Reviving a digital AVO

[Lloyd 1985]

I was sent this AVO DA116 by a fellow forum member, after it was found that the display was faulty, with segments missing. Sadly there was also a piece broken off the glass, which had connections to 3 segments. I was originally intending to replace the LCD with some vintage LED displays, but decided to at least give the old LCD a chance!

On fitting it to the main PCB and turning the meter on I could get most of the display to work, but there were several segments either missing or flickering, the problem got worse when trying to fit the bezel, as the foam pads on it pushed against the edges of the display.

The first thing I needed to do was see if it was going to be possible to get a connection to the 3 segments where the glass had broken away, for this I simply stuck a length of wire in the socket, and licked the end of it, and prodded around on the edge of the glass until something lit up on the display. Amazingly I was able to get all 3 segments back! Obviously a bit of spit on the end of a wire isn’t a permanent solution, for that I used conductive silver paint, 3 little blobs in line with the connector, then once dry a quick check to make sure they weren’t shorted together. To connect them to the PCB I used 3 lengths of enamelled copper wire, and some more conductive paint to stick the ends to their respective points, then some Araldite to secure them to the glass. I left the area where the connections were without any glue so I could check continuity afterwards.

I put 3 stubs of component legs into the display connector to attach the wires to, and one by one soldered each wire in place. The surgery was successful! All 3 segments were working correctly, so I covered the repair with more Araldite. Whilst there I also put a drop of conductive paint on a couple of other problem legs, then ran some more Araldite along the edge of the display over all the legs to secure them.

Since the foam pads on the bezel had caused problems before, I decided to remove them, as they didn’t really serve much purpose, the display is a tight fit in its socket, so unlikely to fall out.

The meter was then reassembled, including re-gluing the front glass window, which had come out. It was given a quick wipe with some foam cleaner, which bought it up nicely. I checked it against my Fluke meter at various voltages, both AC and DC, and they both seem to agree with each other! Occasionally a few hundred mV difference. The high speed resistance range is certainly very quick to take a reading! I have only tested it with a 36 ohm wirewound resistor, as it was to hand at the time, the AVO says it’s bang on 36, while the fluke says it’s more like 38. I think it’s turned out to be a good meter! I’m pleased I was able to revive it!

[Lloyd 1985]

Some more photos…

[G6Tanuki]

Neat!!!

That conductive silver-loaded paint has all sorts of uses. I have employed it to re-connect the grid wire to the top-cap of a valve; the cap had been pulled/twisted off by a previous owner and the remaining bit of lead-out was level with the glass 'pip'. Some people suggested trying to melt the glass down a bit but I was paranoid about the thing either cracking or sucking-in, so a little blob of Elecolit-340 to enclose a small loop of wire, followed by some Araldite, and an EF39 got to live another day!

[The Philpott]

Repair above and beyond. The cost of conductive silver paint has always been an irritant- when you see the price of scrap silver it's easy to believe they're trading on the magic and esoteric name of the holy metal. (Next time you come to use it- it's dried out. Best to sniff it when new to find out what the base solvent is!)

Araldite sticks REALLY well to glass. Not what i expected when i first tried it.

Dave

[Lloyd 1985]

Cheers

It is good stuff, so many uses! I'm sure I have also used it on a valve before, can't remember which one now though! I have used it on another LCD before this, I have a Belkin FM transmitter thing, and the display was connected with that horrible plastic tape with black conductive lines on it, and of course it had started coming off, so I replaced the whole thing with enamelled wire and conductive paint, that got it back to working order!

As for the solvent, I think the main solvent is acetone, it certainly disolves it, my own bottle of it is quite old now, it says it was made in 2019, and to revive it I chucked some celulose thinners in it! I keep shaking it every so often just so it doesn't settle.

[mark_in_manc]

Really nice - well done. And thanks for sharing it - had I encountered this, it would have fallen into my 'it's a black box and it's ******' mental pigeon hole - seeing unusual repairs which I would not have thought of is very useful (and would be more so if my memory was more reliable!).

And having checked out the price of silver paint - I am wondering about experimenting with powdered graphite and also aluminium with varnish to see if I can make anything which works!

[McMurdo]

a spirited effort, impressive!

That meter looks like it started out as an analogue meter and the designers fitted new lcd innards to eke out the mouldings life! Wonder if there's a zeroing screw hole hidden under the front decal.

[Lloyd 1985]

Actually, there is a small round hole behind the dark bit of the front! It’s not central though, it’s almost in line with the small control.

[pmmunro]

Originally Posted by McMurdo

a spirited effort, impressive!

That meter looks like it started out as an analogue meter and the designers fitted new lcd innards to eke out the mouldings life! Wonder if there's a zeroing screw hole hidden under the front decal.

Avo used the same basic case moulding for several instruments one of which was the Model 10 analogue meter. (Attachment 1) The colour chosen for the front panel moulding could possibly be described as "nicotine staining" and might well be mistaken for just that but a "BNIB" sample is more or less the same. This meter was later marketed as the Megger BM10 in black and yellow. The Model 10 may have been for export only originally but as far as I can judge was contenporary with the DA116.

There was also the auto-ranging version of the DA116, the DA117. For some reason the DA117 appears to have been more rarely advertised and is now less common; possibly the design gave problems. My only DA117 sample came suffering from LCD "bleeding" and I haven't been able to find a replacement so I have no experience of using it.

PMM

[Uncle Bulgaria]

Well done Lloyd, that's neat and satisfying to have a mechanical solution to a micro and knotty problem!

[Lloyd 1985]

I may have spoke too soon about it being fixed, the last few days I have noticed it doesn’t like being cold, then again, who does?! It seems to keep loosing a segment on the 3rd digit, but once it warms up it comes back and works as normal, I could happily put up with it, but I might as well have a poke around!

[Backtoreality]

IIRC that was the original fault Lloyd! I have to say I'm impressed with your display repairs, I just thought it would need a replacement display and I had missed out on an auction one by a week!
David

[Lloyd 1985]

Oh, it’s always annoying when you miss out like that! I still haven’t got round to investigating further, it seems to have warmed up a bit over the last week, so the fault hasn’t come back! Maybe I’ll have to bung it in the fridge to induce the fault again!