Author Topic: Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) (Read 15416 times)

beanflying · « **on:** February 07, 2018, 03:53:42 am »

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

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

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

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 · « **Reply #1 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

tautech · « **Reply #2 on:** February 07, 2018, 07:32:06 am »

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

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 · « **Reply #3 on:** February 08, 2018, 01:42:53 am »

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 · « **Reply #4 on:** February 08, 2018, 01:56:43 am »

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.

beanflying · « **Reply #5 on:** February 08, 2018, 01:15:31 pm »

Main range select switch

Tested over 4 weeks prior to shipping

Nixie Backend

Nixie is gang number 1 none of this micro controller rubbish

Resistors are a mix of HP 'matched', Draven or precision hand wound on the voltage set rotaries

beanflying · « **Reply #6 on:** February 08, 2018, 01:20:57 pm »

Nixie pre IC control then off to the plug in cards for high voltage transistor logic.

Bodge 240V cable because I don't have a vintage Belden one.

Inductors various. Sealed and oil filled of some sort but most are now weeping. Can't see that will be an issue.

Do I now need a recorder to go with the 740B

kj7e · « **Reply #7 on:** February 08, 2018, 02:56:38 pm »

You got to love classic 1960's HP constriction.

beanflying · « **Reply #8 on:** February 14, 2018, 11:50:26 pm »

Nice cool day here and the Aluminium soldering bits have arrived. Felder Flux and Bernzomatic Rods. Plenty of cheaper rods on evilbay but you wouls have no idea of what was in them.

Plenty of videos on the soldering or brazing of Aluminium on YouTube most not using flux but I highly recommend it!

Scrub clean and wire brush immediately before doing the job aluminium develops an oxide layer in minutes.

tautech · « **Reply #9 on:** February 15, 2018, 12:04:06 am »

Quote from: beanflying on February 14, 2018, 11:50:26 pm

Nice cool day here and the Aluminium soldering bits have arrived. Felder Flux and Bernzomatic Rods. Plenty of cheaper rods on evilbay but you wouls have no idea of what was in them.

Plenty of videos on the soldering or brazing of Aluminium on YouTube most not using flux but I highly recommend it!

Scrub clean and wire brush immediately before doing the job aluminium develops an oxide layer in minutes.

One of the things I learnt welding alloys was to finish prep with a file as to not leave carborundum contaminates behind that might spoil the weld. Apply same process to cast steels/irons or do the prep with a gouging rod.

Tony_G · « **Reply #10 on:** February 15, 2018, 02:49:44 am »

Looking forward to see the results - BYU is a Uni in Idaho (next state over from me) - Did you get the gear from the US or was it already in Aus?

TonyG

BradC · « **Reply #11 on:** February 15, 2018, 03:02:07 am »

I prep with a set of stainless brushes that I keep in a box (to prevent my wife using them to clean the BBQ). They are exclusively for aluminium use. Between a standard wooden handle quadruple row brush and a set of different shaped dremel brushes and a couple of small single cut carbide burrs it's mostly covered. I generally do a post prep wash with acetone to make sure it's really grease free before welding. Aluminium soldering and I haven't really got on well in the past.

I'll be super interested to see how this works out for you. Are you using an oxy or a gas/air torch?

beanflying · « **Reply #12 on:** February 15, 2018, 04:04:04 am »

Jigged and reasonably straight.

Sourced on evilbay from Utah from a

seller of electronics.

Straight butane is fine for aluminium. Flux made into a paste and scrubbed with stainless brushes.

beanflying · « **Reply #13 on:** February 15, 2018, 04:53:31 am »

Cough cough smokey from the base board charring.

Could do with a bigger torch but now it is tacked I should be able to remove it off the board.

Working better without the flux with these rods?

tautech · « **Reply #14 on:** February 15, 2018, 05:43:56 am »

Quote from: beanflying on February 15, 2018, 04:53:31 am

Could do with a bigger torch but now it is tacked I should be able to remove it off the board.

Yea it's easy to underestimate how much heat Ali sucks....but not as much as copper !

Quote

Working better without the flux with these rods?

If the work is clean and uncontaminated by oxides or oils flux usage can be minimal although the 'correct for product' types are useful as an indicator of correct temp. Good and clean welding glasses let you see the minuscule amounts wet the whole joint.

beanflying · « **Reply #15 on:** February 15, 2018, 08:44:46 am »

Still some more to do but fairly straight and it still fits where is should. Maybe wouldn't like to send it bungy jumping attached by the handle but it seems fine for lifting.

Tony_G · « **Reply #16 on:** February 15, 2018, 04:00:52 pm »

Did you get the input and output attachments with it?

Also it looks like it has a couple of photochoppers as modulators (A16 &A17) - Are they similar to the 419A construction? If so then they apparently benefit from being left on all the time (interesting discussion on the HP Yahoo Group regarding replacement neons as well). Annecdotally some choppers will come back to life if you leave them on for a few weeks (see discussions about the cadmium photocells) - Your mileage may vary on this though (my 419A never came back to life).

beanflying · « **Reply #17 on:** February 15, 2018, 11:24:51 pm »

Similar photo chopper technology I gather and a Mercury reference cell somewhere inside I haven't come across yet. Recalibration and going over it in more detail when I get back together mechanically.

As above I scored an Input connector box but I don't have the output one and another member has the reverse. The connectors seem to have only been used on the 740A and B and the input one has had the extra pins pulled I am assuming to prevent connecting to the wrong side. Most likely this will mean changing out this connector unless I find the other.

edit: Just about finished cleaning up one of the breaks very happy with it. Last time I did any of this was nearly 30 years ago on aluminium irrigation couplings with oxy/acetylene. More heat would be a help here not more temperature.

beanflying · « **Reply #18 on:** February 16, 2018, 05:18:26 am »

Sanded both side frames with 180 grit finish to take out a few scrapes from the the years and to blend the repair a little more.

Vinyl and front panel cleaning time. But first a well earned beer

kj7e · « **Reply #19 on:** February 16, 2018, 05:54:37 am »

Nice job

BradC · « **Reply #20 on:** February 16, 2018, 05:56:32 am »

Quote from: beanflying on February 15, 2018, 11:24:51 pm

Last time I did any of this was nearly 30 years ago on aluminium irrigation couplings with oxy/acetylene. More heat would be a help here not more temperature.

I've had a little oxy/propane rig for years using incredibly expensive disposable oxygen bottles that last not much longer than a fart at $45 each. I used it when I really needed it, but have an array of air/fuel torches that got loads of use. Now Bunnings do no-rental swapsies of decent sized oxygen bottles (by comparison to the 100L disposables) I don't even get the gas/air torch out anymore. More heat, easier to control and the potential for the same heat in a much smaller area.

I've also done some soldering with the TIG. The argon shield helps keep the oxides under control and because aluminium conducts heat so well you don't have to get the solder near the arc. Sometimes silver solder copper with it too.

Nice job by the way!

beanflying · « **Reply #21 on:** February 16, 2018, 06:29:35 am »

I have been looking seriously at a small Tig/Inverter again after avoiding buying one for years. I would put my combined welders bills at $1k'ish in the last 10 years so generally paying someone else has been the better option. Like all things keeping the practice up with welding is one of the keys.

When I got turned down on this one by a 'professional' business and used my $15 evilbay torch and disposable Butane cans to get a result makes you go Mmmmm about current training and the narrow range of skills of some current tradies.

I was taught by a 50+ year old at my first real job because taking the then 3 phase Tig out to the paddocks wasn't really practical and it was what was taught at Tech Schools before TIG's

OMG Starting to sound like an old man 'when I was a boy' ......

tautech · « **Reply #22 on:** February 16, 2018, 06:55:41 am »

I've found LPG torches that you can use on your BBQ bottle quite effective heat sources and cheap to run.
The 'old school' vinyl layers used to use quite large units with 'soft' flames for softening the vinyl when cove'ing under cabinetware toe kicks.
I've got one of each but the smaller one sees a lot more use. Better with a 4kg cylinder than having to hump around a 9kg one.
Something like this one:
https://www.trademe.co.nz/Browse/Listing.aspx?id=1544249384&mkwid=sEIcRBR8m_dc&pcrid=225543738224&pkw=&pmt=&plid=&gclid=Cj0KCQiA_JTUBRD4ARIsAL7_VeXBL3kc2SVVvj_tDa6tHL9VmdrwC2upoOEt5lb5ayv_cAyU0L8RPrgaAqmMEALw_wcB

Cubdriver · « **Reply #23 on:** February 16, 2018, 07:44:30 am »

You did a beautiful job fixing that broken casting! Bravo!

The mercury standard cell in the 740B should be on the right front corner of the A10 board. Here's a screen cap (sorry for the poor quality) from the quickie video I did (and never uploaded until just now) of mine when I first got it a few years ago.

The battery is visible briefly around the three minute mark in the video, in all its corroded glory:

-Pat

beanflying · « **Reply #24 on:** February 16, 2018, 08:27:43 am »

You mean like this crusty little bugga. 0.5mV aint doing much I suspect


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Author Topic: Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) (Read 15416 times)

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