Tag Archives: analogue

Avometer 8 repair

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.

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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:

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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.

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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.

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I used paint applied in droplets with a tiny screwdriver to add weight. You can see it in this closeup of the movement.

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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.

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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!

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Motorboating in Space

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Or, how to stop Zaxxon going thump-thump-thump.

‘Motorboating’ has been a problem in electronics almost as long as electronics has existed. It gets its name from a characteristic thumping or buzzing noise, reminiscent of a motor boat’s engine. It’s a problem which usually occurs in audio amplifiers, and it happens either because of a design error or faulty components. Sometimes a change in an amplifier’s operating environment, such as a radio battery running down, can cause it. It’s loud, annoying, and can even damage speakers,

The reason for the noise is feedback. If an amplifier drives a signal into a loudspeaker, the power for that signal has to come from its power supply. Its power supply, especially if it’s a run-down radio battery, isn’t perfect. Drawing power from it makes its output drop in voltage for a moment. Unfortunately, electronic circuits aren’t perfect either. Their behaviour is strongly affected by their power supply. Connect such a circuit to such a power supply and amplifier, and you have a vicious circle: circuit sends a signal to amplifier, amplifier sends it to speaker and draws more power, power supply affects circuit, which makes another signal which gets sent to amplifier, and so on. It’s called feedback because the output signal feeds back into the input, via an unorthodox route. The circle of feedback can lead to the regular buzzing noise – the motorboating.

Recently I have restored a Zaxxon arcade game circuit board, which dates from 1982 (actually, it’s a bootleg, but the circuit is largely the same). I got it working well, but with one big problem: the sound was accompanied by a constant thumping noise which wasn’t supposed to be there. Here’s a short movie of how it sounded. It’s especially noticeable at the start and end of the clip.

Fans of the game will know that Zaxxon has very distinctive sound. Many video games at the time used digital techniques, often using standard chips, to generate their sound, which gives them a characteristic bleepy quality. Zaxxon is different. It uses what amounts to an analogue synthesizer: a magnificent assembly of timers, oscillators, amplifiers and filters. It has a lot in common with the kind of instruments used in pop music at the time. It makes a glorious, raucous noise.

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But this kind of analogue circuitry has a problem, especially when it’s cheaply built using early 1980s technology: it’s very sensitive to its power supply. Any variation in the power supply basically gets straight to the synthesizer’s output. What’s more, Zaxxon’s loudspeaker amplifier runs from the same power supply as the synthesizer. This lot is a recipe for motorboating, and that’s exactly what happened to my game.

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Of course, we have to assume that it all worked properly when it came out of the factory, but then it would have been running from an official Zaxxon power supply. The one I use in my arcade game test rig may not be as good as the original one, but it’s good enough for most things, and I wasn’t going to change it just to fix this problem. So I had to come up with a modification to keep apart the amplifier power and the synthesizer power.

The traditional cheap and cheerful way of keeping power supplies apart, known as decoupling them, is simply to put a resistor and capacitor between them, like this:

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This decoupling means that variations in one power supply have a smaller effect on the other. It works well, and has been used in millions of electronic devices from the earliest days of radio. However, a certain amount of power is always lost in the resistor. Many circuits don’t mind this, or can be designed to handle it. I tried this approach with Zaxxon,and it turned out that the sound synthesizer doesn’t cope well with a reduced supply voltage. Many of the effects, especially explosions, became disappointingly quiet. I had to find another way.

Arcade games typically use two power supplies: 5 volts for their digital circuits, and 12 volts for the sound amplifier. This gave me an idea: how about using the 5 volt supply to run the audio synthesizer, keeping it neatly separate from the amplifier? Clearly the synthesizer wouldn’t just work from 5 volts: I’d already had trouble with it running from about 10 volts in the decoupling experiment. However, there was a solution. It would be possible to boost the 5 volt power supply up to 12 volts using, aptly, a boost converter. Boost converter modules are cheap and readily available thanks to low-cost far eastern manufacturing. The one I chose had a conveniently adjustable output voltage. It didn’t take long to wire it up. I’d already separated the amplifier supply from the synthesizer, and so I just had to take a wire from the existing 5 volt supply to the sound board, check my work and switch on.

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It worked! The sound was now perfect, with no strange thumping effects, and everything seemed to be at the right volume. It remained only to make the modification more solid, and there was even a handy spare hole  to mount the boost converter in. Job done!

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