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.
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