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Curiosities: Inside a 1970s professional multimeter

2018-08-12 17:32:44,  In: Electronics, Curiosities

Mera V640 is a multimeter made by Polish Meratronik plant in 1970s. It is an analog meter, able to measure voltages (1.5mV - 1500V), currents (0.15uA - 1.5A) and resistance. With specialized probes it was possible to measure high frequencies and even temperature. In 1970s Poland started to make international deals, usually with licenses. Some licenses were bought, like from British ICL for their computer designs, German (western) Grundig (tape players), or Japanese Busicom for calculators, but Poland also manufactured their equipment for export. In 1972 the Masteranger 639 multimeter has been made for Conway, a Canadian electronics company, and manufactured purely for export. The design was successful enough that it was also manufactured for British Marconi company as TF 2650. Year later it was also sold in Poland, under V640 name, after some changes in range switch and adding temperature probe.
These times, export and import with eastern block country was very different than typical transaction, generally Conway paid with foreign obligations which could be used by Polish factories to import foreign components or buy licenses for production.

The meter could be powered with 12 AA cells and one cell for ohmmeter. It was also possible to remove battery cage and install a power supply instead (with power cable breaking after first month of use :) ). What more important, these power supply units have no ohmmeter power - the single AA cell still has to be inside.

The power supply unit is a simple, transformer based one with transistor regulation

To get inside, we have to unscrew the rubber feet. Then it is possible to remove the back cover. It's important not to scracth the internal surface of the case as it has graphite layer for interference shielding purposes.

The biggest area is occupied by electromagnetic mechanism and below we can see a metal shielding. Under this shielding there is a stack of printed circuit boards with range switch.

On both sides we have printed circuit boards, one with range circuits, the second with power and battery test switches, also contains the amplifier.

The amplifier, in form of HLY7006 hybrid circuit, is a Polish hybrid chip designed especially for this meter. In first units, the amplifier was made using discrete components, but this technology, from its nature, has lower repetitivity of parameters than any integrated circuit. So Meratronik quickly switched to integrated technology, with circuit made first in PIE (Industrial Electornics Institute), ZDPUH (Experimental/Manufacturing Plant of Hybrid Devices), PIT (Industrial Telecommunications Institute?) and later in Dolam plant.
Why hybrid circuit instead of purely integrated circuit? In 1970s production of integrated silicon chips in Poland was in its warming-up phase. There were some gates manufactured, there were some structured packaged, but it was too early for a good and accessible amplifier. By its own inertia, these meters, with these hybrid chips, were produced until late 1980s!
If such circuit breaks, a whole meter breaks. There are some replacements, but they are usually own circuits located in a small PCB in place of the original block.

There are a few HLY7006 variants known. First ones were more or less rectangular, with different color variants between semi-transparent white and green (and sometimes the bottom part was green and top was white), rarely with brown parts. The last ones, made by Dolam plant, were made much cheaper way - pins were not gold-plated and not cut from sheet like in ICs, and even plastic polymer was subject to savings - only the main part was rectangular, with characteristic "chimney" - a cylindrical protrusion, in earlier units covered entirely with flat plastic part, containing input dual FET.
Dolam was a factory producing vacuum tubes. Although they experimented with LCD technology even making some parts for measurement instruments, their typical product after era of vacuum tubes were reed switches and relays. Probably expansion of factory in 1970 was motivated by idea that tubes will still be produced, and Dolam needs more plant area for producing microwave stuff. After expansion of transistor tecnhology, Dolam has been left with their relays and microwave technology, so they did many different things, starting from high-voltage switches, ending with assembling radio receivers. Manufacturing hybrid circuits, especially at a dawn of monolithic ICs technology, did the trick and saved them so well that in 1980s they started to manufacture LCDs on Japanese license. Because simultaneously there were even some experiments with first Polish plasma displays, if they could succeed, they would be produced in Dolam plants using vacuum technology they already have - but that never happened.

Some essential links (most in Polish):
Manual and schematic
Description and photos of HLY7006, as well as X-ray of HLY
Extensive history article about V640
Addition to this article, focused on HLY-7006, contains photos of different versions
Calibration of V640, well explained

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