There are at least two dead VT220s in the School of Computer Science. We took two of them apart yesterday in the hope that we could cobble together one working terminal. The logic seemed fine, and the composite video output was working, but there was nothing displayed on the screen. My first thought was a faulty flyback transformer.
I measured the power on the CRT header pins and this is what I found:
pin 1, green, 4.3v , should be roughly 50v to 80v
pin 2, yellow, 6.6v should be roughly 70v to 100v
pin 3, brown, 12v - fine
pin 4, black, 0v - fine
pin 5, white, goes to case ground
pin 6, red, 0v, should be roughly 400v to 800v
pin 7, blue, 0v, should be roughly 400v to 800v
I tested all the diodes, measured the capacitance of all the capacitors. Everything seemed fine. With a lot of advice from the cctech/classiccmp mailing list I found that a 13006 transistor (80W, 8A, 300V) was conducting even when powered off. I'm not sure why this component is faulty. The terminal died during a power cut, so maybe there was a power surge. Perhaps another component has caused it to fail. An easy way to check this is to just replace it, so I've ordered a few of them. Hopefully this will fix it. 13006s should arrive within the next week so I'll see what happens…
<2017-04-30 Sun 10:15> Looking at the circuit diagram a bit more, measuring 0ohm between pin 1 and 3 of Q202 is to be expected given that it passes through the primary windings of a few transformers, so it wasn't really a good test. I will remove it and test it.
A couple of VT220s: