Valhalla 2724A Programmable Resistance Standard

[TheDefpom]

Sure, if I can remember WHERE I put it...

[TheDefpom]

Here you go, a short video showing the unit.

Video is in 4K so to see the most detail go to youtube to see it, rather than the forum viewer.

[TERRA Operative]

Awesome, that's perfect, thanks a bunch!

[hulchm9ib]

Hi do you have the images for the non CPR version?

Thanks

[TERRA Operative]

I just got my 2724A in my hands and took a look inside as Accusource Electronics who I bought it from mentioned they had to replace a capacitor before they sent it to me, as it failed when they dd a final check before packing.

Turns out they replaced the six largest axial caps with Illinois Capacitor brand caps which is totally fine, IC make good capacitors, but the workmanship certainly is not fine at all...
They lifted some pads, completely ripped one off the board and there are some dry joints there too. Seems someone in their service department needs a swift kick in the arse and a some training on how to use a soldering iron.     

Nothing I can't fix, but the point is that I shouldn't HAVE to fix it.

At least the thing works fine, besides the butcher job on the recapping. 



So, moral of the story, DON'T let sellers fix things for you unless you know for certain that they are capable of doing it properly. 

[TheDefpom]

Sounds like you should feed that back to the seller and tell them, check the tantalums too in case one of those blew up too, like it did on me.

[TERRA Operative]

I did send an email with photos. I don't expect much though...

I'll certainly replace tants and the electrolytics that weren't touched yet. Get it to a known state of repair, once I'm back from holiday.

As for now, I'll poke at repairing the bad solder job.


On a happier note, I also got my hands on a brand new in box rotary switch for the cal box now too, the exact part number as used by Valhalla (Philips Centralab PSA-211), and I have some teflon wire too. Just sorting low EMF binding posts at the moment so soon I'll have a switchbox.
The switch was from ebay USA and it seems the seller has a few left as of this post. https://www.ebay.com/itm/156390841118

I also plan to cut a bit of clear acrylic to make a temporary replacement lid with strategically drilled holes to allow for the tweaking of the trimpots with the unit closed during calibration and adjustment. It is mentioned in the manual, but no pictures of it seem to exist and Valhalla were not able to provide info, but it's pretty obvious what to do to make one..

[TheDefpom]

Yes you do kind of need that cover with holes, it is VERY temperature sensitive when trying to adjust it, I was having to slide the top cover back quickly, adjust it, and put it back on, wait a few minutes to see how it settles, and repeat.

[TERRA Operative]

Today I whipped up a top panel from clear acrylic with some strategically placed holes to allow for tweaking the trimpots with the lid on to maintain thermal equilibrium.

Attached is a PDF file with the dimensions. It should print out at 1:1 on A2 sized paper if you want a template, but it's pretty easy to eyeball the holes with a ruler and hand drill.

[coromonadalix]

nice idea for the top case,  maybe 3d print some hole plug ?

[TERRA Operative]

Been playing around, got some things done. Uploading a bunch of stuff here for posterity.

- Rebuilt the manuals to be text based and fixed the schematics a bit too (diagrams are still bitmaps, I might redraw them later as vectors if I feel like it) let me know if there are errors....
- Decompiled the firmwares and found some undocumented stuff, including the ability to automate calibration via GPIB. Made a manual with the info.
- The two firmwares STD = Standard, CPR = Cardinal Point Option (zipped together into a zip file).
- And the decompiled firmwares as .asm files (zipped together into a zip file).

It seems calibration data can't be extracted from the GPIB port, but the socketed NOVRAM chips IC522 and IC523 can be read by an Arduino Uno pressed into service as a chip reader, so I'll add another post with that info once I verify it's a working backup method.

I'll probably write a python script to automate the calibration with a 3458A soon too. First I need a new UVEPROM for the firmware as mine is throwing errors. I think it's dead so it's off to Akihabara for a replacement next week.


Also, there's an amusing thing if you try hacking the firmware in the wrong way (like trying to enable CPR mode on a STD unit I suspect). See page 8-2 of the Firmware Reference PDF for a laugh. 

[TERRA Operative]

And here we go, an Arduino read/write thingo for backing up and restoring your cal and user non volatile memory chips.

https://github.com/JaredCabot/valhalla-2724a-novram

I stuck a ZIF socket on an Arduino Uno perfboard and it works great.



And an updated manuals repository is here (The firmware reference manual has been updated):

https://github.com/JaredCabot/valhalla-2724a-manuals