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!
Reblogged this on AMP.
Pingback: Avometer 8 BLR121 15V battery replacement | martinjonestechnology
Pingback: Wouter's Blog » Blog Archive » Avo Model 9 Mk II
Pingback: Servicing a Fluke 12 Multimeter | martinjonestechnology
Hi again, great work; how did you get it so clean? I have acquired another AVO8, just as filthy!
Hello, i bought a 1959 model 8 mk2 at a flee market today. I measured the mains voltage and it was spot on at 230V. Measuring DC voltage is a problem – it reads too low. A 9V battery which the DMM shows as 9,6V , when Avo is set to 10V range is measured around 7,5V , and at 25V range as 8,5V . By adjusting the slide above the magnet, i managed to get it up to 8.5 and 9V, but still way off the mark. Tried with the voltages from a laptop charger, and a li-ion cell, same thing.
Does the problem ring any bells?
I am soon to graduate as an electrical engineer, so feel free to go all technical 🙂
I haven’t seen those symptoms before, but I’d start by checking all the multiplier resistors for the DC volts ranges. Does it under-read on all the DC ranges or just one? What about the current ranges?
Thank you for your reply,
i havent tested for currents yet, but i will. It reads low on all DC ranges, just depends on what range it is, its not equally low everywhere, but its low.