In part 5 I repaired the damage I’d done to the ‘enhance’ feature. The ‘locate’ button was the next problem to solve. It was really sensitive and tended to get stuck in the pressed position.
Its job is to let the user see where the trace has gone, or where it might appear. It’s necessary because the oscilloscope has dozens of controls – probably more than a hundred – and if any of them isn’t set right, the usual result is a blank screen. The locate button helps by squashing the trace in from all four sides and cranking the brightness up to maximum, so you can see whereabouts the beam is pointing. Here’s a picture of a normal trace (a video output from a BBC micro, as it happens) and what it looks like if I move its vertical position way up off the screen and press ‘locate’. You can see that a squashed version of the trace appears near the top of the screen, so I know I should move it down to see it properly.
The 549 has an extra feature, though. Because it’s a split-screen storage oscilloscope, it’s expected that it will often be used in single-sweep mode: waiting for a trigger signal, then recording a trace once, then stopping so you can see it. While it’s waiting for the trigger, there’s nothing to see on the screen. It would be helpful to have some idea where the trace will appear when the sweep starts, to make sure it will actually be captured.
Tektronix, typically, thought of this problem. The locate button behaves differently when either of the ‘store’ buttons is pressed. In this mode it simply moves the beam into a special non-storage area on the left of the screen and turns up the brightness but leaves its vertical position unchanged. In this way you can see where the trace will start when it triggers, but without trampling on the storage area. Neat. Here’s a picture of the same video waveform as above, correctly positioned but with the timebase in single-sweep mode, storage enabled, and ‘locate’ pressed. You can see the shape of the trace on the left, out of the storage area.
Of course, to achieve this, the locate button has several contacts affecting various parts of the circuit. It’s buried deep inside the scope behind the front panel, and is a pain to get at. I managed to remove it and found that one of the brass contacts which also provides the spring action had fractured and actually fell off when I touched it. That would explain why it was getting stuck.
I considered replacing the switch with something else, or a relay, but realised that by dismantling the existing switch (it was strung together like a kebab with two screws), rearranging the contacts a bit and soldering a couple of them together, I could repair it. It took a bit of experimenting with the paxolin spacers until the action was right and all the contacts did what they were supposed to, but it was a success. After reinstalling the switch, which involved some tricky soldering at close quarters, it all worked as it should.
Time for the finishing touches. I gave the case a good clean and got out the tinsnips to make a new cover for the EHT compartment because it was missing. I had to guess what it looked like from photos, but I added an insulating sheet of plastic on the inside to reduce the risk of a high-voltage flashover from the transformer pins or anywhere else.
With a quick clean of the case and knobs and a replacement mains lead (the old one was perished and cracked and wouldn’t pass a PAT test) this magnificent machine is ready for use once again.