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| Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A) |
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| Dave Wise:
Board v1.0 has some trouble. It's fine in the spacious and quiet A17 position, but in A16 you have to bend the connector pins 45 degrees to avoid nearby board A11. New v1.2 layout ordered. This one has connectors up high to avoid mechanical interference. It's also EMI-hardened because A11 emits high-voltage spikes. |
| Dave Wise:
Pictures of v1.0 board and how it's installed in the chopper module. I'm using a scrap of 0.01-inch clear PET film as the insulator. It was the packaging for some random trinket at the store. My Fluke 895A reads > 2e12 ohms at 1000V. If Henkel doesn't warn me off it, I will secure the film to the chopper housing with Loctite "Stik-N-Seal" Outdoor Adhesive, then secure the board to the film with dabs of the same filling the voids around the PTFE turret insulators. |
| Dave Wise:
Henkel says, they don't test Stik-N-Seal for electrical, therefore, don't use it for electrical. I tested it and it's fine. It's a synthetic rubber in a volatile solvent. With dielectric absorption settled and about 50% relative humidity, I read about 1e14 ohms at 1000V sustained for an hour. Understand, I'm using it because I have it, and it's cheap and abundant. Normally, Guard and In-minus are at the same potential, but the spec allows them to be 500V apart and I don't want to be the weak link. A scope probe bypassed by 10K in the A16 space picks up about 200mV peak-to-peak from A11 (home of HV rectifier A11CR1) when the instrument is idling. This will not interfere with calibration. (I see about 1V at 1000V out.) Anyway, you can calibrate the module outside the instrument - unless your photocells are marginal (in which case why are you doing this?), just select and trim the frequency and leave the pulse timing at default. I made neon-less mounting plates out of polycarbonate sheet. You could also cut up a CD/DVD/BD. I'll include a dimensioned sketch in the writeup. Since 740B#2 contains my input protection re-design, I've replaced A9DS1 with 30V TVS diodes across A17V1 and V2. This will protect them from surges in case the operator accidentally switches range or mode with high voltage present. As soon as I've verified all the program's calibration functions, I'll build a V1.2 board, set its LED drive for A16's photocells, pull out the KVD module, and install A16 and A17. The attached picture shows the A17 powered on the bench. V2 is brighter because that photocell is blind. It doesn't show in the picture but I had to replace the old capacitor because (a) it's too bulky and (b) one lead is too short. I threw together a cal harness, it reuses the 5-pin ICSP jack. |
| Dave Wise:
I have installed my hybrid modems in 740B number two. It's looking good. They easily exceed efficiency and resistance requirements. There's less drift, and less input current. (Compared to previous quad-photofet modems.) After I've recorded some observations, I'll modify 740B number one. I'm continuing to test as time and energy permits, and I'm working on a writeup. |
| Dave Wise:
Here it is. For 740, 741, 3420, and 419, with possibility of repurpose elsewhere. Writeup, MPASM source code and hex, and KiCad schematic and board layout. Please look it over and comment. |
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