AVO VCM MK3. Meter needle not moving when switched on.

[Dekatron]

Nice that it is almost working.

One easy check would be to move the fuse to see if you can reach the required voltage/current by just doing that, otherwise you will have to measure the resistance of the calibration resistor.

[HBWOODY]

I cleaned the fuse and checked the calibration resistor R19 and it measured good 2.96m.
I also did a insulation leakage test using a 1megohm resistor, the meter registered .5megohm on all tests.

[HBWOODY]

After cleaning the fuse and checking the calibration resistor the needle position did not change position.

[Dekatron]

Can you move the fuse to a lower setting, assuming it is at 240V now, or adjusting the mains setting switch one step up or down and see how it behaves then?

[HBWOODY]

Martin
Adjusting the SET switch one step up/one step down moves the needle a little, I can move the needle to the correct point by turning the SET switch 6 clicks to the left.
Tomorrow I will double check the meter resistance and work my way through the circuit again.

[HBWOODY]

Is the meter resistance of 3190ohms ok?

[HBWOODY]

I moved the mains tapping from 240/250volts to 220/230volts, the needle can now be adjusted to reach the SET point but the ANODE/SCREEN/HEATER volts are now too high and cannot be lowered enough using the SET switch.

[Dekatron]

The meter resistance is quite good, only 60 ohms from the stated 3250 Ohm - what is the FSD current and what is the current at the set AC line, the 1mA/V line and straight up at 50% deflection?

How much higher is the Anode/Screen voltages when the meter hits the set AC line?

[HBWOODY]

FSD = 30.5ua
SET = 27.5ua
I will have to modify the meter testing rig to measure the 1 ma/v and 50% deflection meter points. I have noticed that when testing valves the meter ua readings are very close to what are shown on the DMM over the whole meter range.
When I move the mains tap to 220/230volts I get Anode volt readings of 233volts when setting the Anode volts to 200volts position, 13volts higher than specified. These readings were taken when the meter needle had been adjusted to the correct meter SET point.
When using the 220/230volts tap should I have adjusted RV4 to 52volts before measuring the Anode/Screen volts?

[Dekatron]

In the service manual AVO specifies the Anode/screen voltages at -2/+6% tolerance which means that your 233V is just within that range.

When changing the fuse position you will have to redo the calibration as the voltages change for the circuit.

Check the relay contacts so they are clean and so the relay doesn't have high contact resistance through the path that the voltage takes. Also check the on/off switch for high contact resistance.

[HBWOODY]

I recalibrated using the 220/230volt tap and the needle reaches the 85ua on the scale which is the same result I get when calibrating using the 240/250volt tap.
I thoroughly cleaned the relay and checked the switch.

[Dekatron]

If you can correctly calibrate the Grid-volts control with the link open and the fuse either in the 230V position or the 240V position you have a problem that AVO realised in the AVO Mk IV in 1962 - some of the testers need to have the wire for the link connection to RV6 moved from the 125V tap to the 150V tap to be possible to calibrate with the link closed.

You can read about this change in the AVO Mk IV service manual where they have a "Table of component changes" where they specify this change.

So I suggest that you try and move the connection from the 125V tap to the 150V tap for the connection going to RV6 and see if that works for your tester.

I have never seen this change described in any of the AVO documents on the Mk III that I have but I have seen other people discussing it as a solution, but so far no one have reported back to me if they tried it and if it was a success so you are a little bit of a pioneer here if you try it and succeed.

However I do not see any reason for the solution not to work, especially since AVO used in in later Mk IVs and also because it is only about raising the voltage so that the calibration works for the leakage and insulation tests as it doesn't affect the Grid-volts control in any way as the voltage you raise is out of phase with the Grid-voltage.

[HBWOODY]

The 125volt feed to RV6 is taken from the 125volt contact on the Anode switch. Do I cut this connection and move it to the 150 contact on the Anode switch?

[HBWOODY]

I also did a insulation leakage test using a 1megohm resistor, the meter registered 0.5megohm on all tests instead of the required 1megohm. Could this be related to the problem of getting the needle to the SET point?

[Dekatron]

Yes, you can try and move the wire on the switch.

Yes, the Leakage test will probably have to do with this.

[HBWOODY]

The 150volt tap was on the Screen volt switch so I connected it to RV6. Now the needle can be placed in the Set position, Screen/Anode/Heater volts as they should be and it is now calibrated and ready to check some valves. Thank you Martin for getting me to this point. I will look into the leakage/insulation problems next. Hopefully I am nearly finished this project.
My tractor is now working but also needs a little tuning
Haydn

[Dekatron]

Nice!

I'll keep my fingers crossed that the leakage tests show a better measurement than before.

[Dekatron]

For a correctly calibrated AVO Mk III or Mk IV the voltage across the circuit with the 2.96 Meg resistor, the meter, the 200 Ohm measurement resistor and the 1 Meg resistor that you want to check when performing the Leakage test is approximately 106V. You can measure this voltage if you want to make sure that your tester has this voltage - measure from the junction of RV4/R6 to the junction of R3/R4 with the link closed.

The current flowing through this circuit is then some 26.75 uA (106/(2.96M+2453+200+1M)). The spot on the 0-100mA scale that corresponds to 26.75uA is approximately 67 (26.75/39.75 FSD) which is almost spot on for the 1 Meg leakage position. You can measure this current to if you insert a meter in the circuit - just put it in series with the 1 Meg resistor.

[HBWOODY]

Thank you for letting me know how to proceed with the leakage problem.
I have measured from the junction of RV4/R6 to the junction of R3/R4 with the link closed and the voltage is 119volts and current 28.5uA. The needle position is 68 on the 0-100mA scale of the meter.
I presumed the needle should go to the 1 Megohm position on the meter scale (50uA position)

[Dekatron]

Well, it seem quite ok then when it comes to the measurements, with those the needle should land more or less on the 1 on the outermost scale, or just to the right of the 1.

One comment/question though: Is the voltage 119V correct? With the voltage and current you write (119V and 28.5uA) either the internal calibration resistor is higher than 2.96M (closer to 3.17M) or the 1M (closer to 1.2M) resistor is higher - or could it be that the meter you are making these measurements with is not measuring half wave rectified voltages correctly?

Also, I don't understand this sentence fully "I presumed the needle should go to the 1 Megohm position on the meter scale (50uA position) " - especially the "(50uA position)".