AVO DA112 - HELP schematics & manual

[Vincent]

Hello everybody.

This is my first post but I have been a reader of this forum for a quite a while.
What a fantastic resource and such a knowledgeable and helpful community!

But back to the subject.

The patient - one of AVO`s very early digital multimeters - DA 112
Health status of the patient - well... more dead than alive as it stands now.

It has been acquired from a well-known auction site. was quite visible form the photos that its condition was quite bad, apparently not working but nevertheless, as such an item is quite rare, I decided to take a plunge.

Upon arrival... here I must express my very special "gratitude" to the carrier - the only smashed parcels that I ever had all were handled by them.
And so it was this time to add insult to injury so to speak, in addition to quite poor original condition, now I got its case and internal fixings - all broken into pieces.

although it all was quite disappointing, it got me even more encouraged to restore this grandpa.

so leaving the smashed casing issues aside, further internal inspection revealed:

1. the rechargeable battery heavily leaked
2. the meter must have been kept in quite a dumped atmosphere (if not partly submerged in water at some point) - hence the general rust patches everywhere
3. seems that moisturized air and leaking acid from the battery - created some mist that got onto the majority of the elements, resulting in:
- corroded contacts
- corroded elements. particularly electrolytic` aluminum cans (and check out that coil`s shield on one of the photos below!)
- the solder - the chemical reaction with the mist created a thick layer of, dunno how to call it properly but for simplicity let`s call it "oxidation", that you can`t even desolder components as the heat from soldering iron (40W!) doesn`t penetrate thru it at all.

house of horrors...

what has been done so far:

1. smashed case - put together to the best possible condition (there were tons of very tiny bits that was virtually impossible to put properly together and some of them were even lost), epoxy does a great job.
2. general rust form the fascia and internally - scrubbed, treated with phosphoric acid, and lacquered where required
3. Electrolytics - although some checked OK, all have been replaced (as their cans have started corroding away anyway)
4. PCB examined for dry joints (remember the huge oxidation on the solder) - fixed a few. here I must say that back then they did not try to save on solder so all joints have quite a generous amount of it and although the oxidation level is thick, underneath the contacts usually look very well soldered on.
5. Some paper in foil caps will be replaces - got them on order.

NO attempts to switch it on have been made. A bit too afraid to be frank.
Not afraid as in "scared" but afraid like in "disappointed" as by the look of it the chances are it will blow up...

Due to the corrosive atmosphere it was kept in, literally all the components need checking out and the oxidation layer on the solder doesn`t make the job of unsoldering (e.g. to lift op one leg to take proper measurements) easier.

but again, i am not giving up, just yet. It is a piece of metrology history worth trying to save, there are not many of them left out there.

Hence, guys, need your help if anyone happens to have schematics and manual for this rare gem? Looking up on the Internet did not yield anything ...

I will keep updated on progress, it is bit slow, only get time to work on it on weekends, and probably will need to ask for some help along the way.

thx
Vince

PS and of course the photos, we all love them:

[Sinewave]

This is a very interesting meter being one with nixie tubes. I don't have one of these myself, but I hope to see yours being restored.

[Vintage_RC]

Check this thread, might be a lead to a copy of the manual:-
https://www.vintage-radio.net/forum/...d.php?t=133750

[Vincent]

Thank you Alan, I have come across that post. That`s pretty much all that pops up when googling for this model... anyways, I am hoping Jeremy & Peter will see this post, if not I`ll try to PM them.

w/o schematics it will be extremely difficult if not just impossible to restore. Markings on quite a few components (especially micas) have become unreadable so event replacing is difficult not knowing their values.

in the meantime, a few more photos.

last few paper capacitors are going to be replaced over the coming weekend and I think I will give it ago. just powering up w/o blowing up, letting alone proper readings, given its condition will be considered not less than a miracle.

I love that AVO, there is one for selling in Spain, but very expensive in my opinion, they are asking 190 euros.

I hope to see yours restored soon

[pmmunro]

Vincent,

Send me a private message with an email address where I can send you copies of the user and service manuals. They're too big to post here.

PMM

[Vincent]

Hi Peter

Sent you a PM.

Can`t thank you enough. seems that you`re the only one (probs in the whole world) who still has it!
Now I have now a glimpse of hope to get the old-timer back on track

Thank you!

Will keep you all posted. This weekend may be the day when it gets powered up in many many years.

[Vincent]

Alright guys, finally I got some time to work on the beast to move things a bit forward.
After last few caps went it and some extra finishing touches - such as detoxing switches etc, I gave it a go.

Well... contrary to what I widely expected - I did not blow up no magic smoke, sparks or anything, but started up rather smoothly.
The digits on the screen chaotically jumped for first 2-3 sec but then they all settled down at 0.00

Wow, I thought to myself, not even blew up but also zero`ed, what a decent start! so maybe, just maybe - it could actually work!
So set to DC, 10V range, stuck the probes in and grabbed my trusty 1.5v battery and... still the stable 0.00 on the screen
Switched over to Ohms, grabbed 56k resistor - and... still that nicely glowing zero.

see the pic.

Quick check on the continuity of the input signal – defo it reaches the boards…
A bit disappointing, but definitely not a surprise. Again, the fact that it started at all - is more than I expected.
Now, I guess, I will have a VERY long road ahead in trying to find what and where has gone faulty in there.

Now that I have the schematics (thanks to Peter!), there is a glimpse of hope; in their service manual Avo did a pretty good job in giving test points across the boards so is doable and finding a faulty component should be fairly straight forward, in theory. But what will make it 100 times harder is the state of the solder on the boards that is heavily oxidized, so on top of the suspected faulty components, literally any joint can be suspected as dried as well, and a ton of wires going across the boards - and their conducting state is also will need to be checked and confirmed.

to be continued....

[Vincent]

Hey guys

Has been a while since my last update.

A bit of good news. One little step forward has been made. The device is now displaying some numbers. it now constantly samples and shows random figures (something like 1.234) with decimal lamps jumping according to the range switch pressed but not affecting the actual figure.

Well - at least a sign of some live in it

but what was wrong?

The (creepy!) story starts here:

Upon checking the output voltages on the DC-DC converter (the one that supplies voltages for different parts of the device), it became apparent that they all were miles out of spec.

A fix was simple, but this was something that I would not imagine in my wildest dreams.

So a WARNING for those who`ll dare to embark on a similar journey with another example of this meter - the device MUST have a good NICD battery connected at ALL TIMES the device is switched on on mains.

I just threw away the badly leaked original battery and honestly was not even going to have one installed, as I was planning to leave it as a mains only meter.

The schematics were a bit tangled making hard to clearly see the exact way the battery is interconnected in the circuit, but after a bit of jumping around b/w different parts of schematics and drawing my own, to my amusement it became clear that the battery forms a leg of a voltage divider from the mains voltage source (not stabilised, just transformed and rectified) and w/o battery the voltage was circa 2x of that it should be according to the spec.

And to make things MUCH worse, this voltage then directly feeds a DC-DC converter whose range of outputs (something like 200, 100, 30 v) are in direct correlation with it. hence having 2x supply resulted in ridiculous outputs of the DC-DC.

So a battery was purchased and installed. The voltages were then back to being close to the spec so it is time to move on to the next stage - checking the reference voltages. Which, in my suspicion after staring at the circuit diagrams for a few days, could have something to do with all the zeroes I was seeing on the display as one of the ref voltages was fed into the A-D converter.

So having checked them - sure thing, they’re out. I mean completely out, just some mV.

What the... . Okay, thankfully not that many reactive components to check out on the board and here we go: a transistor and a Zener are busted.

Hm... not a big deal to change, replacements are at hand, so done in 15 mins. Powering up - and all the reference voltages are bang on and the screen starts showing numbers and sampling! good stuff

But the question about what caused it just kept me restless. Transistor and Zener giving up the ghost together - sounds like they got fried rather than just failed due to age or smith.

So here goes a little experiment. Removing the battery, power on and very rapid check of the reference voltage board. The supply voltage of the reference board comes from one of the outputs on the DC-DC converter (should be 27.5v) and as I already discovered - w/o battery the voltage is much higher (around 40v) to the extent that it can (and does if left for a while) fry up those transistor and zener. Nothing was fried during the experiment as it was very quick but during which the components became burning hot.

The pic of the fried transistor and zener circled in red - attached.

Now the very big question that I am left with - is that what else across the boards these abnormal voltages from DC-DC converted could have got fried up


---------------

So this is where I am at now.

I still can`t understand who on each came up with this battery dependent design.

Battery do fail, they degrade, they can go short, they can go open. How one can make the whole circuits supply voltages being dependable on that even to the extend when components can get fried - it is beyond my comprehension.

but I am not giving up just yet, show must go on.

Next stage is the A/D board. Quick check on some oscillating circuits check points in there shows nothing good on the o`scope.

to be continued...


PS and another thing to mention to whoever is gonna follow my steps. Whatever service manual you manage to get hold of - may have errors. I recall being stumped checking some voltages on the board against what`s in the doc until I have cross checked with a copy of another edition of the manual where the errors were corrected.