of manual HP · Calibration procedure for HP Model A. Catalog pages, covering HP A. Hp A Pdf User Manuals. View online or download Hp A Operating And Service Manual. View and Download HP A operating and service manual online. DIGITAL VOLTMETER. A Measuring Instruments pdf manual download.

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Difficulty manuql this repair was in inability to run self-diagnostics and lack of reference working meter to trace issues. After repair there are some extra modifications described and tested, to gain performance improvement on this multimeter.

Redistribution and use of this 3456aa or any images or files referenced in it, in source and binary forms, with or without modification, are permitted provided that the following conditions are met:. All information posted here is hosted just for education purposes and provided AS IS. In no event shall the author, xDevs. Used multimeter and instruments manufactured by HP are well recognized and highly respected among hobbyists and engineers, who are maintaining and taking care of old, but still mmanual functional instruments.

First two models ,HP A and HP A sometimes can be found for very low prices on secondary markets, providing bargain entry into high-resolution gear for beginners and students. But often such low-priced units have failures and issues, and likely require repairs and recalibration.

Detail architecture of instrument can be studied from block diagram, represented in operation manual. Later have own divider and amplifier and have separate measurement path for ADC.

DC voltage circuitry also handles direct measurement or ratio measurement switching, amplification of low voltage ranges, such as 1 VDC and 0,1 VDC to 10 and times accordingly, as well as using There is also optional filtering circuits, h can be disabled.

Reference voltage around 6. This is interesting implementation, as modern instruments usually use optocouplers for similar isolation task. Main outguard processor is also handing keyboard processing, display operation and math functions for received data, as well as provide digital interface to computer via GPIB. Power supply PSU is providing power for whole unit, with special care taken for separation between outguard and inguard domains, as these parts taking power from separate, isolated windings in transformer.

Analog circuitry domain implements various precision resistors and hybrids, which help to maintain high performance and stability of critical circuits. Just as honest description from seller, unit was missing one of outer covers and inguard internal shielding covers, and showing garbage on display and front panel LED s:. After arrival unit was switched for proper mains voltage VAC hereprior to connecting power. First attempt to boot meter revealed random front panel operation, without any system or pattern.

Most of keypresses do not change any behavior, and some cause random indication changes. Better quality copy, part 1 and part 2. High-resolution copy of manual HP Calibration procedure for HP Model A. Catalog pages, covering HP A.

HP Model A service notes A. HP Model A service notes B. Binary file32 Kbytes. Visual inspection did not show any serious problems, so we started diagnostics and repair from power supply. Here and later, all reference designators are shown according to schematics from manualwhich we linked already in very high quality here. Usually not bad idea to install higher capacitance and voltage ones, if area 345a size permits.


After this simple operation all power voltages become normal, but this did not solve issues with our HP A. There is more to fix. By having this message on power, initial conclusion that processor, address and data bus, as well as RAM and ROM chips are working properly.

Also display interface logic was counted as manusl as well. Front panel was disassembled, but no visible damages or faulty parts were found. This in turn brings decoder logic into question, as keyboard logic should be scanned 4356a to check if any keys pressed.

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U17 and U19 on A4 were manuao, but meter was still doing same thing. At this point, main diagnosis was reconsidered, as there were online threads, such as [3] and [4]mentioning ROM chips failures. Not even UV 27xx series, disregarding idea of getting into UV rig 3456z buying extra parts. Since it was a low-cost restoration project, some salvaged parts suitable up to task were found. Since new ROM have 32 manua, instead 24 on original circuitry, direct replacement would not work. Also due different pinout definition, some pins from socket were isolated from board, so we can solder wire and reroute signals to proper locations.

IC socket was soldered on PCB in a way, so pin 16 fits pin 12 of original ROMso we get data bus and some address signals match original layout. MX29F was fully erased in programmer, as well as disabled boot-block protection. Sure, most of our readers can adopt this for their specific ROM programming hardware.

Latest Kb was used, downloaded from rfscientific. They also have some other firmware dumps. Somewhat a backup feature for future-proof operation. You can also spot second IC socket on photo. This is due simple reason, as my MX29F had missing data pin, so had to fix with extra bit. Moment of truth… Powering on meter, same problem and chaotic operation.

There is last thing left before actual processor — RAM chips. Again search for obsolete DIP -package chips, without any progress. It had narrow package, so pin socket fixed up from two pin IC sockets was installed into intermediate adapter. Ground to ground, data bus signals to data bus. Pin 21 — wired to address signal A10 taken from U12 pin In original schematics, signal A10 is used to switch which RAM chip is being accessed, and for reliable operation delay line, consisting of logic in elements U12d, U12c, U9f, U9e was used.

Now, after RAM replacement, instrument finally came back to life.

| HP A digital voltmeter restoration

Self-test discovered inguard-controller failure. In my case processor i was installed in socket. Chip temperature was OK, so I just removed chip and installed back in. Immediately instrument went to normal operation and show few counts on display. So I went ahead and replaced old socket with modern high-quality collet-type socket. Right after repair and initial checks, instrument was modified — installed temperature sensor, improved thermal and air insulation around voltage reference zener, and later whole reference module was replaced with DIY triple LMbased module.

Results in the end of this article were achieved after all these improvements completed and installed. Additional temperature sensor LM was installed to allow monitoring internal instruments temperature. This was performed by wired connection, protected with heatshrink tube. Ground return was connected to chassis ground, so this 3465a not introduce noise to analog circuitry.


3456A 6 1/2 Digit Digital Multimeter

Current limit resistor was connected in series and heatshrinked as well. 3456z -port with temperature output was installed on rear instruments panel, using airflow grill.

One of grill sections was cut out with pliers. Avoid any drilling or milling, this easily can cause little metal bits and pieces stuck somewhere on PCB s and cause problems later.

Thermal sensor wiring was routed using existing holes and areas, as you can see from provided photos. Temperature measurement can be performed on both inside meter left photo and outside ambient right photo:. Decided to try some of these notes to implement on this particular instrument restoration project. Since all thermal control and regulation is done internally on-chip, there is only one way to do such — reduce thermal dissipation into surrounding.

There are two main paths for thermal dissipation: This approach will allow reducing thermal coupling between zener diode and circuit boards around, helping to reduce thermal power required to maintain temperatures. To further isolate reference there were foam caps installed on top and bottom surfaces of reference board, like was shown on photographs.

Application of second note from Mr. Dobkin, using multiple voltage reference zeners, was possible to perform just now, in progress of writing this article. Schematics of triple zener reference is shown below:. This circuit allow reducing thermal drift and output voltage noise by square root of three.

Three LM they were deliberately bought used, with old date of manufacture, to avoid initial aging effects were grouped on one side of PCBwhile resistors and operational amplifier LT were populated on opposite side. PCB cutouts were milled around LM zeners, and top and bottom sides were further isolated by foam caps, similar to previous approach with original voltage reference module. Missing outer cover was purchased on eBay, and inner shields were DIY manufactured.

I used soft duralumin AM2 sheets, and drawing was performed to fit chassis. Metal cover thickness is around 1. Finished covers after few weeks of initial testing were darken using ferric chloride. This was targeted to reduce metal emission and improve thermal dissipation. Results are available for comparison: I could not find this specific clock crystal, but doubled and quadruple quartz oscillators were available in local Quartz shop.

I got one with similar case, such as this so original socket could be used. U12 pin 4 on A30 board was carefully desoldered and bent over PCBso this output clock signal can enter clock division circuit. Divider output is routed back to free pin pad on A30 board. Final task of fixing meter — adjustment, or calibration, as it is usually referenced today. Calibration sources and standards required are much less than amount of available ranges. For DC voltage sources of stable 0.