In part 4, we’d got the delayed timebase working thanks to swapping out a pair of 6AU6 valves. But there was still something not working, and it was all my fault. I’d killed the ‘enhance’ feature.

The 549 is special because it’s capable of capturing an event which takes place in less than a microsecond, and storing it so a human viewer, or a film camera, has time to see it. There’s no digital memory or computer here: the storage is done in the cathode ray tube, directly on the screen. By an amazing feat of electrostatic trickery, it’s possible to put the screen into a mode in which electrons hitting the screen at a point will ‘bump’ the phosphor screen at that point into a different state, and it’ll stay like that – it’s a bistable phosphor. Then, by gently sprinkling electrons all over the phosphor from a flood electron gun, the bits which have changed state continue to glow. In this way, as the electron beam sweeps across the screen tracing out the waveform, it can leave behind a visible trail. It’s possible to erase the trail later by winding up the power of the flood gun for a moment, and bumping the phosphor back into its normal state.

This is some pretty advanced physics, and it’s very analogue: lots of bits of metal inside the tube at precisely-controlled voltages throwing electrons around in a high-speed game of ping-pong so they end up in the right places, with the right energy, at the right time. Tektronix, forever at the bleeding edge, wanted to extract the last possible bit of performance from this finely-tuned and expensive system. As the electrons hit the phosphor, it takes a certain number of them to get it to change state. If the beam is sweeping too quickly, there isn’t time for that to happen, and so the trace doesn’t get recorded. However, it turns out that if the whole screen gets a bit of a ‘kick’ from the flood gun before recording starts, it gets more sensitive for a moment, so it can record faster traces. The specifications quote a writing speed of 5cm per microsecond, which is pretty swift. I reckon it’s roughly equivalent to 100 megasamples per second in a digital oscilloscope. Digital scopes took another 20 years to get that good. The 549 was released before Sgt Pepper’s Lonely Hearts Club Band.

This ‘kick’ to wake the screen up is provided by the enhance feature. A couple of transistors generate a precisely-timed pulse just before the sweep to get the screen ready and poised to catch the electrons. Or, at least, that’s what should happen. It turns out that if a clumsy repairer accidentally short-circuits the 500 volt power supply to the input of the enhance circuit, it dies.

I killed D1134, Q1135, Q1145 and D1146 in one swift move. The damage stopped there because the next component in line is a valve, which shrugged off such a minor trifle as an accidental half a kilovolt.

The diodes were simple to replace. I used 1N4148s. The transistors looked uncritical enough, and I put a couple of 2N3704s in. But the circuit didn’t work. No pulse came out, and no enhancement happened. I looked again at the design, and it seems that the gain h_FE of the transistors, especially Q1135, is critical.

The transistor Q1135 is biased by R1133 and the ‘enhance width’ control R1132. Normally, it should be switched on, and when a negative-going pulse from the trace flyback blanking arrives via C1131 on the left, it should switch off for a moment and generate a pulse. However, the gain of my replacement 2N3904 was too high. The current glowing through R1132 and R1133 was enough to keep it switched on all the time, even when the blanking pulse arrived. Result: no ‘enhance’ pulse output.

The manual says that the original transistor is equivalent to a 2N918, which has a gain of about 50. I didn’t have any of those, and they’re long obsolete. But there’s another half a dozen of them on this board doing other jobs, so maybe I could swap some from there and use my newer transistors for another job. A quick look at the circuit revealed this:

Here are two transistors, Q1033 and Q1043, just being emitter followers from the erase controls. It really wouldn’t matter what their gain was, as long as it was enough. A quick swap and…success! Both erase and enhance worked as intended.

It’s very handy that the transistors in this machine are all in sockets. Interestingly, the old ones look like they’re in TO92 packages:

But their pinout is the opposite way round to modern transistors, which have to sit facing the wrong way in the socket with their legs bent.

 

So far so good. There are still a few problems to sort out, though. The ‘trace locate’ button is ridiculously sensitive, so you barely have to breathe near the scope and the trace shrinks into a little rectangle in the middle of the screen, which is annoying, and the whole thing is filthy dirty. Timebase A’s triggering seems a bit jittery at high speeds, too.