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Hewlett Packard 740B DC Standard Digital Voltmeter (and 740A)
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Dave Wise:
I have detailed notes any time you want.

Having accidentally blown up the Germanium Q1 and Q2 years ago, 740B #1 is retrofitted with Silicon, plus mods* to accommodate its different characteristics.  (Currently KSA1010 driving MJ2955 but I want to try TIP42C driving BD912.  I also found some 2N6331's.  I'll drive that one with an MJ2955, similar to HP's own choice.)

* A 1nF/330R in series from collector to base, or remove A7C8; and an RCD "tracking voltage clamp" across T4 primary to tame the leakage-inductance spike.  That's 10uF, 1K, UF4003.

It's a good idea to put a heat sink on A7Q3 if main switch Q1 is Germanium.  A7Q3 has its work cut out for it discharging that big heavy base, and it's the hottest transistor in the instrument.  (With Silicon Q1, A7Q3 barely conducts at all.)
Make sure A7Q2 is working; if it goes open, A7Q3/R7/R9 will fry.

In late production, HP added a freewheel diode across Q1 to prevent transformer ringing from driving the collector positive.  They seem to have used a garden variety rectifier but I discovered that a fast-recovery part like the MUR115 or UF4003 is way better.

740B #1 also has a DIY HV transformer wound on ferrite instead of iron because the original overheated when operated full-throttle overnight.  Its relatively low saturation ceiling forced me to develop a simple mod to the A7 board current limiter which is compatible with the original parts.

I refitted both 740B #1 and #2 with H11F-based choppers, and I recently realized I had matched the modulator parts for the wrong characteristic.  They need to be matched for temperature coefficient.  If this results in a large net offset, it's easy to trim out, by simply inserting a resistor at one or the other end of the front panel ZERO control.  I matched the parts in a toaster oven, 20C-60C, using a Keithley 147 Nanovolt Null Detector to measure the offset.  Out of nine same-lot H11F2's, I found two good matches with less than 2uV drift over 20C-60C.  The mod-demod efficiency increases and it's desirable to reduce AC amplifier gain to avoid overload on the millivolt ranges.  Adjust A3R19 not A3R9, I don't know what HP were thinking there.

While I was in there, I reduced A4 noise by replacing transistors.  I used KSC900.
Beware of the 2N3391 data sheets.  Central Semi, National Semi, Fairchild, and New Jersey Semi call the 2N3391 "general-purpose", but GE (HP's vendor for 1854-0033) is "Ultra High Beta, Low Noise".

The original neon-and-photocell input protection circuit is trouble - the HV cell HP used to disconnect the input generates 10-100uV of offset.  (This is how I discovered the oven was oscillating - offset ramped up and down as oven load affected the light level on the photocell.)  Even if it didn't, the circuit is not usable with H11F's with their 30V absolute max.  I engineered a new circuit based on the Supertex LND150 high-voltage depletion-mode MOSFET which is optimized for current limiting, and a TVS.  There's also an SSR to keep input current out of the KVD in STD mode.  This circuit works better than the original did when it was new.

Years ago I discovered that 740B #1's oven had run away and cooked the PCB inside so I rebuilt it with the two separately-floating circuits on separate boards to avoid leakage.  It's important to keep the reference zener thermally connected to the inner can.  The thermistor was cooked too so I replaced it with a modern part, discovering in the process that control stability can be improved by attaching the thermistor to oven wires instead of the can, to introduce some phase lead.  I still had to reduce control loop gain to keep it from oscillating.  I discovered that 740B #2's oven was oscillating too and reduced its loop gain until it stopped.  The frequency is in the milliHertz; I graphed it with a storage scope while perturbing it with an HP 203A oscillator.

Speaking of leakage, I discovered that T2, the transformer powering the in-guard system, has a lot of leakage between the +34V and -12V secondaries, which messes up low-level readings.  A simple mod reduces the voltage between them and hence the leakage.

When I was doing hi-pot testing, 500V between MINUS and GUARD in VM mode leaked into A2C17 and punctured it.  This proves HP never did that particular test.  Symptom is no reading in DVM mode.
I added a TVS across the new cap.  SMAJ30CA since I was already using that to protect the H11F's.  It can be almost anything, normally the cap has less than a volt across it.

(I've said this one before, I'm repeating it here to put it all in one place.)  Battery leakage messes up circuit operation and must be cleaned.  The battery on A10 can be anything you like.  I used a lithium coin cell with tabs.  It will last for its shelf life.  If you use some kind of power supply instead, it must float 1000V with no leakage.  The battery on A11 should be relocated onto the GAIN CHECK switch.  It can't be 3V lithium as that amount of offset will overload the chopper amp.  I used a silver oxide button with tabs.  It will last for its shelf life.  If it's not 1.35V you need to adjust the GAIN CHECK bogie value.  If you do away with the photocell choppers, GAIN CHECK becomes a pointless function anyhow since the H11F's will last forever.

PROCEDURAL NOTE: When doing INTERNAL CAL, wait several minutes after you switch between 1-9 and 10-12 to allow dielectric absorption in the switches to relax.
Cubdriver:
Dave, thanks for the very thorough writeup!  I will definitely be going through that again when I eventually get the beast onto the bench.

Much appreciated!

-Pat
Dave Wise:
Don't rush into removing all the chopper photocells even if your gain is below spec.  Try modernizing just the demodulator side first and see if that gives you enough boost in efficiency.  I think the main challenge will be syncing the demodulator drive to the modulator, unless you punt and drive both from a new circuit.  (BTW I have PIC source code for that.)  Don't try for a final design, just breadboard something to see if it's enough.  (I'm speaking from the hassle I went through matching pairs of H11F.)   If you do essay to install H11F's, the main amp has to be H11F1 or H11F2 for the 30V breakdown.  (The main amp - even when gain = 1 - has to generate 30V to get the expected slew rate out of A4.)  The meter amp can use any H11F including the 15V H11F3.
Be careful with the mod-demod phasing - Figure 3 in the manual is wrong.  On the demod section of A17, V4 is actually lit by DS1 and V3 by DS2.  The chopper amp overall (2 wire to 926 wire) should invert.

Do be suspicious of A9V2.  Mine was generating on the order of 100uV photovoltaic offset.  I've replaced it with an SSR; Ixys CPC1981Y works fine and leakage is negligible.  Standex SMP-1A40 would also be great.  You can wire in the LED in place of A9DS2 if you add series resistance.  I'm sure HP would have used one of these SSRs if they existed in 1965.
Dave Wise:
I'd like to add one thing.  The H11F1 characteristic that needs to be matched is not offset - by manipulating resistors A10R6/R7/R13, you can null out at least 1000uV.  The important thing is drift.  You want a pair of H11F1's that drift in the same direction, the same amount.  This is even more tedious than offset matching, but I was able to get several satisfactory pairs out of about 20 candidates using my toaster oven as a cheap environmental chamber.
Dave Wise:
NOTE: Everything in this thread is applicable to the 740A, 741A, and 741B, as well as the 740B.  It may be useful for the 419A Null Detector as long as power consumption stays low.

NOTE: With respect to chopper phasing, the 740B schematics are a confused
muddle, including my well-intentioned annotations.
For now, my definitions: Both Main and Meter.  Physically, cells are
numbered top looking down V1 thru V4 counterclockwise.  V1 is mod series,
V2 mod shunt, V3 demod shunt, V4 demod series.  V2/V3 lit by neon DS2 or
LEDs DS2/DS3, V1/V4 lit by neon DS1 or LEDs DS1/DS4.  DS3 and DS4 are
internal to the H11F1 demodulator switches.

I have new findings.  I am grading my photocells by scoping them against a current source, adjusting illumination so R-lit is 20K, and recording the time it takes the cell to recover to 100K.

Recovery is a straight line at normal illumination, which means that T10 (HP's figure of merit, see AN-919) is meaningless unless you also adjust illumination so all cells have the same R-lit.

Of my sixteen cells (two 740B's), eight look very promising.  I think I can use photocell modulators along with H11F1 demodulators.  The latter's near-ideal behavior lets us do a very important trick.

FINDINGS

1. Asymmetric Demodulation Timing
We must turn on series switch V4 at V1 on and keep it on throughout V2, to route the entire half-cycle waveform to the A4 integrator.  This boosts efficiency by around 30%.
Use a microcontroller.
Four states.  V1+V4, V4, V2+V3, none.

2. Customized Modulator Drive
We MUST tune the light intensity on each cell, which rules out neon.  I suggest poking LEDs down the empty V3/V4 wells.
You should insulate and guard.
With custom drive currents, modulator pulse width is not critical, just set 50% dark ratio (beta) - on both channels - and forget it.

Doing this gives me another big boost in efficiency.  My results are comparable to my estimate of original new-instrument performance.  Don't go overboard, remember HP put lower limits on V1-V2 average series resistance.  (160K on A17 - main, 350K on A16 - meter.)

3. Asymmetric Modulation Timing (partially true)
Bright short pulse V2, dim wide pulse V1.
E.g. A17#1V1 treated as V2.  100uA 5ms is about 10K valley and
it hits 100K about 1.8ms after turnoff.  300uA 500us is also
10K valley, but it hits 100K about 1.6ms after turnoff.
dR/dt is almost constant for a given Rlit, slowing down as the cell is driven harder.
So if you maintain a constant Rlit, you will have essentially constant dR/dt
regardless of pulse width.
On the other hand, a shorter pulse makes room for more dead time,
so it is still useful: brighten V2 and shorten V2/V3, stopping short of
degrading cap charge, and lengthen V1/V4, stopping short
of degrading Rin.

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