IBM 5151  -  CRT's Cathode Heater

When diagnosing a faulty 5151 (with no display), one good initial thing to check is that the cathode's heater is operating.

In a lot of monitors, the orange coloured glow of a heater that is operating can be seen if you look into the monitor via the top air vents.  Sometimes you will need to do that in a darkened room.  That rarely works for an 5151, even in a completely darkened room (no light whatsover).

Note that in the 5151, the heater gets power whenever the 5151 has AC mains power applied (i.e. no video/sync signal is required for heater operation).

To see if the heater is operating, one method is to open the 5151 and look at the window per the photo below.
Opening up the 5151 presents three issues:
  • Safety issues;
  • Physical instability (see note 1);
  • Possible physical damage by power transformer (see note 2).

Note 1: With its cover off, the 5151 is physically unstable (except when face-down).
Note 2: The case comes apart after removal of two screws topside, and six screws underside.  On the underside, do not remove the two screws that are covered (photo), because removal of those together with the remainder will result in the power transformer and its mounting plate coming loose within the case and causing damage.

  xxxxxxx WARNING:

Opening the 5151 will expose you to dangerous voltages and currents.
Those threats can still be present long after power has been disconnected from the 5151.
Before opening the 5151, adequately educate yourself as to the hazards.
Some hazard examples at here.