I’ve recently been bringing this beautiful old scope back to life. It’s one of the last models in Tektronix’s legendary 500 series of oscilloscopes, which were in production from the 1950s until the early 1970s. It has the extraordinary build quality and attention to detail of that era of Tektronix equipment, and was seriously expensive. If you had £2000 burning a hole in your pocket in the late 1960s, you could have bought an E-Type Jaguar or a scope like this one. The 549 is particularly unusual because it’s the only one of the 500 series which has a storage tube, so it’s capable of storing any signal you can capture using any letter or 1-series plugin, which makes for a very versatile instrument. It’s also full of cosy, warm, glowing valves.
This particular instrument has clearly had some repairs done in the past. There are some untidy diode and resistor replacements in the power supply, and the 4kV EHT supply has had a rewound transformer fitted and various other components replaced. I’d seen it switched on before I got it, but had never seen it produce a trace. The photo below shows the modified EHT supply, complete with new transformer.
I switched it on for a quick ‘smoke test’. There was no smoke, but little else either: the power indicator and graticule illumination worked, but there was nothing to be seen on the screen even when I set the controls to settings which should show at least a spot.
The first job was to check the power supplies. These Tektronix scopes have an elaborate regulated power supply arrangement which provides -150V, +100V, +225V, +350V and +500V rails. They were all present, if a tiny bit low, but a quick adjustment of the -150V supply (to which all the others are referenced) brought them within spec. So far so good, but still no trace.
It turns out that this scope, unlike the 535A I used to use every day, has transistors in it as well as valves. Though they don’t glow nicely, they still need to get power from somewhere. In this case it’s an extra -12.4V rail I hadn’t noticed before. My meter measured zero volts, which was definitely wrong. The 12.4V regulator only has three transistors in it. Probing around the circuit showed that almost every voltage was wrong in it. The two smaller transistors were in sockets, so I experimentally wiggled them and they both fell off in my hand! Their leads had corroded right through during storage. That wasn’t going to help the power supply work. According to the circuit diagram they were NPN types and didn’t seem to be doing anything exotic, so I got a couple of trusty BC107s, conveniently in the same TO18 package, and put them in. Switch on – and the 12.4V rail is back. Success.
Now we have a trace! The trouble is, it’s rather dim. Very dim, in fact. And somewhat skewed off to the right hand side – the horizontal position control can’t bring the left hand end of the trace much further than the centre of the screen. But timebase A works, the controls on the plugin (I’m testing using a 1A1 which I know works) do the right things, and the trace can be focused. Even better, the storage functions seem to work: I can record the trace on the screen and erase the upper and lower halves of the screen independently. That means the tube (which is irreplaceable) is almost certainly OK. Curiously the stored trace is much brighter than the ‘live’ one.
But why is the trace so dim? I looked at the setup procedure in the manual, and it turns out that the ‘intensity range’ control is already at maximum. Someone’s been here before. Maybe the repaired EHT supply isn’t working well? I rigged up a pair of AVO 8s in series as an ad-hoc 5kV meter (don’t try this at home, dear reader) and it set easily to the required 3700V. One secondary leadout wire of the replaced transformer has a habit of sparking to ground, so its insulation will need attention. But the EHT is correct, so that’s not the cause of the dimness. More investigation required – see part 2.