Side project - IBM 3278 terminal restoration - looking into the high voltage power supply

REMOVED THE HV POWER SUPPLY AND OPENED IT UP TO INSPECT

The supply sits below the CRT inside the terminal. A connector brings 44VDC and a logic signal that disables the high voltage until the terminal logic is initialized and working properly. It also carries 400VDC to an accelerating electrode and a similar voltage to the focus electrode inside the CRT. A separate wire with a cap is plugged onto the side of the CRT to energize it with 18,000VDC. 

The supply consists of a small printed circuit board attached to a metal heat sink. The connector mentioned above, from the terminal, plugs into this board. Another connector has wires that go inside the metal box under the heat sink. 

Inside the metal box, I saw that everything was potted - sealed with a material that hides the parts underneath and keeps them from shifting about at all. This is helpful with very high voltages as parts that might shift due to handling of the terminal could come close enough to arc to other parts. The potting keeps all parts at their design distances. 

Unfortunately, the potting makes it very difficult to inspect, test or repair what is inside. A quick check with an ohmmeter shows infinite resistance between the anode cap and ground. I suspect some part has failed inside the potting resulting in a total lack of the 18KV power. 

THIS DOES NOT USE A FLYBACK TRANSFORMER FOR HIGH VOLTAGE

Most television sets make use of a flyback transformer to generate the high voltage for the CRT. The horizontal oscillator sweeps the beam across the tube, then at the right end of the line, the flyback transformer produces a powerful pulse to make the beam race back to the left extremely fast. This transformer also produces the anode voltage for the tube. If the horizontal oscillator is not running, there is no high voltage in this sort of design.

The 3278 terminal does have a horizontal oscillator which sweeps the beam across the tube, but it is not used to generate the anode voltage. Instead, as long as the 44VDC supply is present and the disable/enable logic signal allows the HV power supply to work, it will produce 400VDC and 18,000 VDC. It generates AC with an oscillator in the power supply, to drive transformers that step up to the target voltage. 

POTENTIAL SOLUTION FOR THE LACK OF 18KV POWER

It seems logical that the presence of 400V shows that the oscillator and driving circuits are working in the supply. It seems wasteful to create a second oscillator to drive the 18K stepup, thus it should be present since we have 400V coming from the supply.

This suggests to me that we have an open winding in the transformer or an open circuit in some voltage doubler components, but those are all inside the potting. If I can't fix that, I could install a replacement supply of 18KV. 

I did some quick checking and found availability of supplies that take 115VAC input and produce 18KVDC output, which would probably cost me about $100 or so to buy. I would add a method of disabling this driven by the disable/enable logic signal that does to the IBM HV power supply, thus my 18KV will only be delivered when the terminal is initialized. 

Before I commit to this, I should look at the outputs of the other circuitry in the terminal. If it is not producing scans and video output, the restoration might become unattractive. If I can see horizontal and vertical scanning plus a varying stream that would modulate the beam, then I could be more comfortable moving forward with the new power supply.

I will begin to plan the keyboard translator that will allow me to communicate with the terminal in spite of the fact that I don't have the 3278 keyboard - it was poached by keyboard pirates long ago.