AVO mk2 valve tester meter

[oceanobsession1]

Hi, is it possible to test if the meter is reading low without removing it by using a multimeter set to current.
When you set the needle at its mains mark the shunt should put a specified current to hit that mark. If that current is above the specified for the meter, then its lost its magnetism and reading low. So can I take that reading from the rear of the meter, it will cost £100 to have the meter rebuilt, which is ok, but it would be nice to check it first.
Can anyone advise me cheers phil.

[Herald1360]

Can you disconnect one side of the meter rather than remove it? I don't think what you describe (reading volts/amps across the meter will be helpful).

If so, then a series connection of 1.5V battery, suitable resistor to limit current to nominal meter FSD and a DMM on current should give a very good idea of the state of the meter.

eg. if meter is 1mA FSD use a 1K5 resistor to give about 1mA. DMM will display actual current and meter indication can be scaled from any linear scale it may have. Use a slightly less than new cell to keep applied voltage <1.5V

For other FSD scale resistor value as required to keep nominal current just below FSD value.

Edit: a bit of goggling suggests meter is 100uA FSD, so use 15K resistor!

[karesz*]

Be carfull pls, AVO MKII need more less current, as usually the MM-movements, ie 8MKII has only 37,5uA FSD. I`m sorry, have "overseed" that its for MKII Tube-Tester...
Karl

[Herald1360]

karesz* speaks wisely! Though the mk2 valve tester meter does not appear to have the extreme sensitivity of the later 30uA ones.

I believe 37.5uA refers to the meter in the AVO8 rather than the valve testers.

A safer approach to my suggestions above might be to use a 25K pot and wind the current up from zero until the AVO meter hits FSD then compare with DMM reading.

[oceanobsession1]

Thanks for your help , I'm thinking about buying a milliamp meter for checking
the main meter setting , the one that you put into the link in the rear by the
fuse holder , this one on ebay looks good what do you think


http://www.ebay.co.uk/itm/Meter-Mill...gAAOSwkNZUl0Ji

or should I buy a cheap analogue multimeter
cheers phil.

[Herald1360]

I wouldn't spend £20 on something where you would get the same accuracy and a lot more functionality from a DMM at anything from £5 upwards.

For a reasonable cheap analogue multimeter see here:

http://www.maplin.co.uk/p/mt-2017-la...FYIIwwodBIkJmg

I still reckon that to check the meter properly you need to get inside and disconnect one side of it from the instrument so nothing else can upset readings.

[oceanobsession1]

Hi again, I think you are right ive been looking at buying an analogue mm, and if I were to purchase old I could be buying something with magnetism problems or inaccurate. Sanwa seems to be a good buy but there are Chinese ones being made, they work on a fet principle, anyway what do you think about these

http://www.ebay.co.uk/itm/Sanwa-Mult...okcSvHPkZL_kGg

cheap me phil.

[oceanobsession1]

Purchased the Sanwa JP-12D. I hope it does what I need, I've also got the fluke but it don't do DC current me phil.

[Herald1360]

For info:

http://park15.wakwak.com/~smart/muse...nwa-jp12d.html


JP12D looks like a reasonable basic 20k/V dc multimeter. It has a 3mA range that should be usable to check your valve tester meter to within about 5% which is probably good enough.

[oceanobsession1]

ok I got the analogue multi meter and checked out the current reading at the
link whilst running up an el84 and as I thought the reading on the avo was 40
mv , and 24 mv on the multi meter , so 8mv down , so now I'm going to remove the meter and use a 1.5v battery and potentiometer to check the current to the meter when removed , phil.

[Radio Wrangler]

No, that won't work either, Phil.

An ordinary multimeter has 'DC' and 'AC' ranges. On 'DC' everything is simple and it reads the direct current component, or the average current value if the current is fluctuating.

On 'AC' it gets more complicated. It rectifies the AC input, then the movement responds to somewhere between the average rectified value and the peak rectified value. The maker fixes this problem by choosing scaling resistor values so that the pointer indicates the RMS current when the input is a clean sine wave. So ordinary multimeters on AC read accurate RMS values only on sinewaves.

"True RMS" equipped multimeters have an analogue computer circuit which squares the input voltage or current, then a capacitor averages the resulting waveform to get the mean value, then the analogue computer extracts the square root of the capacitor voltage, then the meter shows the result. This gives the RMS of whatever wave shape the AC applied has.

The metering in the Avo VCM is none of these.

The valve is tested using AC voltages from switched taps on a transformer. Valves only conduct in one direction, so the anode current is off for a bit over 50% of the time, and then it's a distorted version of the top half of a sine for the time there is anode current flowing.

The meter in the VCM responds to the mean DC off this peculiar waveform, and AVO have scaled the meter to show what the DC current would have been if a normal valve had been tested with DC conditions!

No ordinary meter, AC, DC or true RMS will read the same as this.

Avo printed correction factors in their calibration procedures, and provided terminals so a DC ammeter could be inserted and used.

There are three flaws:

The characteristic curve of the valve will change the wave shape at the anode current meter, and this will affect readings. This is smallish for good valves, and Avo took it into account in their data books.

The accuracy of everything depends on the accuracy of the mains voltage. Avo took this into account by having many fine steps of mains voltage taps on their transformers, along wit the 'Set AC' meter function.

The shape of your local mains waveform will affect readings. There is no correction for this in the VCM. In most domestic situations, there are so many rectified loads in all the surrounding houses that the mains waveform is badly distorted. One way out of this would be to power the VCM via a resonant type of constant voltage transformer.

The Avo VCM is fine for its original purpose of checking whether a valve is worth putting back in a piece of equipment, and for comparing valves you're matching. In all honesty, it isn't a piece of extreme-precision measuring equipment, though it certainly looks like one. Once you know what's going on inside one, the degree of trust placed in them by people buying and advertising valves is either very funny or very sad, and I can't decide which!

If you want high accuracy for testing valves and the ability to check against calibrated voltmeters and ammeters, you need to test under DC conditions which means building your own rig. Your results will be accurate, and you can even get calibration traceability this way, but it doesn't have the street-cree on auction sites and the consequent price magnification factor that 'tested on an AVO' brings.

David

[frankmcvey]

Excellent post, David - it's certainly furthered my understanding. Adam Savage and Jamie Hyneman may have a job for you in California!

Cheers,

Frank

[David Simpson]

The meter movements for AVO VCM's Mk's 1 & 2 have an FSD of approx. 450uA. The Mk3's & 4's and the CT160 are 30uA. The AVO8 MM's are 37.5uA.
If someone ever has to substitute a VCM's meter with an analogue or digital MM - as soon as "Test" is selected - full sinusoidal Va is present on at the meter's terminals. So take extreme care.

Regards, David

[oceanobsession1]

Thanks very much to the kind folks who have offered help and support cheers
phil.

[oceanobsession1]

Ok folks , I have managed to borrow an avo megger m7027 that reads microamps and using a 3k3 resister and a 10k pot and 1.5 volt battery I have taken some measurements , if fsd means 100 ma then the digital meter reads 208ua then checking at 50ma the digital reading is 218ua and at 25ma the digital reading is 118ua , every time the gauge comes back to zero its in a different position and you have to readjust the zero , also it seems to make a big difference at what angle the position of the gauge is , so its off to hertz meters
for repair . me phil.

[Radio Wrangler]

It looks embarrassing that the meter movements in any AVO product should be so often troublesome, but there are two factors:

1) Avo didn't want to put any active circuitry in the VCMs whether for cost, complexity or long-term stability reasons. So to make useful leakage measurements, a very sensitive movement was needed. However, the model 8 AVOmeter has a sensitive movement and seems much more reliable....

2) The metering in the VCM has to measure anode current over a wide range and there is little protection against over driving the meter by selecting the wrong range.

The VCM in the right hands was reasonably reliable and not much trouble, but a lot were used in industry by employees who didn't personally own the machine and therefore had no vested financial interest in treating it kindly. A lot were used by people with inadequate training. Plenty were cared for by small businesses where the owner used it, or oversaw employees.

Nowadays a lot of VCMs are in the hands of guitarists and audiophiles. Some of whom have spent a lot of money on them, and haven't a clue about how to use one carefully. Modern people have become a bit too used to the idea that things don't usually allow control settings which will damage the machine (When people make remarks to me along the lines of 'it shouldn't let you damage it' I usually ask what sort of steering their car has )

Several times we've had people appear here who have bought VCMs and tried using them for 'running-in' valves. This has resulted in VCMs with burned-out transformers as well as other damage. THey are only rated for a quick test, and the guitarists and audiophiles seem particularly focused on power valves which just makes disaster even more likely.

One telling bit of evidence are the films of valve factories like Mullard at Blackburn. Notice that there are no endless lines of people with AVO VCMs testing valves... they used custom made panels designed to run valves up to their full ratings and to hold them there for a while. They used DC HT as well to make calibration easier. Valves getting deeper analysis went on curve plotters.

David

[oceanobsession1]

David , thanks for your help and advice , I'm struggling a bit here as you can
see , but I'm slowly taking things on board , and getting the meter working
correctly seems like a good move . me phil.

[David Simpson]

Phil, & any other guys new to AVO VCM's - Some time ago, during recuperation from a shoulder operation, I drew up a very simplistic diagram of how an AVO CT160 works. The 160 uses the same 30 microAmp meter as the Mk3 & Mk4, and the associated circuitry is similar. I.e. they are all just use the 50Hz Peak to Peak(P-P), mains AC(via a transformer), for the voltages applied to the valve's electrodes.
Much confusion arises from using external multi-meters, -which just try to read RMS values of full sinewaves or crudely half-wave rectified pulses. AVO, in their wisdom, about 65 years ago, decided to scale their Anode current(Ia) & Grid Voltage(-ve Vg) switchgear as though they were full Book DC values. Actually, a valve might momentarily,(every 20milli-Seconds), be running at those values, but they are definitely not(approx. 1/2) the smoothed DC conditions under which they would work in radio equipment.
In fact, just about all old valve testers & characteristic meters, from around the world use AC. Jackson, Hickok, Mullard, Taylor, etc.
AVO's MK 3's & 4's, 160's & 163's will, if fully in calibration,(a lengthy & confusing procedure), provide tabulations for an Ia/Vg graph which can get close to an accurate one from a fully DC Tester. But they were designed mainly for giving a "Go/No Go"(Green or Red meter zone) indication of a valve's condition. Plus heater continuity & inter-electrode insulation.
Internet speculation, and some over-enthusiastic folk have elevated these pieces of equipment to cult status. They are now pretty ancient, and would benefit from full overhauls & conversion to DC operation. "Whanging" of meter needles left & right must be avoided at all costs.

Regards, David