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

[HBWOODY]

(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)".)
I thought that when doing the leakage test with the 1Megohm resistor the VCM meter needle should be on 1M on the outer scale of the meter, it is just to the right of the 0.5 mark.
I will check the calibration resistor and the 1Megohm resistor and also take measurements with a the DMM's

[Dekatron]

Is it possible for you to take some photos when you are doing the measurements ? If so take photos of the scale, the DMM/Instrument with the measurements and also of the resistor you are using plus what position the switches are in?

Can you try another 1M resistor, also try adding a few of them in series to see what measurements on current and what deflection you get on the meter so that you can see if it is a linear behaviour or if any of the resistors are incorrect.

[HBWOODY]

I selected the 1Megohm resistor and it measures 1001ohms.
I am at the moment working out the circuit route on the end of the circuit selector.
I cut one end of R15 (2.96m) and it measures 3megohms, do I have to cut both ends to get a true reading?

[Station X]

Originally Posted by HBWOODY

I cut one end of R15 (2.96m) and it measures 3megohms, do I have to cut both ends to get a true reading?

No.

[HBWOODY]

When checking the meter resistance and adjusting the meter for FSD should the 10k shunt R35 be in place?
R36 measured 201.5 ohms, is that acceptable or should I replace it.

[Dekatron]

Measure the shunt and metere separately so that we can see what they each are.

What DMM are you using when doing these measurements?

[HBWOODY]

I have two Fluke 25 multimeters. I also have a Keithley 169 which I sometimes use.
Is 201.5 ohms be ok for R36?

[Dekatron]

Do you get the same resistance for R19 (2.96M) on all three meters?

Likewise for the other measurements like voltage and current, are they similar on all three meters?

I don't think that the Fluke 25 are True RMS meters like the Fluke 87V so you might get incorrect readings on the half wave rectified voltages with these. I had similar multimeters from other brands earlier and that's where I learned the difference between them and the Fluke 87V so I switched to Fluke 87V to get correct measurements on half wave rectified voltages in AVOs. I couldn't find any direct info on the Keithley 169 either but it didn't look like a True RMS meter from what a quick Googling resulted in.

If you know someone who has a Fluke 87V that you can borrow I can recommend you to do so and compare the measurements so that we know that they are correct (assuming of course that the Fluke 87V you borrow is working correctly). I had one Beckman 3030 RMS DMM which was more than 3% off when measuring half wave rectified voltages in the AVO, it was even up to 10% off on some ranges.

R36 should be 200 Ohm +/-1% so that is within that tolerance.

A photo of the scale when you are measuring the 1 Meg resistor would be nice to understand what you mean with the needle being on 0.5 Meg.

[HBWOODY]

I looked at some other Avo VCM meters and noticed the 1megohm position on the scale corresponds to about 68uA on the scale so the Mk3 is showing the correct reading, on my meter the 1Megohm corresponds with about 50uA. So it appears my Mk111 is showing the correct leakage results (will find the correct scale for the meter).
Does the 118volt mean that correct checking of valves can not be achieved until I find why it is not showing 106 volts.

[Dekatron]

Isn't your AVO Mk III equipped with an original meter with correct scale?

You can't expect correct measurements in an AVO if it isn't calibrated correctly with the correct voltages/currents present on all controls and the transformers - that's why AVO specified that you should use specific meters that had been calibrated so that the measurements were correct.

If all components in an AVO are within their tolerances you should get an approximate voltage of 106V, but that can vary by a small amount and the AVO will still be correct within its tolerances - however 119V seems a bit to much to me as that is over 10% to high.

[HBWOODY]

I do have a few spare meters and I foolishly presumed it was suitable for the MK3. I can swop the scale so that it has the correct scale (not ct160)
The positives are that the leakage tests are correct, The SET position is as it should be, heater/screen and anode volts are good. One remaining problem is the 118V which should ideally be 106V. As Martin mentioned it may be a false reading from my Fluke 25.
Is it advisable to modify the MK3 to the revised circuit (diodes replacing valves etc.)
Is it ok to use a trimmer on R19 to make it easier to achieve the necessary 2.96Megohm.
I feel (hope) I am close to finishing the project which I would never have come close to without the educated advice I have received.

[Dekatron]

I wouldn't convert it to silicon diodes if it works as it is.

You can always convert it later if you want for some reason. If you decide to replace the valves with diodes you'll have to recalibrate it and you should preferrably also add a 100k/2W resistor to common ground from the V2 cathode so that it works the same way that AVO designed their Mk IV. Just have a look at the schematics of the Mk IV and you'll see how to connect the 100k resistor - you can even add a diode and 100k/2W resistor for the screen voltage while you are at it, but that means that you'll have to cut a wire and find a good place to add those extra components (diode and resistor).

You can use a trimmer for the 2.96M if you want, AVO did so in one of their CT160A revisions. Otherwise you can just buy a few new 1% resistors to get to 2.96M with good enough tolerance.

Maybe you can borrow a Fluke 87V from someone to check the voltage you have and then compare it with your Fluke 25 at the same time.

[HBWOODY]

Would I be correct in thinking that the higher 119Vdc is due to using the 150volt transformer tap instead of the 125volt tap to feed RV6?

[Dekatron]

No, the tap has nothing to do with this as long as every resistor is correct in your VCM and as long as RV6 can be adjusted to set the needle on the SET AC line correctly. The current flowing through R19 and the meter is what makes up this voltage drop, and with the higher value you need a higher voltage to reach the SET AC position. What makes the higher voltage is partly due to your R19 being to high in value and/or your Fluke 25 not measuring a composite made up from two different half wave rectified voltages.

With the link closed you have a composite voltage made up from two half wave rectified voltages of different peak voltages which normal RMS meters can't measure correctly, you need a True RMS meter for this (or a meter that has been calibrated for this). A technician at Fluke explained this for me a long time ago and wrote me a very nice email with all of the maths and equations plus explanations on how their different meters would display these measurements (unfortunately I lost this email in an ISP email server crash many years ago) but he explained in detail that I needed a True RMS meter for these measurements to show correctly on the display without having to calculate the deviation for normal RMS meters. Normal RMS meters works very well for normal sinusoidal voltages and some even for half wave rectified voltage (but not all on half wave rectified voltages) he explained, but like my beckman RMS 3030 it didn't show correctly on half wave rectified sinusoidal voltages. Most normal RMS meters only handle normal sinusoidal voltages, True RMS meters handle more curve forms like triangualr or square-pulse waveforms as long as they are within the limitations of the meter design, the RMS/DC converter possibly with the help of firmware in a microporcessor have their limitations too.

R19 sets the current flowing through the meter partly for calibration purposes to make sure that all transformer voltages are correct but also for the resistance measurement scale in the Insulation/Leakage tests - the meter works just like any Ohm-meter in this position.

[HBWOODY]

Thank you for that explanation Martin. I will invest in a Fluke87 when I get a chance.

[Dekatron]

Don't forget to adjust R19 so that it is closer to 2.96M than the 3M you have now, otherwise the voltage will be to high with whatever meter you use. Those extra 400k from R19 at the current of 28.5uA you measured for the 1M position is still close to 11.4V to high.

[HBWOODY]

Finally found time to check out voltages and adjust the meter and R19 to their specified figures using the newly arrived Fluke 87V dmm to check the measurements.
The meter resistance is now 3250ohms
R19 now as close as I can get it to 2.96mohms without using a trimmer and I now get a reading of 106v when measuring from the junction of RV4/R6 to the junction of R3/R4 with the link closed.
Needle SET position is now fully adjustable and calibration has been done so now All I need now is a standardised valve to check the Mk3's accuracy. I did have one from Nonode but foolishly shorted the heater.
Can now see light at the end of the tunnel.
Thank you again for getting me to this point.

[Dekatron]

Really nice to read that you got all the measurements right now.

I'll keep my fingers crossed that you find a standardized valve to check against, there were a few made some time back here at the forum that I hope someone can pass on to you so you can compare your Mk III measurements to other peoples measurements.