AVO 8 mk5 problems

[ORAWA01]

Hi All

Just got my first AVO meter. It has all the batteries at the back, and they are in good state with the right voltages.
But when checking DC voltages, and Ohms, the meter doesn't respond at all.

The movement of the meter seems OK. It moves about when tilted, and the needle comes back to the zero position softly.
The leads are checked, and they are OK works fine on other MMs.

What would be the steps to check for the problems?

Thanks

[Sinewave]

Quick check, does the meter movement move very very freely on all ranges? Or will it move very freely with right hand control set to DC and left to 0.05mA range, then dampen up nicely when the left hand is set to off?

[ORAWA01]

The hand movement was not much free. When the unit is tilted, it slowly & reluctantly moved forward about an inch, then came back softly.
When the left dial was set to off, the movement was not happening at all even when tilted.

Will do further checks with the dial settings as suggested, and update later when done. Thanks.

[pmmunro]

The traditional Avometer, including the Model 8 Mark V, is a "passive" instrument on all but the resistance ranges for which the 1.5V cell and the 15V battery are needed.

If you don't have a circuit diagram, it would be best to get hold of one and have a good study of it. Two versions are attached below. From what you write, I'm assuming that you don't have a lot of experience of these meters; please accept my apologies if this is not correct.

The basic instrument is a microammeter. The moving coil coil and its swamp have a resistance of 2666.7 ohms and the sensitivity (for full scale deflection) is 37.5 microamperes. R9 to R15 form a "universal shunt" with a resistance of 8000 ohms which is connected across the moving coil and swamp on all DC ranges. The current in the shunt will thus be 12.5 microamperes at fsd making 50 microamperes in total. The sections of the shunt select the current for fsd on each range. Note that the movement has 100mV across it for fsd.

To measure voltage, resistors R16 to R24 provide a chain of "multipliers". At 50 microamperes, R16 will drop 2.9 volts so that again there is 100mV across the movement. Each of the other resistors in the multiplier chain drops the difference between the selected voltage range and the next lowest one when fsd current flows.

If the current ranges work correctly but the voltage ranges do not, it is probably R16 which is open circuit. If some of the voltage ranges work, the multiplier resistor for the highest faulty range will be open-circuit.

If neither current nor voltage ranges work, it is possibly the function switches or flexible PCB tracks which are faulty. The function switches are the printed switches on the bottom board of the meter. If segments are damaged, the only answer is a replacement board which would have to be from a donor meter unless you can devise a means of repairing the tracks or pads. The interconnections use flexible printed circuits which are easily damaged by excessive flexing - damage here is quite common. These flexible circuits need care in soldering but broken tracks can be by-passed with conventional wiring. If several tracks are damaged it is probably best to make up a small conventional harnes to replace the faulty flexible circuit.

A service manual is available either on line or one can be supplied by a forum member.

PMM

[Superscope]

I think the first thing I would check if you haven't already done so, is
the Test Leads you are using.
If they are the original AVO ones, they are very prone to intermittent or
High Impedance connections, especially if you are using the Conical Probe ends.

Try to check all the Ranges if possible, AC, DC Current and Voltage and Resistance.

If all Ranges are completely Dead, the worse case scenario is that your Meter Movement
is open Circuit, but if your lucky, it might be a broken connection elsewhere.

Check the Fuse, although I don't think the Fuse on a Mk V covers the Voltage Ranges.



Ian

[ORAWA01]

Yes, this is my very first AVO meter, so everything is unfamiliar to me.

Will carry out further checks on the movement with the dial settings, and if no progress, then will have to open it up, and have a good look inside on the PCB and the switch connections too? And will check those resistors, if they are in good condition.

Thank you for the schematics.

[ORAWA01]

Yes, the fuse is there next to the D size battery, and it is OK.
I just tried DCV and ohms readings last night, and absolutely no movement on all settings.
I shall try ACV and Ampere checks tonight.

The leads used last night were from my other Analogue Meter, and it is very cheaply made one, but it fitted OK. The leads work ok with other meters, so it must be the AVO8 problem, I thought.

[Boulevardier]

Just a silly thought - because you say it is your first Avo 8. Have you checked that the overload cut-out button hasn't popped up? That's happened to me, and I haven't noticed it.

Mike

[ajgriff]

To save you the trouble of looking for it a PDF copy of the manual is attached.

Alan

[ORAWA01]

Good point.

The Cutout point button is there, but not doing much when pressed. I tried pulling it out, but it felt as if it is not connected to anything. It just stayed pulled out not doing much, and nothing was happening, so I pushed it back.

I was wondering what it was for.

[ORAWA01]

Originally Posted by ajgriff

To save you the trouble of looking for it a PDF copy of the manual is attached.

Thank you for the full manual.

[Sinewave]

Sounds like your cut out is fine. Don't pull up the cut out, you could break it. My son decided to do that to one of mine, so there's another Avo of mine 'for parts'.

[pmmunro]

Sinewave,

What parts of what model of Avometer cut-out are broken? Rather than scrap it, I could have a look and see if I have the parts you need. Send me a private message if you want,

PMM.

[Sinewave]

Originally Posted by pmmunro

Sinewave,

What parts of what model of Avometer cut-out are broken? Rather than scrap it, I could have a look and see if I have the parts you need. Send me a private message if you want,

PMM.

Thanks, however I've plenty others which work, so I haven't even looked inside this one. Perhaps one day I'll get around to it.

[ORAWA01]

I have tried to measure ACV, DCV, Ohms and Currents, but nothing worked. No movement. It is stone still.
I also connected my Insulation Resistance Meter, and sent 500V DC into the AVO8, but noting.
The insulation meter reads OL, no connection.

On my other DMM with the Insulation Meter, the DMM reads 561V DC, and the insulation meter reads, 10.8M ohms.

[Superscope]

I think you will need to check the Movement directly.
Bare in mind, that if it is Ok, it only needs 37.5 uA for FSD.
Anything significantly higher than that risks damaging the Movement if it is not already OC.

As I alluded to earlier, the worse case scenario is that the Movement is OC.
Unfortunately, if that is the case, your AVO will be a Spares Mule going forward, as you
are unlikely to find a Spare Working Movement. Not impossible, but highly unlikely at a sensible price.

Isolate the movement and feed it with anything up to 37.5 uA and see if it responds.
A quick dab across the Movement with a Multimeter on Ohms range to see if there is pointer movement would be Ok, but make it a brief Dab as the current supplied by the Multimeter might be relatively high for the movement.

In Summary, if the Movement is Faulty, your AVO is unlikely to be economically repairable.
If, on testing the movement you get a response, then we can take a step by step look
on where the Circuit Fault might be.

Let us know how you get on.


Ian.

[ORAWA01]

Thanks for your detailed info and advice.
I will watch a few youtube videos on AVO8 servicing, and will try to assess the problem further.

Will update with any progress or findings.

Thank you.

[stuarth]

If an open circuit movement was the only problem, the AVO wouldn't read open circuit at the terminals on voltage ranges as reported, the universal shunt would give some continuity. In fact, the resistance into the terminals on DC voltage or the higher DC current ranges ranges would be almost unaffected by an open circuit movement.

Open circuit at the terminals is more likely to be a switch, flexible PCB, fuse, or cutout problem. It's worth trying to measure the resistance into the AVO terminals on all DC voltage and current ranges. If they are all open circuit, you could set the AVO to 10V DC, connect a battery, eg a PP3 to the terminals, then use your DVM as a signal tracer to probe inside the AVO to find where the open circuit actually is.

Stuart

[ORAWA01]

Great info and advice, Thank you.
Will try that out, and see how it goes.

[Sinewave]

If you're working on this tonight I could put some photos up if you need any assistance.