AVO 8 question.

[The Philpott]

Odd. Did these meters arrive from the same source?

Dave

[sparkymike]

Hi Dave, yes they did ,strange to say. !! (Local to me.)
Mike.

[The Philpott]

Aha..So if the Fluke meter agrees, then perhaps whatever's been 'mended' has been mended twice, then the mender has run out of ideas. We'll see what happens when you've changed a rectifier!

Remember i'm not too far away.

Dave

[sparkymike]

I just downloaded the service instructions for the model 8, so that might be useful later. Loads of helpful tips in there.
Mike.

[sparkymike]

I did a resistance check from the left hand probe socket up to the rectifier and result was within the stated tolerance of the 99K and 150 K values.
Am I right in thinking that is nearly a direct route to the movement apart from the 670 coil and switch contacts.?
Next thing to do will be a good clean of the switch contacts. The reset had a film of corrosion on the points but now that reads good.
Mike.

[The Philpott]

I can't actually remember Mike, not having had this particular fault.

[sparkymike]

I am also wondering if the movement magnets have lost a bit of power.
This might cause meter needle to swing more than it should. Is that possible ?
I will change the rectifier out first before I try another movement.
Mike.

[pmmunro]

Originally Posted by sparkymike

I am also wondering if the movement magnets have lost a bit of power.
This might cause meter needle to swing more than it should. Is that possible ?
I will change the rectifier out first before I try another movement.
Mike.

It depends on what you mean by swing.

If you mean that the pointer oscillates more than normal before settling to the final value (reduced damping), it is very likely that the chain of resistors that should be shunting the movement is open circuit. The possible fault could either be an open circuit resistor or a switching fault. This would also give a high than correct deflection.

If you mean that the deflection of the pointer is less than normal, one possible cause could be loss of magnet flux. These meters have a magnetic shunt which can be adjusted to give the correct value of deflection, but only a range of a few percent.

PMM

[sparkymike]

I have now noticed that one of the two dial plate holding screws is missing, so I will be removing the movement to replace this. I don't know if there is enough room behind the plate for this screw to have dropped out on its own, so I suspect it has been got at before me.!!
Now while movement is on the bench, is there a method of checking it for accuracy.? ie. How did they do it at the factory.
One other non connected question, but something I have wondered at for some time.
How did the original pattern maker produce the mottled effect on the case front ? If it was done freehand, it must have taken days/weeks to do.
Mike.

[The Philpott]

I believe the facia finish is simply moulded. They had managed to get bakelite moulding to quite a sophisticated stage with fine detail. (Earlier ones are sometimes found to be warped, have inconsistent colour, little lumps in the surface or a spalled surface.)

Well the movement is a 37.5uA unit shunted up to 50uA; so have you the equipment to deliver very low current? (I have never tried this so get a second opinion in any case!)

The magnets were also fairly stable at this time of development, so unless they've suffered impact or been touched with ferrous tools things are hopefully within adjustment tolerance.

Dave

[Superscope]

Originally Posted by sparkymike

Now while movement is on the bench, is there a method of checking it for accuracy.? ie. How did they do it at the factory.

In a Word Yes!

There are Two different Tests that should be done on an Avo Movement .

First, is the Movement sensitivity, and this is achieved by checking that FSD is reached
exactly at 37.5uA

This requires an accurate Current Source at this level obviously.

Second, is the Scale Linearity, and this will require a small circuit that would need to be
accurately calibrated when built.
This checks the 10,20,30,40,50,60,70,80 & 90% Scale points once the 100% (FSD) is
confirmed.

The Factory would have done exactly the same way we would do it now.



Ian

[sparkymike]

Thanks for the info.
I did a resistance check with my digital meter by connecting it to the probe terminals on the Avo and set it to AC and got the following results.
1000Volt 1.017 Meg.
250 V 250K
100 V 99,6K
25 V 6.28K
2.5 V 65 ohms.

The actual movement plus the small bobbin coil mounted on movement frame
I measured at 3.3K

Both faulty AVO's read approx the same.
I would be interested to know how the above readings compare to a known good meter.
Mike.

[The Philpott]

1000v - 1.013 meg
250v - 254k
100v - 100.5k
25v - 24.7k
10v - 9.54k
2.5v - 245 ohm

Avo 8 Mk III

[pmmunro]

The basic movement and swamp characteristics are:
Current for fsd 37.5 microamps
resistance 3333 ohms
voltage for fsd 125mV

These have to be correct because all the other component values in the meter are calculated around these.

The sensitivity of the meter's alternating voltage ranges is 1000 ohms per volt/1mA (for fsd) except for the 2.5 V range which is 250 ohms per volt/10mA.

To judge if the values you are reading at the terminals are correct you would need to supply an alternating waveform and measure the voltage and current at the terminals. Using a digital multimeter, which will use a direct current for resistance measurements is unlikely to give a meaningful figure. It will vary between different DMMs anyway.

For the higher values of alternating voltage, multiplier resistances, R2, R3 & R4 in the attached diagram, will predominate.

PMM

[sparkymike]

I missed out the 10 volt which was 1.01K
Mike.

[sparkymike]

Thanks PPM for the AC circuit. That will be a great help.
I seem to remember seeing a picture with all the resistors/coils with id labels on them. I am sure it was one of the AVO's , but perhaps later type. I will have to look on my external overflow had drive.
Mike.

[stuarth]

Are we talking about two significantly different versions of the AVO 8 here?

The Philpott resistance values are consistent with 1000 ohms/volt (1mA) for AC voltage ranges from 10V to !000V fsd, and a step up transformer for the 2.5V range only, resulting in a lower ohms/volt value for that one range.

The circuit diagram in post 34 shows a step up transformer for 2.5V 10V and 25V ranges, so an impedance value lower than 1000 ohms/volt for those 3 ranges. Sparkymike's resistance values seem consistent with the circuit.

Stuart

[The Philpott]

A good point. The 8/II does indeed use the transformer for 2.5, 10 and 25v AC whereas the 8/III only uses the transformer for 2.5vAC. (There's a small capacitor across the 2.5v resistor also)

It seems they did take a thoroughly holistic approach to improvement, that is to say the difference between MkII and MkIII is not what i initially thought, ie just an extra fuse and more windings superceded by component resistors!

Dave

[stuarth]

So Sparkymike's resistance values are probably OK for his meters.

AVO might have made the changes because they introduced more modern rectifier diodes which did not need the voltage boost for a linear scale except on the lowest (2.5V) range.

If that's the case, the moral for us here is that you can change an old oxide rectifier for germnium diodes, but not the other way round. As if you would.

Stuart