Digital Test
Hewlett-Packard 3440A / 3445A plug-in

Manufacturer: Hewlett-Packard
Model: 3440A / 3445A plug-in
Year: 1965
Schematic: RE
Functions: DCV, ACV
Display: NIXIE
Digits: 4
Logic: Ge-PNP

An early digital voltmeter utilising a conversion technique no longer popular. A ramp is generated spanning a positive and negative voltage range. A gate controlling the clock input to the counter string is tripped at the input level on the ramp and at zero. Which trip comes last determines the polarity indication.

The shortcoming of this technique is that the accuracy of the displayed result is dependant upon both the slope of the ramp and the frequency of the oscillator feeding the counter. They must be mutually stable to maintain accuracy, however they are implemented as independant electronic units. The test switch and adjustment on the front panel provide for tweaking the slope of the ramp to bring it into consistency with the count frequency.

Early use of FETs around the input stage.

The 3445A plug-in provides a fairly quick auto-ranging capability.


- Unit Log -

Serial Number: 439-02100
Year of Manufacture: 1965 (Counter boards stamped with "Tested Mar 65", component date stamps of 6415 to 6510).
Date of Receipt: 2005
State upon Receipt: Semi-functional. A little flaky and inconsistent between ranges. Grubby, plastic coatings on panels are sticky. Very minor corrosion on some exterior surfaces from some deposit. Foam on PCB bracket decayed. A couple of transistors and a variable resistor had been previously replaced.

Date: 2006 Apr 21
Procedure: Cleaned, reassembled. Aluminum side frames swapped in reassembly for esthetic improvement. Disintegrated plastic cable clip on 3445A input shield cable replaced.

Date: 2006 Apr 22
Procedure: COMP. BALANCE adjusted for zeroing. Generally functional. Ranges do not want to adjust to be consistent with each other. Occasional flakiness on AC display on 100V range. Occasionally, display will stick at some reading.

Date: 2006 Apr 23
Procedure: Signal ground noticed to be connected to chassis ground (should be isolated). Traced to metal segment of AC shielding cage in 3445A plug-in contacting chassis. Design/manufacturing flaw. Segment adjusted to avoid contact.

Date: 2006 Apr 23
Procedure: Signal wire in AC shielding cage of 3445A observed to not be in contact with solder junction. Manufacturing flaw, wire was too short to go through junction and ended up being butt-soldered. Wire replaced and resoldered.

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Early Digital Test Equipment