AVO 7 voltage reading low.
 

Afternoon, first post here. I've been tidying up my workshop and stuck on the back of a shelf was my father's Avo7 meter, which must be at least 60 years old as I can remember watching him use it as a small boy.

Sentimentality stops me from throwing it away and I'd intended to put it on Ebay in the hope that it might be of use to someone - checking completed listings I don't expect to make a fortune, but at least it wouldn't end up in landfill.

I know nothing about the Avometer and very little about testing them, but Google advised me that even without batteries fitted the Avo should (if working) indicate DC voltage. I've just tried it with a brand new 9 volt and a 1.5 volt battery and the 9 volt battery is just under 9 and the 1.5 one only about 1. My digital meter reckons the actual readings are nearer 9.5 and 1.5 volts respectively.

I was just curious as to whether this indicates damage inside the meter - if I'm going to sell it I'd like to be honest about what I'm selling.

Thanks for your time.

Alan C

Re: Avo 7 voltage reading low
 

Hi Alan,

Welcome to the forum.

The first thing I would check is the Test Leads.
If you were using the original Avo ones, they are known
to sometimes go High Resistance and this would certainly
reduce the readings.


If the Test Leads check out Ok (Test them on your other Meter)
then check if all the other DC Voltage ranges give similar reduced Readings.
If they do, try exercising the rotary Range Switches and the Q Knob.
The Q Knob needs to be pulled out first before it can rotate, but
exercise the Pull out as well.

If you can give us the Serial Number, we can tell you the exact
Year the Meter left the Factory.

If it's not the Leads, or dirty Contacts sucessfully cleaned by exercising,
then sadly yes, the fault will likely be a dirty contact inside requiring a
little physical help in the cleaning department.
The Classic Avometer is basically just a bunch of Switch contacts.

If you fancy the challenge, I'm sure we could talk you through a
few checks to isolate the problem.

Definately don't throw it away.
The Model 7 is still a useful Device to have around.



Ian

Re: Avo 7 voltage reading low
 

^^^ Wot Ian said. It's very likely to be fixable.
Exercising the Q knob may expose a connectivity problem..that's an easy one to remedy, but if the stacks of leaf switches that are operated by the rotary range selector switches are out of alignment that's a slightly more fiddly job.

One thing against disposal on Ebay/Etsy (or anywhere else online for that matter) is that these meters can end up being modified into table lamps, bluetooth speaker modules or nixie clocks. These buyers are not really the sentimental type, it's merely a means to a profit. If you're bothered about what happens to it this is something to bear in mind.

As you already have a digital meter you may find that this Model 7 complements it or acts as a back-up and second opinion. We should be able to find you a scan of the manual for it, (but this won't tell you how to fix the low readings!)

Dave

Re: Avo 7 voltage reading low
 

Ian,

Thank you for the welcome and comments.

Initially I did try with the original - very past their sell by date -leads, then tried with two fresh short pieces of wire, no difference in the readings at all.

Just tried exercising the Q knob but again no difference. I don't mind a challenge at all, please tell me where to start. I've had it to pieces once already to remove the rusted in 50 year old D battery that my father had left wired in!

One question - there is a screw on the lower right side of the case side which I think had a washer of some sort, and which I seem to have mislaid. The screw did not appear to be a case fixing screw, more an adjustment type - does its omission matter?

Serial number ( I think - on bottom of face) 7946 -312230

Regards

Alan C

Re: Avo 7 voltage reading low
 

The little grub screw with the 'top-hat' washer was a tamper evident seal so that the factory could tell whether the meter had been opened whilst still under warranty. It was filled with black pitch at the factory, which had to be dug out to reach the grub screw.

I believe your serial number relates to a Model 7 from September 1946.

Dave

Re: Avo 7 voltage reading low
 

-Are you OK with measuring mains voltage on the 400v AC range without endangering yourself? If you can get a reading of around 240v AC then i think that the low readings on DC volts are likely to be dirty or maladjusted leaf contacts on one of the rotary selectors.

It's possible to study how they move with the meter opened and disconnected from all sources of power- rotate the switches and have a look under good lighting.They may need cleaning (isopropyl alcohol on a strip of rough card, the card inside a loo roll is abrasive enough for this) Don't use abrasive sandpaper or nail board- it's too aggressive.

I can put a couple of photos on to make this clearer.

Dave

Re: Avo 7 voltage reading low
 

-This is a Model 8, but same principle. As you can see a piece of paper behind the switch stacks (together with good light and magnification) is needed. (There will be more than one stack of switch contacts in there- there's 3 or 4.)

With the meter open it's sometimes possible with the use of a mirror to tap a leaf switch with a cocktail stick while measuring a low DC voltage, and see the reading fluctuate- then you have found where your problem is.

I'm not guaranteeing these switches are the reason for your low readings, but it's a likely candidate.

Dave

Re: Avo 7 voltage reading low
 

Dave, Yes I’m quite happy to have a go at measuring AC, will try tomorrow and depending on the result have a go at cleaning.

Once again, appreciate your time in responding.

Alan C

Re: Avo 7 voltage reading low
 

No problem. Occasionally i've had to bend one of the leaves slightly to get a good contact, sometimes accessibility for this can be a problem...but we'll burn that bridge when we come to it!

Dave

Re: AVO 7 voltage reading low.
 

One thing that has not been checked is the condition of the batteries used to check the voltage ranges.
A flat battery may well read good on a modern DVM.
Try the batteries with both meters at once just to be sure.
Service manuals for valve radios give voltages with an allowance for the test sample an analogue meter takes from the circuit. Modern DVMs often read higher.

Re: AVO 7 voltage reading low.
 

If it's any help here is the circuit of the Avo 7.

Peter

Re: AVO 7 voltage reading low.
 

Morning,

Tried measuring AC voltage - digital meter reading 253 volts, Avometer nothing at all (I did change the dials over before anyone asks!). Dumb question though, I'm assuming as for DC measurement, you don't need batteries fitted inside the meter??

Also checked the 9 volt battery again and it was reading 9.7 volts on the digital, but only 7 on the Avometer.

Any suggestions?

Thanks

Alan C

Re: AVO 7 voltage reading low.
 

Good point about the batteries, I had excluded that on the basis that the batteries on test were quoted brand new- but it's always worth a simultaneous meter check.

My photo accidentally captured a black cylindrical spacer between certain leaves, which is prone to make a break for freedom if the switch is mechanically 'adjusted'. I think that the Model 7 has at least one of these- the circuit diagram suggests so.

Dave

Re: AVO 7 voltage reading low.
 

Just an update, took meter from case to take the photos below and ses what was involved with cleaning (looks difficult!), while it was out of case tried testing 9 volt and 1.5 volt battery and this time got good readings (nearly 10 volts and 1.5). Tried AC mains but no reading. Put back in case and connected up 1.5 volt battery to the internal battery compartment in case it made a difference reading AC (just clutching at straws as you will see), again no reading.

Attach a set of photos for judgement - I've no idea if this is normal condition or "it's wrecked, just throw away":)

Re: AVO 7 voltage reading low.
 

Post No-12

Hi Alan, no batteries are needed for any of the AC/DC current and voltage ranges- only the resistance ranges.

-There could be more than one of those miniature switch contacts that are not making or breaking properly..i would explore this before thinking about other components.

The wandering reading when you input 9.7v DC hints very strongly at switching, BUT ALSO:
examine what is happening with the Q potentiometer mechanism- on my model 7 the tiny silver spike in the end of the prod had come loose (but the leaf contact under it could also be corroded or dull in your meter)

Dave

Re: AVO 7 voltage reading low.
 

The Q prod in exploded form.

The silver contact in the end isn't supposed to come out- this one came loose and the meter readings were all over the place.

Dave

Re: AVO 7 voltage reading low.
 

- A non-invasive way of diagnosing an intermittent leaf-switch problem. While measuring something grab the rotary switch at 12 o'clock and 6 o'clock, push at the top and pull at the bottom, then vice versa. Repeat with the other switch.
Although there's only a few thou of movement, sometimes this is enough to disturb the leaf switches.

Dave

Re: AVO 7 voltage reading low.
 

Hi Alan, i don't think Post No.14 showed up when i did my contributions earlier today, so my comments may seem out of sequence.

The meter is a bit grubbier and contaminated inside than we'd hope, but it's not a dead loss. There's some modern looking insulation in there so it's had some attention in the past. The general appearance backs up the age of the meter. Cleaning up the black horseshoe magnet with blu-tak then dabbing on an uber-thin layer of 3 in 1 oil with a cotton bud to prevent further rusting would be a good idea to start with. Don't go near the actual meter movement.

(Cotton buds amongst the wiring are not a good idea as they're too 'grabby' and it's easy to fracture the thinner wires)

I'm glad you're getting intermittent results on your DC voltage test- it means it can be fixed!

Dave

Re: AVO 7 voltage reading low.
 

Note that your first photo shows the bimetal coil that is responsible for thermal compensation. It's purely mechanical and a bit cranky, the wiper arm can be seen to move immediately even if you just breathe warm air on it.
Dave

Re: AVO 7 voltage reading low.
 

Confession time….that could be because 60 years ago a small Alchad played with his father’s Avometer , can’t remember what I did, but it had to be sent away for repair. I was not flavour of the month:).

Alan C