Even more than its predecessor, the B3-34, the Elektronika MK-61 was a Soviet calculator whose design was clearly inspired by Hewlett-Packard's highly successful line of programmable calculators. The MK-61 is a Reverse Polish (RPN) calculator, with a four-level stack and a fairly well designed keyboard that includes a gold and a blue shift key. If this isn't enough, the overall appearance and feel of the calculator is somewhat similar to Hewlett-Packard's Spice series of calculators such as the HP-33C.

I used the word "inspired" intentionally: while there are many similarities between the MK-61 and HP calculators, the MK-61 is clearly not a copycat clone. Despite the similarities, the MK-61 is most definitely not an HP calculator, as evidenced by its different programming model, unique features, and function set.

That said, the MK-61 is a surprisingly well-designed calculator. Its display accuracy is limited to an 8-digit mantissa, but within those 8 digits, the calculator delivers accurate results; the manual, in fact, (just like HP calculator manuals of the same vintage) does include a discussion of the expected accuracy of all built-in functions. This is in stark contrast with Western calculators such as the National Semiconductor NS 4525, or the Sinclair Cambridge Programmable, both of which have built-in functions that are laughably inaccurate, sometimes delivering results with only 4 digits of precision.

The construction of the MK-61 is also fairly good. Judging by its external condition, one of my MK-61s has seen some rough times, yet it remains perfectly functional. The documentation is low quality in appearance, but high quality in content and thoroughness.

The calculator has a reasonable storage capacity: 105 merged program steps and 15 memories are sufficient for many tasks even in the absence of permanent storage or continuous memory.

There's one thing, however, that this calculator isn't: it is certainly not a fast one! The display goes blank after every keystroke for a noticeable fraction of a second, and a simple operation, such as taking the exponential of 1, takes more than 2 seconds to complete! Programs of a similar design take up to five times as long to execute on the MK-61 than on my HP-65, which happens to be more than ten years older.

Its crawling speed notwithstanding, I found the MK-61 a very pleasant calculator. Despite the unusual cyrillic nomenclature, it didn't take long to get used to its keyboard, and within no time, I was writing short programs on it without even glancing at the manual. Perhaps the most striking "feature" is the seemingly arbitrary keycodes in program memory; unlike the case with most Western keystroke programmables, keycodes on the MK-61 do not appear to be related in any way to key positions on the keyboard.

I am sure there are many mysteries that I haven't discovered yet, but I have already been able to port my Gamma function implementation to this beast. This is yet another one of those implementations that requires several constants to be placed in memory after the program is keyed in. To calculate the Gamma function of any argument (including negative non-integers) enter the number and hit the C/П button. The program demonstrates the use of indirect addressing and loops, two of the advanced features of the MK-61.

M2: 76.180092
M3: -86.50532
M4: 24.014098
M5: -1.2317396
M6: 1.208651e-3
M7: -5.3952394e-6

00    0E    B^
01    01    1
02    14    <->
03    5C    x<0
04    15    15
05    0E    B^
06    25    \(\circlearrowright\)
07    12    ×
08    25    \(\circlearrowright\)
09    25    \(\circlearrowright\)
10    25    \(\circlearrowright\)
11    01    1
12    10    +
13    51    БП
14    03    03
15    49    x-П 9
16    14    <->
17    48    x-П 8
18    06    6
19    40    x-П 0
20    08    8
21    41    x-П 1
22    01    1
23    Г1    K П-x 1
24    69    П-x 9
25    61    П-x 1
26    10    +
27    01    1
28    11    -
29    13    ÷
30    10    +
31    5Г    L0
32    23    23
33    02    2
34    20    π
35    12    ×
36    21    √
37    12    ×
38    69    П-x 9
39    13    ÷
40    18    ln
41    69    П-x 9
42    05    5
43    0-    .
44    05    5
45    10    +
46    18    ln
47    69    П-x 9
48    02    2
49    23    1/x
50    10    +
51    12    ×
52    10    +
53    69    П-x 9
54    11    -
55    05    5
56    0-    .
57    05    5
58    11    -
59    16    ex
60    68    П-x 8
61    13    ÷
62    50    C/П
63    51    БП
64    00    00