In part 3, the scope was working, with a trace that seemed to do the right things. But the delayed timebase was dead. It’s a handy feature that effectively lets you select a part of the waveform on the screen and zoom in to magnify it, and in today’s digital world it seems almost inconceivable that it was possible to do such things with analogue electronics. Mind you, it takes quite a lot of analogue electronics to achieve it, especially with valves. The 549 contained an outrageous 53 valves when I counted them. In the photo below, the swung-out panel on the right is basically devoted to the delayed timebase.
There’s a part of the circuit called the ‘delay pickoff’ which is responsible for selecting which part of timebase B’s sweep to magnify or, more accurately, when to trigger timebase A. It contains a comparator, which compares the sweep voltage as the beam scans across the screen with another voltage, which you set by turning the ‘delay multiplier’ knob on the front panel. When the two match, it sets a flip-flop, which triggers timebase A. In this way, you can choose where on the screen to start the timebase by turning the knob. Handy.
I started probing around with another oscilloscope to see what was going on, or not going on. The circuit diagram in the manual is full of helpful voltage readings and waveforms so you can see what should be there at various points.
In this case, I found that there was a nice sweep sawtooth on the grid of V414 at the left hand side of the diagram, but instead of a sweep on pin 1 of V428A (the blue waveform), I just had about -120V. The grid of V428A was also at about -125V, when it should be held at -100V by R425 and R426. Aha, I thought, R425 has gone high in value! These old carbon resistors do that. So I checked it – no, it’s fine. Then it must be V428A that’s faulty! I even proved it by removing it from its socket, and the voltage at pin 2 returned to -100V. I borrowed a known good 6DJ8 from elsewhere and triumphantly fitted it. Switch on and…no change. Hmm.
If V414 and V424 both have 225V on their anodes and -120V on their cathodes, they’re clearly not conducting much, or they’d probably have melted. I swung open the panel that they’re mounted on and had a look at them. V424 wasn’t glowing, and felt cold! I gave it a wiggle in its socket and it started to glow, and immediately the voltage at its cathode went up to about +30V. Found the problem! But still there was no sawtooth waveform there, just a voltage which varied as I turned the delay multiplier control.
I carefully cleaned the pins and sockets of V414 and V424 but it didn’t help. I thought I’d try swapping them over, to see if anything changed. For no particularly sensible reason, I did it with the power switched on, and I’m glad I did: as I put the second one back into its socket, I noticed purple flashes from inside it! That’s not supposed to happen. One of the 6AU6s is very sick. Here’s the guilty party.
I borrowed a 6AU6 from another scope, and there was an improvement: the delayed timebase started to work, but the range was all wrong. It would start the delay half way across the screen, and it was impossible to adjust it properly. I replaced the other 6AU6 as well and it worked much better. I was able to adjust the delay start and stop controls so each notch on the delay multiplier knob corresponded to one division on the screen, just as they are supposed to. Here’s a picture of the delayed timebase working. It’s in ‘B intensified by A’ mode, and you can see the brighter section of trace.
Time to go shopping for a pair of 6AU6s.