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| Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) |
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| 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 :palm: |
| 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. :) |
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