The Avometer 8 multimeter has lots of useful ranges, including a special high-resistance range which can measure resistances of up to 20 megohms. This is handy, but it needs a special 15 volt battery. The battery it’s designed for is a BLR121, which was once fairly common but is now dying out. The BLR121 is just about still available but it’s expensive, and since the meter is likely to last a long time I wanted a battery which would also last, and be easy to replace when necessary.
An old solder reel, a bit of copper pipe, and five common-or-garden lithium coin cells is all it took. The coin cells are CR2032, which are 20mm in diameter, and they fit just neatly inside the solder reel. I cut down the reel to form a tube about 35mm long. The stack of five cells is about 16mm long, so I filled the remaining space with a bit of copper pipe cut to about 22mm long. This is what the assembly looked like:
It fitted just neatly into the battery compartment of the meter. The spring contacts are very convenient because you can fit more or less any shape between them!
It works perfectly, and it’s cheap. The CR2032 cells are available for less than 50p each if you shop around, and they have a capacity of around 200mAh. The BLR121 replacements I’ve seen have a capacity of only 40mAh, so the lithium replacement should last about five times longer. Not bad.
Here’s a gratuitous picture of the Avo in use checking the power supply of a BBC Micro.
The Avometer 8 is a British electronics icon. For probably half a century it was the standard high-quality multimeter, found in every factory, workshop and laboratory. Though an analogue meter seems like an anachronism in today’s digital world, it’s still useful for some tasks, and there are decades’ worth of service manuals and test procedures which still call for measurements to be made using an Avo. They only stopped making them in 2008 because some parts were no longer available.
This one was given to me by a colleague who got it as part of a deal when he bought a secondhand broken TV. I’m not making this up, I promise. It (the meter, not the TV) was made in 1964 according to the serial number. It really didn’t work when I got it. It read about 30% low on all ranges, the pointer kept sticking, and it hardly ever returned to the same zero point on the scale. I was on the point of scrapping it, but decided to save it because it’s got stickers on it from the lab I used when I did my degree, and I was encouraged by advice from the people of the UK Vintage Radio forum. I opened it up:
It’s clear that everything is hand-built, and should be quite serviceable. The problems with this meter seemed to be in the movement itself – the sensitive, fragile coil suspended by precision bearings in a big magnet – rather than the electronics. The movement is so delicate that I was worried about wrecking it rather than fixing it! However, it’s only held in with two screws, so I could take it out and see what needed doing.
In the picture above, the movement has been taken out and is standing on top of the rest of the meter. With the benefit of a little advice, and a very handy article from the Amateur Radio Relay League in February 1943 called ‘Rejuvenating Old Meters‘, I set to work.
The gap in the magnet in which the coil is suspended was full of tiny iron filings. They’re not supposed to be there. They get in the way, causing the coil and thus the pointer to stick, and they short-circuit the magnetic field, reducing the sensitivity of the meter. I cleaned them out in the recommended way using a little piece of Blu-tack.
The bearings suspending the armature were way out of adjustment: it rattled and caught on the centre pole-piece of the magnet, again making it stick. I adjusted the bearings, centring the hairsprings and the coil in the gap and just taking up the slack so it could move freely. The bearings in the Avo are sprung, so the armature is never quite rigid, but there should be no rattle in it.
Things were looking up, but there was still a problem. The movement wasn’t balanced, so the position of the pointer was very sensitive to which way up the meter was held. The pointer assembly has three little arms, one opposite the pointer and two perpendicular to it, to which it’s possible to add weights to balance the pointer. It’s a very delicate operation. You have to hold your breath while doing it, since the slightest draught sends the pointer swinging wildly. This picture, reproduced from the Rejuvenating Old Meters article, shows how the balancing is done. First, the meter is set to zero while lying horizontally. Then it’s turned to stand vertically. The tail weight is adjusted with the pointer horizontal, and the side weights are adjusted with it vertical.
I used paint applied in droplets with a tiny screwdriver to add weight. You can see it in this closeup of the movement.
It doesn’t take much – the balance is incredibly sensitive.
Now to test it. The bare movement is supposed to take 37.5μA for full scale deflection. With a power supply and a resistor, I gave it a current of 37.5μA and it worked! I couldn’t tell whether it was exactly right because the naked movement is so sensitive to draughts that the pointer was never quite steady, but it was close enough for me.
I reassembled the meter, sticking the glass (yes, real glass!) back into the case as I went, and was delighted to find that it was now working – no sticking, it returned to zero every time, and was fairly accurate. It read about 1% low, though. That’s within its specification but I thought it could do better. Fortunately the Avo designers made the meter adjustable to fix such errors. There’s a shunt on the magnet which can adjust the magnetic field a little to compensate for the slight loss of magnetism as it ages. It’s the piece of metal with the slot in it, held by one screw, in this photo of the top of the movement.
A couple of millimetres to the left was all it took, and the Avo now reads correctly to within 0.5%. Not a bad result, considering the only tools required were a screwdriver, a bit of Blu-tack, and some paint. Try that with a faulty digital multimeter!
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.
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 hFE 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.
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.
In part 2 I established that there was something wrong with one of the windings on the replacement EHT transformer, so the grid voltage was either too low, leading to a very dim trace, or too high, leading to a trace which was too bright. The only way to fix this problem was to modify the transformer. Knowing that it’s a tricky device which has to handle high voltages, it was with some trepidation that I took it out of the scope and dismantled it.
Whoever made the transformer knew what they were doing. They’ve taken care to put a gap in the core to prevent it saturating – notice the plastic washers used as shims between the two core halves. Looking at the circuit design, I believe that this power supply is running as a flyback converter, so a gap in the core is required. Everything is nicely taped up and varnished, and there is paper tape in between the layers of the windings, too, for good high voltage insulation. I was relieved to see all this: it makes me feel confident that a repair is worth doing.
It seemed clear that one winding, the one generating too large a voltage on the grid, had too many turns on it. I had to work out how many turns to remove. I’ve been designing some flyback converters for another project recently, so I could make an educated guess. I’d expect a transformer like this one to run at somewhere around 1 turn per volt, maybe more, maybe less. I want to reduce the grid winding’s output by about 50 volts, so I thought I’d start by removing 20 turns from it.
I put the transformer in the oven to soften the varnish so I could dismantle it (150 degrees C for about 15 minutes). Unwrapping the winding, removing 20 turns was easy – they were all on the first layer, so I only had to unpeel one layer of tape. I temporarily put the transformer back in the scope and tried it. Success! I could follow the instructions for setting the intensity in the manual: set to single sweep, so the trace should be blanked, turn the front panel intensity control to maximum and adjust the internal intensity range control so the spot is just visible. It was well within the range of adjustment of the control.
With the EHT working properly, I checked the voltages: the cathode is fixed at -3700V. For normal trace brightness the grid is at about -3750V, and about -3800V hides the trace entirely.
Incidentally, I was mistaken about how the intensity control works in my comments in part 2. Having studied it more carefully, the EHT supply regulates the cathode voltage to be -3700V. The intensity control varies the voltage on the ‘top end’ of the cathode winding between about +10 and +100V so the power supply has to work more or less hard to maintain -3700V at the cathode. That results in the voltage on the grid winding varying by about the same amount, so the intensity varies.
I also sorted out the horizontal sweep problem. I found a racing car in the relevant bit of the circuit diagram, but it seemed to be missing in the scope itself.
That wasn’t the problem, though. After checking the voltages around the horizontal amplifier, it became clear that R352, a 402k resistor which feeds a voltage from the normal/magnified registration preset control into the horizontal output stage, was open circuit. I replaced it and the trace looks good now, and can be adjusted properly.
Now to find out why the delayed timebase doesn’t work. Oh, and I slipped with my high voltage probe whilst checking it on the 500V connection on the storage PCB, and shorted it to another pin, destroying two transistors and two diodes in the ‘enhance’ circuit. Oops.