Eltra calculators

Eltra was radio factory in Bydgoszcz, Poland. established in 1920s, nationalized in 1945, they made first Polish consumer-grade transistor radio mini-receiver in 1959. Since 1970s till 1980s they were manufacturing calculators. They started with calculators based on VFD Japanese tubes (K-741 and K-741M, difference is in keyboard layout) and Mostek chips, later using LED displays imported from western countries. They used western chips first, later using CEMI Polish circuits. All their calculators used 9V battery or 6 1.5V AA cells. It was possible to use 9V power supply with jack mono plug too.

- Their first mass-produced calculator with LED was K-764 or K-784, later re-named to Brda 10U. Some units have K-764 and K-784 markings, some just Brda 10U.
- Brda 11U was Brda 10U with Polish components and little different keyboard.
- K-765 was NOT Brda 11U. It was more like K-741, but with LED display and keyboard similar to Commodore SR-36, but with simple operations only.
- Brda 12U was more popular, it used 6 1,5V AA batteries, was built around the same NS LED display and Polich IC.  Had a new casing and keyboard.
- Brda 13U - today very rare - white and brown, used different casing.
- Brda 14S used the same casing as Brda 12U, but had more scientific functions (Polish chip MC14008).

 Some Elwro calculators were prototyped in Eltra Brda 12U casings.

Brda 10U

Type Simple pocket
Display: LED
Keyboard: Metal membrane.
Chip: MOS MPS7541
Display driver: ITT 548-5N
Batteries: 9V
Operations: +, -, /, *, Decimal Point, %,sqrt, memory.
One of the first Polish pocket calculators with LED display made in Eltra. Allows to perform simple operations and has memory. LED display is manufactured by NS. This display has quite bad quality and drains battery, so it uses battery saving option - after a minute of inactivity it displays only a single dash in the middle. Can be used with 9V DC power supply.
 
Carrying cover Keyboard is different than in 11U Inside

 

Brda 11U

Type Simple pocket
Display: LED
Keyboard: Metal membrane
Chip: MOS MPS7541
Display driver: CEMI UCY74549
Battery: 9V
Operations: +, -, /, *, Decimal Point, %,sqrt, memory.
Brda 11U was 10U with different keyboard. It uses Polish circuits too. As Brda 10U, it could be used with external power supply. Its keyboard was made with small membranes pressed with plastic keys. These membranes were metal, so it was quite rigid, but keys had to be pressed a bit hard.
 
Numbers Battery saving mode Error: divide by zero
Overflow error Memory in use (notice the dot) sqrt(-3) error
Underflow error Inside Mainboard
 
Display Rear of keyboard  
 

 

Brda 12U

Type Simple pocket
Display: LED
Keyboard: Metal membrane.
Chip: CEMI MC74007
Display driver: CEMI UCY74548
Battery: 6x1.5V AA
Operations: +, -, /, *, Decimal Point, %,sqrt, x^2, 1/x, memory.
Brda 12U was popular and widely manufactured because it used only one western part: LED display. All chips were Polish, it had more capabilities than Brda 10U or 11U, and it could still be powered from batteries or 9V power supply.

Brda 12U Schematic
IC sheet with application schematic
Designation Numbers, minus and memory sign Battery holder
Mainboard from different unit Mainboard - solder side Rear of the keyboard.
   
Keyboard - membrane    

 Power supply unit for Eltra/Unitra calculators.

A typical linear power supply for Eltra/Unitra calculators, giving 9V DC at maximum 300mA. The output is in 3.5mm mono Jack plug, positive going to the center pin. Inside, a linear regulator based on transistor and Zener diode is used.
This is probably not original power supply unit, because its designation only looks like Elwro, but definitely is not "Elwro". Original ones had "Unitra" markings embossed on casing.

Power suply units were common for these small, battery-operated calculators because on one 6F22 9V battery they had relatively small working time, only few hours.