Advice about repairing an IBM 5151

tony duell ard at
Sun Aug 30 01:36:46 CDT 2015

> I am trying to identify why my IBM 5151 display has no picture when
> connected to a normal MDA card in a IBM PC 5150. So armed with an
> oscilloscope, digital multi-meter and the SAMS Computerfacts for it I
> started to investigate. First, the card does send out signal and that
> signal does reach the board inside the monitor. I checked the power
> part of the circuit, all diodes and the transistor check fine. I
> probed some of the vertical and horizontal transistors and there is a
> signal there too. Then on the video board both TR19 and TR20 have on
> their collector and emitter (respectively) a signal (95Khz). What else
> can possibly be wrong?

Do you mean no picture or no raster? To me the 'picture' is the video
modulation (that makes different bits of the screen light or dark). If you
turn up the brightness and contrast controls do you get any illumination on
the screen?

I am going to assume you don't, since that is the more common problem.

First check the internal 12V (or so) supply. Is that present and correct. Note 
it mght be low due to an overload somewhere else in the monitor, for
example flyback transformer problems.

Is the CRT heater glowing (can you see an orange glow from the end of the
CRT neck)? If not, and if the 12V supply is there, then check the CRT and its
socket. There may be a series resistor too, check that.

Now check the CRT voltages. If you have an EHT meter, check the final
anode voltage (on the rubber connector on the CRT flare). Expect
about 10-12kV here.

The CRT pins (from memory) are as follows (All voltages guessed wrt ground): 

1: Control grid (10's of V, +ve or -ve)
2: Cathode (10's of V +ve)
3,4 : heater. One is ground, expect 12V or so on the other
5: Control grid (see pin 1)
6,7 : I call them anodes, you call them grids :-). Expect a few hunded volts on each pin.

What voltages do you measure?


More information about the cctech mailing list