AVO Mk-II Valve Tester restoration.

[marcvos80]

Although the meter amplifier would be the best solution future wise, stubborn me still wanted to check out the effect of adding extra magnets to the meter. I already ordered some different sizes of magnets... so what the hey.. Let's try!

The effect was quite simple.
By adding two 1.5mm - 10mm magnets the movement is right where it should be! 440uA @ FSD with an impedance of 114.8 Ohm.

I will be fitting the meter to the AVO with the magnets and check again in a year or so what happened to the FSD, to see if it it stable or not.
If not, I can always add the meter amplifier as discussed earlier.

[Dekatron]

Nice!

I see that you put them flat on the magnet, if I remember correctly Euan and other people who tried this always put them on the sides of the magnet - maybe that is why your placement worked better.

Which magnetic flux direction do these magnets have? South on one side and north on the other or sideways?

[marcvos80]

I saw another post on the forum with the magnets mounted like this, so I haven't tried putting them on the side of the magnet.
These have a "normal" flux, so north on one side and south on the other side.

If I flip the magnet it will work the opposite way and make the meter even less sensitive.

[Dekatron]

May I ask where you bought them and which type you ended up using?

Good to know which direction they are magnetized. I must have missed that other thread on the mounting.

[marcvos80]

I ordered them at a "local" dutch internet shop for magnets https://www.magneetgigant.nl/

I've ordered several different thicknesses (1.5mm, 2mm, 3mm and 4mm) of round neodymium magnets (normal flux) with a diameter of 10mm; 2 pieces of each thickness.
Total was something around 6 Euros, shipping inclusive.

I've ended up with using 2x 2mm on each side of the magnet.

Btw. it was a post from you (in a thread), Martin, which pointed me the the placement of the magnets like this
https://www.vintage-radio.net/forum/...hlight=magnets
Somewhere in the above thread there is a link to the picture.

[marcvos80]

Question. Does anyone know the calibration guidelines from "Hein Ros"?
Is this guideline he mentions ".. If erroneous adjust connection (slider) to R7/R8 accordingly ..".
What slider does he mean? R7 and R8 are two fixed resistors in my meter.

[marcvos80]

Ah, I think I already found how it works...

"R7 & R8 (fixed resistors) (Sets Vg) Three points possible, junction of R6 and R7, junction of R7 and R8, and at the end of R8."

So it can be set to 0 Ohm, 5k or 10k.

https://www.vintage-radio.net/forum/...2&d=1393113427

[Kevin Hoyland]

Hello.
Sorry it has taken a long time to put the photos on . I have more if you need them Kevin.

[marcvos80]

During testing and calibration I've noticed something strange.
Calibrating the net voltage and Gm went perfect, no problem there.
I'm trying to calibrate the anode current and "nothing happens".
I've connected a 2k5 WW pot between the Anode and Cathode, connected my AVO 8 in the Link, meter set to test and nothing happens - no current flow.
Anode voltage comes up (I've connected an other Volt meter to the dummy load) and corresponds with the rotary switch values.

When turning down the load (starting at 2k5) I can see the voltage over the dummy load going down, so something is happening, but there is no current flow through my Amp meter and no movement on the meter scale.
If I increase the dummy load nothing happens, until I increase it too much.
Then the Cut out relay trips.

Am I doing something wrong here? Or is there a fault somewhere in the tester?
Any thoughts?

[marcvos80]

Originally Posted by marcvos80

During testing and calibration I've noticed something strange.
Calibrating the net voltage and Gm went perfect, no problem there.
I'm trying to calibrate the anode current and "nothing happens".
I've connected a 2k5 WW pot between the Anode and Cathode, connected my AVO 8 in the Link, meter set to test and nothing happens - no current flow.
Anode voltage comes up (I've connected an other Volt meter to the dummy load) and corresponds with the rotary switch values.

When turning down the load (starting at 2k5) I can see the voltage over the dummy load going down, so something is happening, but there is no current flow through my Amp meter and no movement on the meter scale.
If I increase the dummy load nothing happens, until I increase it too much.
Then the Cut out relay trips.

Am I doing something wrong here? Or is there a fault somewhere in the tester?
Any thoughts?

My bad! I've made a fault in setting up the current measurement link...
Check first.... then measure..
Lets continue calibrating!

[marcvos80]

Alright… all calibration measurements looked good. The meter calibrated quite fine.
But now I ran into something I don’t understand.
If I setup the meter for a normal test, it worked fine (see photo), the EL34 measured 75mA. Perfect!

Then I tried a different tube (12ax7), also a good result.

When I then turn down the Set zero and press the mA/V button nothing happens.
That doesn’t seem right…

Now the most strange thing is that when I wanted to check if the quality test also doesn’t work on the el34 the measurement is going berserk!
When it first settled nicely at 75mA, the needle now shoots quite rapid to maximum scale and a loud humming noise is coming from the tester.
I quickly turned it off and checked all knob and dial settings again.. everything was set to the book..
I don’t understand what is wrong here.
Any thoughts of you guys?

[retailer]

Are you operating correctly, it's been a while since I used mine but I believe once you establish the anode current such as EL34 75mA you back off the meter to zero with the set zero control and pressing the mA/V button gives the Gm or Ma/Volt reading on the meter, after backing off and before you press the mA/V button you may need to adjust the meter range selector switch to a suitable range for the valve being tested - in the case of an EL34 you would expect around 11mA when the Ma/V button is pressed.
If you are just wanting to test a valve to see if it suitable to remain in service ie REPLACE/GOOD then after backing off the meter to zero, set the meter selector to mA/V and turn the SET mA/V knob to the expected mA/V for the valve under test, pressing the mA/V button will see the meter needle move into the GOOD section of the meter if the valve is serviceable. Don't be in a hurry to throw away valves that show as REPLACE, they may still work ok in certain circuits.

[marcvos80]

Yes, those are exactly the steps I've done (as I recall correctly).
After backing off (meter goes to zero) and then pressing the mA/V button nothing happened....

I will repeat the measurement this evening to double check the procedure and report back.

[marcvos80]

Okay, I've set the tester aside for a few weeks but checked some things today.
Here's what I've found:
- Anode current reads fine (checked with an dummy load on the meter and with an external amp meter in the LINK)
- Testing any tube is going wrong, too much current flow: Meter needle goes in the right corner and the tester starts to hum as "if she is gonna blow".

I think there goes something wrong in the screen supply.

If tested with a good EL34:
First set everything according the tube book, but leave the screen voltage @ 0V.
Rollers at 126540310
Va=250V, Neg grid = 13,5V, mA/V=11

If start the screen supply at 0 Volts and switch upwards step by step I get an anode current of 75mA @ 125V Screen voltage.
If I then go further up the current goes way too high. Normally it should read 75mA @ 250V screen voltage (now half that value).

Note: with the screen voltage op 125V at the dial I only measure a voltage of 52V at pin 4 of the EL34 (screen) and 264V @ pin 3 (anode)

I've triple checked all settings on the rollers and dials, so that is fine.
Does anybody have a clue?

[marcvos80]

Btw. Voltages measures are AC.
As I think of it, the voltage screen should be right since it is rectified without any capacitor to make a steady DC voltage.
With my RMS meter a should measure somewhere around half the AC value

[marcvos80]

Does anybody have a clue on what could explain the excessive current draw?
Any help would be appreciated.
Thanks!

[Dekatron]

Could it be that the tube you are testing is oscillating?

If a tube starts to oscillate all manner of strange things will start to happen while it is being tested.

[marcvos80]

I've tried different EL34's and they all give the same result.

[David Simpson]

Marc, its been highlighted on many many AVO VCM threads in the past(see "Search") that these AVO testers operate on full Peak to Peak mains derived 50Hz voltages on the Anode, and half-wave rectified pulses on the Screen & Grid. NOT DC. (In fact, just about all makes of vintage valve testers & valve characteristic meters(VCM's) operate in this manner).
Also, as Martin says, they can be prone to "PO" - parasitic oscillation when testing high Ia o/p valves, particularly "high slope" ones. Careful fitment of ferrite beads under the valve holder panel is recommended. Or make an IO extension valve holder box with ferrite beads fitted.

Regards, David

[ms660]

So far as I can make out the screen grid is supplied with unfiltered DC, its unloaded average value being approx. the screen grid's selected DC voltage multiplied by 0.5, the RMS value being the RMS voltage supplied to the rectifier multiplied by 0.707.

A check of the unloaded average screen grid supply voltage in the later AVO VCM MKIV is done with an ordinary AVO moving coil multi-meter set to measure DC voltage.

Lawrence.