1.  Simple choke - has 2 leads, usually used with a starter (occaisionally
  a "push-to-start" switch).  There is normally no capacitor.

  When they fail, there is generally no repair other than rewinding
(extremely unlikely to be worth the time and effort), and sometimes the
core is welded together.

2.  High leakage reactance autotransformer - has 3 leads, and is
effexctively a choke combined with a step-up transformer.  Same story as
1.

  A variant is the 1-lamp trigger start ballast, which has filament
widnings added.  It is still essentially non-user-serviceable.

3.  The USA-usual "traditional" dual-4-footer - high leakage
autotransformer with a capacitor in series with the secondary.  Also has
filament windings and a small capacitor (in the ballast case along with
all other parts) to leak a small amount of current around one lamp to help
start the other when both are not conducting but voltage is applied.  The
2 lamps are in series with each other.

  If the lamp-series capacitor (which is not electrolytic) shorts, lamp
current will be somewhat high and ballast input current will be very high.

  There are similar "trigger start" ballasts for 2 2-footers.

  The case is usually packed with tar.  This one is also essentially
non-user-serviceable.

4.  Electronic ballasts - more repairable than the others, but only by
those who know how to troubleshoot and repair them *safely*.  Capacitors
may store charge, and improper repairs may add a fire risk.
  It may be difficult to verify that replacement parts are of
suitable grades (temperature rating, life expectancy at given temperature,
voltage and frequency, also failing less catastrophically, any flame
retardance/resistance) and with suitable characteristics (such as losses
sufficiently low to prevent overheating - and capacitors have 2 main
losses).
  Also, one is unlikely to repair a ballast for less (including value of
time) than to replace it.

  Bottom line:  Fluorescent lamp ballasts are generally unrepairable.

 - Don Klipstein (d...@misty.com)