Author: Sergei Frolov. Please return to me this page with corrections of English grammar.
On this page you find out about the undocumented and interesting features of soviet calculators. The materials of this page are prepared on the basis of clauses from popular magazines, and also proceeding from personal experience of the author of clause. If you know something else, please, send materials to me.
It is forbidden to use materials of this clause in any purposes without the coordination with the author.
MK85, MK85M 
Bug  All these calculators are assembled on the same chip. In these calculators function of squire root from a negative number does not cause an error. Instead of an error on the indicator the result of the function is output. 
Hidden  At pressing simultaneously keys "CP" (clear memory) and "Cx" in the calculator the not
documented character (blank) is entered. This character is equal to digit 15. If from the gated in character to subtract 1, it is possible to receive the not documentary characters appropriate to numbers from 10 up to 15: (10), (11), (12), (13), (14), (15). 
Features  In this calculator there is one specific function named (e^ix). After executing this function the register Y stores value of SIN X, and the register X stores value of COS X. Perform of division function cause to calculate Tangent of number. 
At reading a command of check of a condition or contents in register X are perceived by the managing device as a negative number. As the representation 0 can appear only after performance of the operator of change of a mark , it is necessary to avoid input of this operator before the operators of transition on a condition.  
Bugs 
In calculators of early issues the following bugs are observed: At addition of number containing in a mantissa seven "9" and more of four units in eighth (undisplayed) for fractional mantissas the digit, and greater on exponent there is a bug. For check of the calculator it is necessary to combine 9.9999999 and 10. If the result is equal 120, at calculations it is necessary to take into account this feature. 
At execution of single operators of
calculation of elementary functions and operator in the digits of one of the registers (which
number depends on a kind of function and values of argument) of the ring stack of
numerical memory the signal of overflow is brought, that results to errors at use of this
register for a storage of operands. For check of the calculator it is necessary to clear
memory, by switching off a power, and after inclusion of a power to press keys . If displaying a zero, the calculator is free
from a examined lack, otherwise displaying: at function evaluation between calling to stack it is necessary to take into account it overflow the registers or not use has flown down memories. 

The transition to the subroutine is not carried out, if the code of a command BP is brought in a cell of program memory with the address 55, 65, 70, 80, 91 and 92, and the operator is carried out, which code is equal to the index of transition to the subroutine. For check it is necessary in a working mode to press keys . If on the indicator displaying 11, the transition to the subroutine is carried out normally (the cell with the address 55) is checked, but at displaying 99 transitions to the subroutine from the specified addresses are not carried out. 
Features 

Bugs 
Features  In the calculator MK71 there is one very remarkable feature. It is
connected to switching of the switch of degreesradiansgrads in an average rule  between
degrees and radians or between radians and grads. The calculator passes in very unusual
mode of operations reminding work with the calculator MK51. First, now numbers in the calculator have a mantissa of length 8 instead of ten, though the missing digits are stored in memory, but are not visible. Secondly, some buttons of management have other values! The key with the image of degrees now calculates return values of number, in a combination to a key F  factorial. The key 1/x  switches a way of calculation of trigonometrical functions (degrees  radians  gradients). On the indicator they are displayed by accordingly badges "F", "P", "K"! In a combination to a key "F" the key 1/x now enters into a mode of statistical calculations. The key "hyp" now enters the information in degrees, minutes and seconds, and back in a combination to a key "F". For indication of number which is taking place in memory, mode of a key "F" and the calculations with a constant are used segments of the most left digit. 
Features  It is known, that the calculators have some limit of accuracy of calculations. In the maintenance instruction about it is usually obviously told. However on this calculator on the indicator only right figures of result. That is the result can take all eight categories, and it is less. 
In this calculator there is a function of calculation factorial. A feature is that the result of factorial is calculated by consecutive multiplication of numbers: 1 * 2 * 3 *.. * n. For example, the result of factorial from 69 is calculated for 5 seconds.  
Bugs  May be that error conditions are do not blocking work of calculator. Error condition can be reset by pressing any key. 
Features  The register of a position of a comma has a size in 16 digits. It enables to work with numbers with the orders up to 16 exponent. Thus the comma dies away. 
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