I like to repair old broken test equipment. I have a small collection (about twenty items) of multimeters, scopes, function generators and some other odd pieces. I got most of them from ebay, broken, and then fixed them to use them. More recently however I have just been fixing them for fun and don't use them much. I have mentioned this before on my blog, when I restored a multimeter from the 1940s (the oldest in my collection). In that case I was focussing more on the wooden case which is unusual for me. I'm usually just doing electrical repairs on them. You can see it here.
Anyway, this post is about my AVO DA116. I did this repair a while ago and almost forgot to post about it. I bought a broken AVO DA116 from ebay. For those who don't know; AVO made a few digital multimeters before the name AVO went away. So far I have three digital AVOs. I now have two of them fully working, the other uses nixie tubes and still needs fixing.
Here is a quick pic of the AVO after the fix.
So, what was wrong with this meter? When I first tested it, I switched it on, but couldn't see anything on the screen. It was as if there was simply no power to the meter at all. I spent about half an hour trying to debug what I thought was a power issue. I was looking through the manual at the schematics (yes this meter came from that era) to try and find where a power fault might occur. After checking all of the outputs of the power board, I concluded that all of the power circuitry was working fine. Then I noticed that the LCD looked rather dark. I almost couldn't believe that I hadn't noticed that the LCD had leaked. If you don't know how LCDs work, go google it now. It seemed that the liquid in this LCD had simply covered the entire screen. You could just about make out the difference between a segment that was on and one that was off if you caught the light correctly. This was the only fault with the meter, I checked all the ranges I could, they worked fine. The only problem was that it was almost impossible to read the LCD.
At this point I thought that it was game over. I'll never find a replacement for that particular LCD. Until this point I had been probing around at the power board at the back of the meter. This is easy to access with the back removed. I needed to get to the LCD to see what could be done. Unfortunately this AVO wasn't designed for reparability. To get to the PCB with the LCD on it you have to cut or desolder some wires.
Once I was in, I found that it was fairly easy to remove the dead LCD. It was sat in a kind of free-form socket with solder on the four corner pins. It took a little effort desoldering but most of the socket pins were untouched. I also found that the LCD was rather large and that if I could get a smaller one with all the same segments on it then I could make an adapter that would allow it to fit in the same socket. So I found some on farnell and ordered five of them. They'll be handy for other projects too and I expected to fail the first time.
These LCDs were slightly shorter and much narrower than the dead one so they could fit on an adapter board. The next step was to make a map of which pins controlled which segments. Then I could use the diagram from the datasheet for the new LCD to figure out how to wire up an adapter board to go between the new LCD and the existing socket.
So, I put the LCD on some pad board along with two rows of headers to match up with the socket. To test the theory that it would work I wired up the common line first and then wired up the minus sign segment to see if I could get that to light up.
After seeing that this worked, I wired up the rest of the LCD. I did this with some really thin enamelled copper wire. It's not the easiest thing to work with when soldering, it took me more than an hour to wire it all up.
I had to make at least one mistake. A dry joint on one of the pins made this segment not light up. An easy fix.
The new screen is almost perfect. I think the decimal points are driven slightly differently and so the contrast is not as good on these segments with the new LCD. It's still easy to read though. With the addition of a small section of grey card I was able to hide the fact that this screen was narrower and would allow you to see the PCB behind it. Now that I had a readable LCD in place, I needed to reconnect all of the wires that I cut to get into the case. I wanted to leave this in a repairable state so I soldered connectors on instead of just splicing the wires in place. This may cause a slight difference in some high current measurements, but I can calibrate it if I need to.
I can't remember how much I paid for it now, but it was fairly cheap as it was sold 'untested'. Now it is fully working and it fits great with my collection. Here are some pictures of the meter after the repair was complete.