A - Front panel buttons not all working  (Read 2350 times)

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Offline Jeff_Birt

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Taking apart the HP3478A - Front panel buttons not all working
« on: September 17, 2016, 02:31:23 am »
I have a HP 3478A which works but some of the front panel buttons are wonky. The rear plastic cover and main metal cover are both simple to take off but getting to the actual front panel looks tricky. I can see that there is a plastic piece with two screws that connects the front panel to the main PCB (the LCD itself is on a ribbon cable). I can see some card edge looking traces on the PCB where the plastic piece is so I'm guessing there are contacts in the plastic piece that are pressing on the main PCB.

It looks like you need to slide the whole main PCB/front panel out of the metal frame as one assembly. I can't quite figure out how to get the plastic surround piece over the three threaded bosses on each side (for rack mounting.) There does not seem to be enough flex in the plastic to pry both sides open enough to clear the bosses.

I thought I would ask if anyone else has taken one apart before trying it and maybe breaking something. I found the owners manual but the section that is supposed to be the disassembly procedure is just pictures (and crappy ones at that) which is no help. Any pointers?
« Last Edit: September 17, 2016, 03:20:21 pm by Jeff_Birt »
 

Offline Jeff_Birt

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Re: Taking apart the HP3478A
« Reply #1 on: September 17, 2016, 02:39:04 am »
Of course I found the instructions right after I posted :)

When I saw 5-11 in the manual I thought that was the page number, that was the paragraph/section number. Now I think I can do it without breaking it :)

 

Offline bitseeker

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Re: Taking apart the HP3478A
« Reply #2 on: September 17, 2016, 02:55:46 am »
Yeah, it is a bit confusing the way the section-paragraph numbers overlap with section-page numbers.

It appears that you need to push the outer bezel back toward the motherboard, bringing the front-panel assembly out the front along with the motherboard. I'm looking at my 3478 right now and that looks like it'd work since, as you said, the bosses block the bezel from going out the front. Yes, steps n. and o. make sense now, after seeing the arrangement on the chassis.
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Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #3 on: September 17, 2016, 03:41:30 pm »
I changed the title to reflect the nature of the problem, i.e. not all of the front panel buttons are working. The service manual is at: http://www.ko4bb.com/manuals/index.php?dir=HP_Agilent/HP_3478A_Multimeter . Look on page 64 (as shown in your PDF reader) for the instructions to take it apart. It is easy once you have the idea of what needs to happen.

The problem I found was that the small black plastic block that connects the front panel to the main PCB was loose on the front panel. The heat staked plastic pins had broken. The plastic piece encloses a nifty gold plated zebra strip. I first thought about just soldering wires form board to board but then I thought I would try a more elegant fix that retained the mechanical connection between the boards.

Here is the front panel and main PCB being slid out.
 

Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #4 on: September 17, 2016, 03:47:43 pm »
Here is the front panel with the missing black plastic piece's home circled. I found a small screw to replace the heart stake post that was completely missing and it was shortened so it did not poke through the front panel. The black plastic piece was super glued into place and the screw installed. The screw insures the plastic part in lined up properly and super glue was applied to its threads and head so it will not come loose. I should note that I used the glue sparingly so it did not run down and get on the front side of the board and foul a switch pad.
 

Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #5 on: September 17, 2016, 03:52:34 pm »
If you have taken one of these part before you may have noticed that I glued the black plastic piece on upside down! DOH! A moment of panic but it was discovered soon enough to pull it up and flip it around, Whew!

Now I'll give the glue time to cure and then slip it back together to see if it works. It is does I have the urge to pull it back apart and apply epoxy around the periphery of the black plastic piece to ensure it does not ever come loose again. Or, I guess I could just isolate all the loose wires and unbolt the regulator from the frame and plug it in. That would allow testing without reassembly.
 

Offline bitseeker

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #6 on: September 17, 2016, 05:24:18 pm »
Thanks for the update and photos, Jeff. I haven't encountered this kind of front panel issue before, hence never took one apart to this extent. Nicely done (apart from the upside-down bit ;D). :-+

By the way, is that the factory-original battery on there?
« Last Edit: September 17, 2016, 05:25:56 pm by bitseeker »
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Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #7 on: September 17, 2016, 05:58:44 pm »
The ICs have date codes from '86 and the unit had an Agilent repair/calibration sticker from 2003. I think the battery was replaced when it was serviced based on how the solder joints look. It was above 3V by a fair bit as well. I can't see any sort of date code on the battery though.
 

Offline bitseeker

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #8 on: September 17, 2016, 06:01:45 pm »
Sounds OK. If it was at or only slightly above 3V, you'd probably want to change it. As a stock broker would say, it just depends on your risk tolerance. :-DMM
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Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #9 on: September 17, 2016, 06:44:08 pm »
I just reattached the front panel back on, isolated all the loose wires and pulled the regulator from the metal from to hook it up. Amazingly all my buttons work now! I'll add a bit of silicone to bottom of the front panel/black plastic piece connection for added piece of mind. Epoxy would work too but it would be impossible to remove at a later date if this patch job fails.

I just measured the battery again and is is about 3.02V. I may go ahead and replace it in the near future when I can add it to another parts order.
 

Offline bitseeker

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #10 on: September 17, 2016, 07:07:40 pm »
Sounds good. There are threads about the process when you get the new battery.
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Offline Jeff_Birt

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #11 on: September 17, 2016, 07:39:53 pm »
Here is the final part of the patch job, some black silicone around the bottom of the back plastic piece (and over the added little screw). This is just for a bit of added insurance. It is now all back together and all the buttons work!

 

Offline bitseeker

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Re: Taking apart the HP3478A - Front panel buttons not all working
« Reply #12 on: September 17, 2016, 07:55:26 pm »
Success! :-/O :-+
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