Advice about repairing an IBM 5151

Vlad Stamate vlad.stamate at
Sun Aug 30 15:22:36 CDT 2015


Thank you for answering, I appreciate your help. Answers inline below.

> On Aug 30, 2015, at 2:36 AM, tony duell <ard at> wrote:
>> 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?
The screen is entirely black no matter what I do with brightness and contrast controls.
> 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.
I am getting 14V out of the transistor in the power supply. Voltages seem to check correct there according to the SAMS schematic. Except for one point earlier in the power supply that is supposed to be 25.9V but I get 28V.
> 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.

I see the orange glow.
> 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.

I do not have a tool that can measure that high of a voltage.
> 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
3: 12V
4: Ground
> 5: Control grid (see pin 1)
> 6,7 : I call them anodes, you call them grids :-). Expect a few hunded volts on each pin.
6: 7.33V
7: 10.29V
> What voltages do you measure?
I checked all those with respect to ground ( pin 4).

> -tony

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