HP 6644A refurbishment – front panel, back protector (#P14F1)

Time to finish the HP 6644A revamp!

tl;dr for the last post: Here’s where I stopped refurbishment after cleaning and replacing caps and fan:

First off, those leads stick out quite a bit at the back as well. I can’t really say which diameter I used, judging from the yellow cable lugs these were 4mm² for the power lines and 1.5mm² (red lugs) for the sense lines. Apart from not fitting the old protector, that one already has seen some damage. Probably because it is so tiny…

So: Redesign. Twice, as the first version that closely resembled the original design had really flimsy legs that suffered the very same fate. Second version is here for download in DesignSpark format: hp6644back_v2

It’s, uhmm, massive, but that’s what I wanted. I think it does not cover the screw holes perfectly as I changed printers between v1 (“too wide, doesn’t fit, argh”) and v2 (“cut off another piece, still too short”), but it does mount nicely. For anyone that doesn’t need that much space for wiring: Just chop off something from the top. As it is getting thicker there, the piece will likely get much more flexible with that.

Here’s the part in comparison to the original HP lid:

Mounted to the chassis: (looks much more massive as it is due to black plastic actually being black, lacking reflections on the rounded parts)

And finally in mounted position in the rack – does protect the connectors and the wire harness during and after mount, doesn’t hit the wall, perfect!

During the process I also replaced every outside screw with modern stainless Torx ones, there are two different kinds on the sides of the case (the strap needs countersunk ones, the other side has more space), one holding the rack mounts, and one for the back panel connectors. Fuck that nasty Phillips crap…

Now for the more difficult part, the front panel connectors. As shown previously, there are holes in the front panel that almost fit standard 4mm jacks. Not quite, and as I wanted double output connectors, I just had to take out the drill.

I asked for another front plate with nice surface finish, and my colleague got back to me with an assortment of five different ones. Well, somebody had fun with his 3D printer…

Click the gallery for detailed views. As far as I can remember he tried different nozzles (layer thickness), fill patterns, as well as “ironing” the completed print with zero material deposit and also ironing an incomplete top layer which caused it to collapse. Some were shiny, others matte, one could decide for any of these prints!

Here’s also the CAD files in RSDOC and (exported) STL format for download.

I ended up using the rough print with incomplete top layer, as it has such nice texture. The 100% fill around the holes isn’t fully visible after mounting, and you also cannot touch that tiny ring. Two of the others were glued together as drill guides, and another was used for the final cleanup. A fourth one was cut and used on the other side as some sort of washer, for a more stable mount. I did not fill the center voids, as I’m not terribly excited to create three moulds or to permanently fix the connectors to the case.

Drilling in progress:

It always helps sticking in the jacks to see how much material is left to remove. As you can see, they do fit already, but won’t align just yet. That’s part of the original HP mould – maybe they realized this and never made the front panel an option?

Front done:

Back almost done, those printed sheets shatter everywhere when mutilated with my beloved side cutters!

The top mounting hole was in place, but the jack wouldn’t screw down properly due to a bit of the front mould being in the way. As that’s not terribly important for the structure, I negotiated with my Dremel. Dremel won.

And that’s all seven mounted. I wonder: Is there a China clone of the original fastening tool for those jacks? It’s the SKS MW SEB (VS) part and they charge 25€ for a tube of steel with two bits sticking out. I used the DIY one from work, we got a couple of them made in our metal workshop waaay back…

Surface isn’t 100% flat as I added some hot snot before mounting. I don’t think that was necessary, probably wouldn’t do that again. But the few tenths of a mm aren’t really bothering me now. I’d say a metal part in some old computer beige also would look nice, don’t ya think?

It’s not done yet, as the metal chassis does fit 6 connectors, but the least important (…GND, what else?) on top has nowhere to go. Again, nothing a Dremel cannot handle. I just cut out a triangular piece. However, clearance is reeeally narrow. One could avoid this with a thicker front plate, or spade terminals instead of my M4 screw terminals (not sure about bending these!) – or another quick Dremel run. I cut off the terminals at the very end of the threaded part. They have a solder can at the top and a bit of spacing underneath, that’s gone now and the clearance is sufficient for my taste. Top one didn’t need the cut as it is centered – and also connected to chassis ground.

At this stage I also shortened some screws on the bottom of the case (likely from the massive heat sinks), those were sticking out just a wee bit too much, scratching the case of the 19″ drawer underneath the power supply when sliding it in.

Add the connectors, move them to the middle, screw in tight and push the front assembly into place. Et voilà:

The short circuit blocks from SKS/Hirschmann fit nicely in ANY configuration, so I reached my goal of standard 19mm spacing all around. The 6644A works nicely, actually with and without the sense wires attached. It somehow adds around 0.1V if the sense tap is missing, which will do for most cases, but I don’t want to rely on that figure. I keep those jumpers in place, that’s why I bought them and that’s why I have two output terminals each. I might need another pair for my electronic load, and a single one for the HP. That would be for referencing one output to earth, as this lab supply is obviously floating by default.

Still considering painting the 19″ mounting brackets in matte black, but that’s a really quick thing to do, even now after fully mounting it. Maybe when finalizing the 3U case shown above. That aside – my HP 6644A is fully pimped now, what do you think :yahoo:

Both 3D models now available on thingiverse, too: 4128255 + 4128097

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[…] a amp meter would skew the results, but the HP 6644A offers remote sensing (especially easy with my custom front connectors), so I just put the voltage reading line after the multimeter, set it to 3.15V and enabled output […]