AVO meter Model 8 Mk2

[popshammett]

Hi, Just bought an AVO 8, fitted new batteries, and when I test on the ohms range (resistance & ohms) I expected the needle to go from left to right when leads shorted together, but needle only goes about 30% across the scale.
Also when measuring DC volts (battery) the reading is LOW ...
Any suggestions as to what may be causing this fault ...
Thanks in advance for any help or suggestion ... pops hammett

[The Philpott]

1- Sorry to be 'Captain Obvious', but have you tweaked the appropriate adjuster knob on the front to obtain full scale deflection of the needle as per the instruction plate on the rear?

2- How much is the DC volts reading falling short by? Depending on your reply it could be wear or maladjustment in one of the stacks of leaf switches.

Dave

[Superscope]

Hi,

Welcome to the Forum.

I suppose the first question is, have you got a Circuit Diagram for the
8 Mk II and how are you with pulling stuff apart and general fault finding?

The fault you describe, could be due to any number of Dirty contacts inside.
The Avo meter has dozens of brass contacts.

It has Three Leaf Switches (each having multiple contacts), a Rev MC Switch,
a Cut out Switch and thats before you get to the Rotary Range Selector Switches.
It could be any or all of these depending on what environment the Meter has
lived in most of it's life.

I would suggest starting on a Faulty low Voltage DC Range and simply working
through the Circuit Diagram for that one Range to find a High Impedence or
open circuit switch contact.


However, if you have access to a 50uA (or lower) Current Source, it would be
best to start on the DC 50uA Range as this will test the Meter Movement Directly
and remove most of the other components from the circuit.

Worst Case scenario is your Meter Movement has lost it's sensitivity, and
unfortunately it is unlikely you can get that repaired now.



Ian

[popshammett]

Thanks for the reply ... I have tried the three Pot's on the front for each of the ohms ranges, to no avail.

On the 50v DC range the 15v battery reads about 5v.

The needle moves freely when moved , and returns to zero position after.

Any help much appreciated ... pops hammett

[popshammett]

Thanks Ian, I will give your suggestion a try ... get back to you later today ... Pops

[Superscope]

Yes, let us know and we can work through it with you if necessary.
I assume you do have a circuit?
They are easily available for the Mk II.

Didn't want to put a Dampner on the Fault finding, so I should point
out, that low sensitivity Meter Movements are actually not that common,
they are quite robust for the most part.


It is most likely, you are going to find one or more dirty contacts, which is not
surprising really for a Meter now approaching 55-65 Years old.




Ian

[The Philpott]

{Good idea to test with the (very low) current source.}

When measuring the 15v DC source, you could try grabbing one of the rotary selector switches at 12 o'clock and 6 o'clock and rocking it that crucial 15 thou or so in and out of the facia..(then repeat with the other one) This can sometimes show up leaf switch issues, indicated by sudden needle movement up or down!

Ian has covered most other bases. It's always worth tapping the cutout switch if an errant reading is suspected.

Dave

[popshammett]

Hi Guys, So, I have cleaned the contacts on both rotary switches that operate in the ohms ranges (DC & AC Switches), also cleaned contacts on Cutout switch, and the Rev M/C button works.

All the components ie Resisters and coils etc look OK, nothing looks over heated or burnt, no dry joints that I can see or loose wires floating..

I do have a Model 8 Mk2 circuit diagram .....

Tried to rock the rotary switches but they are very firmly set ...

Look to me like I have a component failure, but where ??

I see there are diodes in both the 15v & 1.5v circuits, my electronics Know how is very limited, what are they for ??

Any more thoughts would be most welcome ....pops

One more question ... where are the leaf switches, only contacts I can see are part of the rotary switches ... Thanks ...pops

[The Philpott]

Yep, those are the leaves i refer to, operated by the cam rotors on the rear of the selectors.

My next thought was dry joints or parted joints but you've looked for them..
Dave

[Superscope]

Hi Pops,

The Diodes (Should be Two of them) are the rectifiers for the
AC ranges only.

Have you checked the Test Leads first? if they are original Avo ones,
these are known often to go open circuit or High Impedance.

Test as many Ranges as you can, including AC if you are happy to do so,
and make a note of which ones are reading Low.
This will help narrow down the Fault if only a few Ranges read Low.


You can't just look for a Dry Joint, they often look quite normal
from a visual inspection, but are failing electrically.

The Avo meter is simply a bunch of mostly Brass Switches and Resistances,
so faults on DC and Resistance Ranges can only be Dirty Switch contacts,
broken Wires or an insensitive Movement.

AC faults can be the above and/or faulty Transformer or Diodes.
But we will get to that later.


So, before we go any further, we need to know what Equipment you have
avalable for generating Current and Voltage, if any ?
I assume you have a Digital Multimeter available?

What you can then do, is set up a Test Voltage, select the suitable Range
on the Avo and work through the Circuit for that Range only with your
Multimeter and find out where the Voltage is higher than it should be.
i.e. You shouldn't be seeing Voltages (above a few uV) across any closed
Switch contacts. Likewise, you shouldn't see any Voltage across two points
that are supposed to be Hardwired together. That would indicate an open circuit
or High impedence joint.


I will try to Dig out a Mk II circuit later, then, if we know what Equipment you
have available, we can step through some tests

Ian

[Sinewave]

With meters like this, it is best to test the current ranges FSD first, you could really be chasing your tail looking for other issues if the movement has lost sensitivity.

[Superscope]

Yes, this was already mentioned in post #3, but since the
OP does not appear to have a Current source available,
which most people wouldn't have, they are going to have
to go the long way round and do the testing with Voltages.

Even an insensitive Movement can be revieled with basic
Voltage checks, but probably easier in this case to check out
the rest of the Meter first.



Ian

[Sinewave]

I know it was mentioned, but I mentioned it again as the OP could really be chasing his own tail with this one unless it's checked. He could make one with a psu and some resistors. I would say it's easier to check the movement first.

[The Philpott]

-If a second meter of the same design is available it's possible to parallel them then feed them both approx 25uA by passing current from a PP3 from (for example) left hand to right hand, using yourself as a variable resistor depending on the pressure from your fingers, the size of contact patch, length of path, etc.
This will show up a glaring inaccuracy.
Dave

[popshammett]

Many thanks for all the suggestions, Not sure I am up to understanding some though .... I do have a working AVO 8 Mk1, with new leads,

60 years ago I used AVO's when I was in the RAF, but time has taken a toll on my memory.

I bought this Mk2 in the hope of giving it to my grandson who has shown an interest in electronics, sadly as I said before I had hoped it worked, the seller said it worked 4 years ago, but sold it as untested.

Let me see what I can do with your current advice and I will let you know how I get on. Again many thanks for your help and interest. pops.

[stuarth]

Does your Mk1 AVO read correctly on a 9V or 15V battery?

If so, you can use it to do a basic test on your Mk2, the Mk1 and a battery provide a current source with a measured current below 50uA.

Set the Mk1 to a DC voltage range appropriate for measuring the battery voltage and connect it in series with the battery. This meter and battery pair are now a current source, you can read the current by taking the reading off the AVO 0-100 scale and halving it (the AVO 8 is 20kohm/volt, I.e. 50uA full scale.

Set the Mk2 to its 50uA range and connect it in series with the Mk1 and the battery.

The Mk1 should read slightly lower than it did when measuring the battery without the Mk2 in series because of the voltage drop across the Mk2. The Mk2 should have exactly the same needle deflection as the Mk1 because both meters are in series and have the same current flowing, and both will have an fsd of 50uA. If they have the same deflection, the suspect Mk2 movement and its shunt are OK. If the Mk2 reads lower than the Mk1, the problem lies with the movement or its shunt.

Let us know the result, and we can advise you further.

Stuart