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Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)

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beanflying:
Lots to love about this precision boat anchor. 1000V dc precision DC voltage standard and NIXIE tubes combined with proper clicky rotary knobs  8)

Manual is here http://www.hparchive.com/Manuals/HP-740B-Manual.pdf
And some early references to it's predecessor the 740A  (without the Nixies  :'( ) to it here http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1965-05.pdf

Application Notes on the use of 740B and 741B (** Worth a read for everyone) http://hpmemoryproject.org/an/pdf/an_70.pdf

So why buy it - because I stumbled on it on evilbay and it looked very neat and tidy. Price was reasonable so punch the button and wait.

First photo below reveals an unhappy surprise on opening the package  :rant: Second Photo after a few hours warm up helped me smile as the 50 year old + Tracked my 34401a and month old 121GW to the last digit on the 10V range. The 100V and 1000V ranges are out of Cal but not horribly (under 10mV at 10V)

The tougher part of the purchase was to try and find the unobtanium input and output boxes and leads. CUBDRIVER has been looking for the input Box for a few years for his but seems I tin potted one 3 days after buying my 740B and snagged that too. Sorry again Pat  >:D So anyone knows of a source of these Pat needs an input one and I need an output one.

Plenty more photos and full teardown to go including some metalwork fixups, calibration and possibly modding the high voltage inlet and outlet plugs to handle the 1000 VDC. There is a Mercury 1.35V cell to arrange a stable replacement for as well.

beanflying:
Due to the weight of the beast the broken handle isn't likely to be needed to throw it around much (most likely will rack mount it). Went and had a chat to a local Aluminium welder today about TIG'ing it back together. His feeling and a few of the others working there was don't. But either brazing or soldering would be as good or better and lower risk.

So started the process this afternoon with some careful localised heating after first giving it a general warm up. Like Cast Iron old cast aluminium is brittle as you might be able to pickup on the sheared surfaces so anything you do should minimise thermal shock so NO quenching and post work heating is generally required to relieve stresses.

Some Aluminium fluxes to be chased up now as they aren't really that common anymore. If you are looking at this some are Nitric acid based and some of the solders used to be Cadmium based, the first is dangerous and the second can kill you.

Pre and post bending/heating got the handle sitting back fairly square again and the bent Rack Ear got put under the press earlier today  :-+


tautech:

--- Quote from: beanflying on February 07, 2018, 07:15:08 am ---Due to the weight of the beast the broken handle isn't likely to be needed to throw it around much (most likely will rack mount it). Went and had a chat to a local Aluminium welder today about TIG'ing it back together. His feeling and a few of the others working there was don't. But either brazing or soldering would be as good or better and lower risk.

So started the process this afternoon with some careful localised heating after first giving it a general warm up. Like Cast Iron old cast aluminium is brittle as you might be able to pickup on the sheared surfaces so anything you do should minimise thermal shock so NO quenching and post work heating is generally required to relieve stresses.

Some Aluminium fluxes to be chased up now as they aren't really that common anymore. If you are looking at this some are Nitric acid based and some of the solders used to be Cadmium based, the first is dangerous and the second can kill you.

Pre and post bending/heating got the handle sitting back fairly square again and the bent Rack Ear got put under the press earlier today  :-+

--- End quote ---
The 'right' guy can certainly weld this but there'll be some rework (filing) needed.
Instead of welding as advised (wisely if skill levels aren't there) there are indeed high strength solders available.
Magna peddled their products in this part of the word as perfect for this kind of work.
http://www.magnagroup.com/products.php?id=3&lang=en#info_30

beanflying:
Thanks for the link. Interesting NZ has a distributor but no one in Oz.

While I wait for the solder gear time to lift the covers.

The likely origional owner I guess. Seems mine is exactly one year older than me so based on this the 740A may have only been sold for 2 or 3 years. No cooling fans so very clean inside. Dont think these Caps need changing either.

beanflying:
Couldn't figure out what the strange tool clipped inside was for? Easy when you crack the covers. Most of the boards are double sided with single sided load. The oven looks like the cover may come off but as it is likely crusty foam inside and the stability is great unlikely I will lift the lid.

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