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

[Cubdriver]

That would be the critter.  I didn't bother to try to measure mine, though half a millivolt or less wouldn't surprise me...

-Pat

[beanflying]

I think the sensible solution is to abandon a battery all together based on the replacement options. A small Daughter board in that space with an additional cal pot hole drilled in the lid and a modern reference fitted with appropriate dividers if needed. Temp Co shouldn't be a major issue as that bay will be heated by the reference and it is at least 30 minutes to get near being stable. So give the new reference an hour or two before tweaking the cal pot and it will be nice and toasty too and well above any ambient variations on temperature. 

[Cubdriver]

I think the sensible solution is to abandon a battery all together based on the replacement options. A small Daughter board in that space with an additional cal pot hole drilled in the lid and a modern reference fitted with appropriate dividers if needed. Temp Co shouldn't be a major issue as that bay will be heated by the reference and it is at least 30 minutes to get near being stable. So give the new reference an hour or two before tweaking the cal pot and it will be nice and toasty too and well above any ambient variations on temperature. 

I was thinking along those lines too, and did some searching back then and seem to recall finding a couple of methods people had used.  Think I was initially looking for mercury battery substitutes - they're used in old school light meters too, so there's a market for such a thing. I recently got a pair of Chinese voltage reference boards (must have seen them here), adapting one of those into the circuit may be a good option.  The instrument has been sitting on its side in the hall for must be three years now waiting for me to work on it...

-Pat

[beanflying]

Rough idea but a 4.096V reference is easy to get a stable one that will drive a few mA without a buffer. Drop the output into 1/3's with the best precision resistors (4-5k total but not important) you can sensibly afford gets you 1.36V with no trimming. Any Pot you add to tweak the last bit can be any sort as it's influence on Tempco will be tiny.

Most of these References will need 10-15V so finding a 12V rail should be simple.

[beanflying]

One of the other things I was going to chase up was the old Belden/HP lead. Evilbay pricing shipped to Australia would be $50+ Based on another recent acquisition a 735A from the same era having a factory fitted IEC plug I think a change is sensible.

Maxim reference IC ordered with the few supporting bits for the 1.35V replacement.

[tautech]

One of the other things I was going to chase up was the old Belden/HP lead. Evilbay pricing shipped to Australia would be $50+ Based on another recent acquisition a 735A from the same era having a factory fitted IEC plug I think a change is sensible.

Always.......when it can be done neatly !
Sometimes an IEC socket with an inbuilt fuse or switch can be better to fully hide the OEM chassis punching.

[beanflying]

The switch/fuse ones are good for a cover up  I made a template a while ago for my label printer helps keep it neat and also reduces scratches from doing the job. Catch bucket under it keeps the mess contained. Normally the nibbler it easy but the rear lip meant mainly filing to size and the dremel is to messy.

[Cubdriver]

Nice job fitting that C14 connector!!  Those PH-163 power cords are enough of a pain to find here in the US; i don't even want to think about trying to get them where you are.

-Pat

[beanflying]

Part of that reason for none locally is this one I removed is rated at 125V 15A  more like 5-8A for safety so perhaps for gear being shipped to the sensible part of the mains voltage world  may have been fitted differently?

[SeanB]

The caps you will be replacing are the wet tantalum types with rubber seals, as they will be very dry by now. Sad, as otherwise wet tantalum is a very good cap, just not the lower cost versions with a rubber bung seal. Good thing is that in most cases you can replace with a regular modern Nichicon or panasonic ultra low ESR capacitor of the same value and same voltage, and in almost all cases this will work.

[beanflying]

Thanks, board by board pull and sort out coming up over the next few months so caps etc.

The Battery replacement is next on the list now the physical repairs are sorted that way I can hack a temporary output test box and run through the cal cycle as I go.

[beanflying]

Just a few photos of Input cable unobtainium. If anyone has a couple of plugs in their junk bins Pat and I would love to hear from you 

Deutsch MDR07-7P-090 is the number. HP for safety given the 1kV DC potential has pulled some of the pins on the input side.

[beanflying]

Mercury cell replacement lashed up on the bread board tonight. 1.35xxx from 10-12v input. Resistors are low tempco Vishay's. Maxim 6341 csa for the 4.096 reference.

Will pop it on a board in the next day or two and add it to the 740b providing I am not struck down for adding an IC to a classic HP 

[beanflying]

Good enough for Australia 

Just need to figure out where to mount the Mercury Battery replacement now. Temp is fairly stable here today and it hasn't moved a digit in over an hour.

edit yes I have now soldered the cap leg 

Running well on LiPo power at this stage. I need to sort out either a floating supply for the reference or figure out if the resistor divider network can be changed to allow a voltage compared to a circuit ground ..... The circuit diagram is a little interesting 

[beanflying]

Must remember RTFM more carefully 

Seems there is actually two Mercury reference cells in the 740B.  I did order two references so another to be made and crammed in here. Underneath the unit on board A11and under a guard cover hidden by wire and another board  is the loop gain check circuit 

Even with desoldering a coax that goes through the aluminium this is as far as the board moves. 

[Cubdriver]

Oh, snap!  Good find!  I'm watching this closely as mine will need a similar treatment at some point.

-Pat

[beanflying]

Oh, snap!  Good find!  I'm watching this closely as mine will need a similar treatment at some point.

-Pat

Got it out, pre wired the reference wires in place and put it back together again. I need to print out the circuit diagrams and try and figure out a voltage reference point. The simple option will be drop a couple of isolated 12V supplies underneath near the main inputs. Plenty of room but adding more stuff isn't ideal.

When you get a chance could you crack the top on your output box and take some similar snaps to my input one? Based on the snowballs chance in hell of either of us getting the ones we need I now have the Tellurium Posts to make a counterfeit box. I would prefer to make it fairly close to the real deal if possible.

[Cubdriver]

When you get a chance could you crack the top on your output box and take some similar snaps to my input one? Based on the snowballs chance in hell of either of us getting the ones we need I now have the Tellurium Posts to make a counterfeit box. I would prefer to make it fairly close to the real deal if possible.

Will do.  I need to figure out where I stashed it first, though...

-Pat

[factory]

When you get a chance could you crack the top on your output box and take some similar snaps to my input one? Based on the snowballs chance in hell of either of us getting the ones we need I now have the Tellurium Posts to make a counterfeit box. I would prefer to make it fairly close to the real deal if possible.

Here are pictures of the Output box & connector (part no. MDR07-7PB-090), note the polarization slot in the connector is in a different place to the Input one.

David

[beanflying]

Getting back to this after leaving it on the rack for a while. Ran across some gorgeous Renderings of the 740B and some other HP gear.  More HP renders here  https://www.flickr.com/photos/144543740@N05/43372501065/in/photostream/

I think I am making an output box but may casually keep looking for the real deal.

[beanflying]

Just a first pass Model of the Output Box (I will also tweak it so a matching input box can be made). The Plan is to 3D Print it after finalising dimensions of components to be used in making a clone box for the unobtanium.

The base will be a drop in fit for the genuine box and I have modelled in the slight taper on the side wall that would have been to clear the part from the injection mold. Generally it is size for size of the real thing.

[Dave Wise]

Good job so far.  I should send you my notes, I have fixed several problems, some of them basic to the design.

The batteries are not critical, you can adapt for various voltages.  Your precision reference module is overkill but since it's in place, fine.
A11BT1 will crap all over the board when it leaks.  Really it has no business being there.  The effluent will contaminate the board and allow conduction between the main output and the gain check circuit.  I used a standard button cell with tabs spot-welded, and relocated it onto S10 GAIN CHECK where it should have been all along.  If the new cell is different from mercury 1.35V, just calculate the correct meter displacements for minimum and maximum.  This battery is never used in normal operation, so shelf life is the most important characteristic.

When you press GAIN CHECK, the battery goes in series with the signal, which perturbs the output to a degree depending on the amplifier open-loop gain.  This is used to monitor the condition of the modulator and demodulator photocells in A17.  I replaced mine with H11F1 photofet optoisolators, so my gain will be stable for generations to come and I only needed to check it while troubleshooting the new assembly, which has much higher efficiency.  I adjusted a resistor value on A3 (not the one mentioned in the manual, that's bogus) to bring the gain back down to normal.

A10BT1 powers the ZERO ADJUST control, and its voltage is not critical although it's nice if it is stable over temperature.  I used a 3V lithium button cell and added a series resistor to drop back to normal voltage to maintain the same adjustment range as before.  Current drain is a constant 200 nanoamps which means the battery will last for its entire shelf life.

One of my two instruments lacks cables, and I have given up on finding original connectors.  I will construct cables, and for connectors use whatever I can find that meets spec for insulation resistance and voltage withstand, or maybe just hardwire them.  Since I have triax cable but not the custom-made stuff HP used, I will run one cable for input, arranged +, -, guard from inside to outside, and run a separate wire for chassis.  For output, I will use two triax.  Number one will be + out, - out, guard, and number two will be + sense, - sense, guard, with again a separate wire for chassis.  Have to make do with what I can get.

Dave Wise

[beanflying]

Thanks for the Info.

Progress has stopped at present along with the supply of Output/Input Boxes everywhere  So when I have a test print and design I am happy with I will put the STL files here so anyone can either print them or have them printed for theirs.

Hoping to clear a dedicated space so I can leave a few bits of test gear refurb and repair out without blocking the day to day in a few months. In chicken and the egg style I first have to make new trusses and replace a second Garage Roof before that happens 

[Dave Wise]

I built my boxes and they worked out great.  I used a generic type of hobby box from Amazon, high-voltage triax cable with spiral wrap to hold it together, and Amphenol CPC Series 2 connectors.  Shell size 11 just fits, with flange behind the panel and
secured with countersunk flat-head 6-32 screws.  To keep the WARNING HV neon lamp bright at moderate voltage, I put it in series with half resistor, half current limiter made of an LND150 depletion-mode MOSFET.

I used cheap binding posts as proof of concept.  If warranted, I'll replace them with good ones later.

I'm pleased with the way the cables and connectors work.  They're not push-on pull-off like the old Deutsch ones, but they only need a quarter-turn to mate and unmate.  I used some of the input-jack contacts to guard the input-plus line.  I have verified that I'm still at least 10 gigohms at 1V and up.

The two plugs mate either jack, but I haven't got them backwards yet, partly because the cables are clearly different.  (Output is two triax while input is one.)  Anyway I wired them so it's mostly harmless.  The only thing I couldn't fix is if you put the output box on the input jack, the plus-sense terminal goes live with plus-out voltage even if the output jack is turned off.  It's a side effect of the guarding.  I could paint the input and output with complementary colors if I was more worried.

[beanflying]

Sounds good Dave but



Other than that can you link or provide a bit more detail on the Triax bits. I have all but given up waiting for the second unobtanium box for mine.