AVO valve tester meter servicing

[HBWOODY]

It seems like there is an intermittent short on one of the hairspring connections. Do not think my hand is ready enough to attempt desoldering.
Can an appropriate swamp resistor purchased?

[Dekatron]

If the moving coil resistance is close to 3250 Ohm in your meter then you can't fit a swamp resistor as it should be closer to 2600 Ohm to fit an appropriate swamp resistor with the correct resistance to counteract the resistance change in the moving coil when it is heated.

[HBWOODY]

Measured the meter resistance and worked out the correct value of the shunt resistor and the needle does not reach FSD at 30ua. Am I correct in thinking the magnetic flux is low and that I will need to fit a pair of neodymium magnets? If the answer to that question is yes is there any information in this forum to help me fit these magnets? I have clicked on Mr Yutaka links but they do not seem to be working.

[Station X]

Have you tried moving the magnetic shunt?

[HBWOODY]

Yes, moving the magnetic shunt helped a little but not enough.

[Dekatron]

What resistance did you get on the moving coil?

What current does it need to reach full swing (FSD)?

Any chance of a photo of the meter movment in question?

[HBWOODY]

The meter measured 3044 ohms.
I could get a FSD with 30ua without the addition of the shunt. With the shunt the needle was in the area of the 1 mAv line.
I will open up the meter and take a photo tomorrow and check to see what was needed to achieve the FSD.

[Station X]

A shunt should increase the FSD. That's what shunts are for. To decrease the FSD for a given voltage a series resistor is needed.

[HBWOODY]

Will go through the meter checking procedure again tomorrow. I must be doing something wrong.

[Station X]

Are you referring to the 10k shunt? That shunts the meter to give an FSD of around 40uA. Send 30uA through the combination and you'll get 75% of FSD, which isn't far off the 1mA/V line.

[HBWOODY]

Yes it is the 10K shunt I was referring to which in my case is is 7.9K due to the meter's resistance. Thank you for pointing that out. This morning I will check how many ua are needed to get it to FSD.
Now having to deal with another problem, the needle is not moving to the Set position when I switch the Mk3 on. Did voltage checks, heaters and anodes which were correct then when attempting the grid voltage setting up could not find the expected voltages to adjust the neg grid volts, hopefully an easy one to sort out. Will check that part of the circuit when I get a chance.

[Station X]

The bare meter movement should have an FSD of 30uA, which in post #27 you say you has been achieved. Given that its resistance is 3044R a swamp resistor of 206R will be needed to bring the resistance up to 3250R. 206R seems rather low though, something around 600R is more usual.

Shunt this with 10k and you should get an FSD of 39.75uA which is correct.

[HBWOODY]

Ran through the meter test again results are:-
FSD 33uA
Meter resistance 2453 ohms
Shunt. 18.5K

[Dekatron]

I think that you should test your meter with the meter test circuit I have included, without any swamp-resistor (series resistor) or shunt resistor in place so that you get a correct figure for just the meter movement (if you haven't already done so).

This will give you correct figures to work from to get any series resistor or shunt resistor correct for your particular meter.

[Station X]

So the meter's resistance has changed from 3044R (post #27) to 2453R ( post #33) and the shunt resistor has changed from 7.9k to 18.5k?

[HBWOODY]

the discrepancies are down to my inability to get my head round the maths etc.
I have assembled Dekatrons meter tester circuit and will do exactly as suggested by DeKatron.
No doubt the result will be different as I did include the swamp resistor when attempting earlier measurements
Graham you will have to excuse my ignorance in these matters. I am learning
(slowly) and hopefully the next set of measurements will be the ones which will make sense to you experts whom without your help I would never even got this far and believe me I am ever so grateful for all the help I have had on this forum.

[Dekatron]

The maths for the meter is not that complicated, I'll try to explain it below:

Meter internal resistance according to AVO: 3250 Ohm (moving coil plus swamp resistor)
Meter FSD according to AVO: 30uA

This results in a voltage drop of some 97.5mV across the meter, which we need to keep even after adjusting it with series resistors and/or shunt resistors.

Adding the external 10 kOhm shunt means that 9.75uA flows through this shunt (97.5mV / 10 kOhm = 9.75uA). The resistance seen by the circuit is the internal meter resistance in parallel with the 10 kOhm shunt which results in a resistance of approximately 2453 Ohm ( 3250*10000/(3250+10000) = 2452.8302....).

This means that a meter with the 10 kOhm shunt in parallel will draw 39.75uA (30+9.75uA) and will have a total resistance of approximately 2453 Ohm.

If your meter has an FSD below some 39uA and an internal resistance below some 2500 Ohm (39uA * 2500 = 97.5mV, the limit of course being 39.75uA and 2452.8 Ohm which also results in 97.5mV) it is usually still possible to use it with correct series resistor and shunt resistor but you'll have to calculate these values properly and you might have to unwind some turns from the swamp resistor (if it is in place) - there are of course situations where FSD current multiplied with the moving coil resistance is outside of the total of 97.5mV and in these cases you'll have to use an opamp or adjust the magnetic shunt or add magnets.

[Station X]

I'll just add to that, that the 10k shunt is an integral part of the tester and with a normal meter there's no need to calculate its value, it's always 10k.

The swamp resistor on the other hand is inside the meter and its value will need to be calculated to give a total resistance of 3250 Ohms without the shunt.

[HBWOODY]

Meter movement 2269 ohms.
Swamp resistor 675 ohms.
FSD with 30uA current.
So with a shunt of 10K and a resistor of 306 ohms wired in series the meter will perform as it should.
I will give it try out this evening.
It was thinking that the shunt had to be calculated which was confusing me so thanks for pointing that out.
Thank you again for your help and patience.

[Dekatron]

Did you find a swamp resistor? Because in post #18 you said your meter didn't have one.

Otherwise your calculation is correct as long as the internal shunt resistor in the AVO stil has the correct value of 10 kOhm.