MacGyverS2000 may have it mostly right. It definately sounds like a tantalum capacitor (rectangular with yellow case) 4.7uF but I think it is 50V since you mention thyristors (high voltage-type circuits). 5V is more like you would find with processors or low-voltage logic circuits.
There are three sizes listed for 4.7uF 50V at a distributors site. 2312 (6032 metric), 2915 (7338 metric), 2917 (7343 metric). The sizes work like this - "2312 (6032 metric)" means: 0.23 inches long (end lead-to-end lead) x 0.12 inches wide, or 6.0mm x 3.2mm
If the "K" has a line above and a line below, then it is a Kemet part with 10% tolerance (for the 4.7uf value).
Tantalums are used for low-leakage, long life, and sometimes for low ESR. Tantalums are polarized - they only go in the circuit one direction. Tantalums have little ability to handle over-voltage, and no ability to handle reverse voltage. Reverse voltage or over voltage usually results in the part blowing apart.
Since you don't know if ESR was a factor in the original part selection, I would just go with a medium ESR rating. Low ESR devices are generally used in switch-mode power supplies for better filtering. ESR in this value range from 250m Ohm to 1.5 Ohm. A Kemet T494D475K050AT with 600m Ohm (assuming your size is 2917) is $2.58 each at Digikey (
A lower ESR part will cost double.
