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"GREAT" Meter! The HP 419A DC Null Voltmeter, [RESTORATION]

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You asked me to take a look at the power. I didn't because the neon lights were on and their waveforms were perfect. Instead, I started disassembling the device to get at the additives he identified as problematic. During my disassembly, I checked for continuity and on tutorials of to make sure I hadn't severed the connections. To my horror, I noticed that the now converted impartial battery percentage was no longer attached to the toolbox. Have you gotten away with this mistake correctly for years? It does not matter. The impartial attachment of the battery to the case stopped the vibrations.

I found the comment about Paul Carlson “not understanding” neons, earlier in the thread, amusing and ridiculous in its assumption. Just because someone specialises in a certain electronic technology type, how on earth a stranger is able to ascertain that this is ALL they know about, is daft. If I’ve ever seen an expert of experts in electronics, it’s Mr Carlson.

Assumption is the mother of all %^%#%# as we know.

Dave Wise:
I have two 740B's.  Some years ago I built chopper modules using H11F1 photoFETs, but matching temperature coefficients is really tedious, so I decided to try a hybrid approach, with photocell modulator and photoFET demodulator.

I made a board which fits on the chopper module.  It should work on the 419 and 3420 as well as the 740 and 741.

PLEASE POST PICTURES OF THE 419A CHOPPER MODULE, showing the demod-side terminals.  Thanks.  I may need to modify the board layout.  Or just tell me.  True or false: the demod side has terminals in all four corners.

I will post a writeup soon, at topic "Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)".  It retrofits LEDs in place of neon, with several axes of adjustment for timing and light level so you can get the most out of your particular photocells.  It also retrofits photoFETs in place of the demodulator photocells.  Removing two photocells lets you pick the best parts for the modulator.

My 740B's yielded twelve good cells out of eighteen total.  The remaining six relax to high resistance too slowly to give adequate efficiency at the chopping rate.  This degradation in loop gain can cause slow response on sensitive ranges.  In some cases, large input signal makes the output drop to zero ("foldback") or even reverse.

I hope it's useful.

Dave Wise

(I find Carlson unwatchable; he trumpets falsehoods with narcissistic confidence.)

Dave Wise:
All materials posted at topic, "Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)".


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