AVO MK4 shows too high current

[Stewen1]

Hallo ! Today I found this forum and I am very happy for that . I have
registered and this is my first posted thread: as a retired tech and as a hobby
I repair radios , HiFi amps and guitaramps mostly equipped with tubes . To be able to measure anodecurrent ( adjust bias) for example in powertubes I usually solder in 1 or 10 ohms , matched resistors between cathode and ground .
To be able to make these controls without this modification I have made an adaptor from a smashed , faulty EL34 tube and an octalsocket . A precision 1 ohm resistor is inside this unit between pin 8 and pin 8 . A multimeter in mV
mode can be connected over this resistor so for example 40mA through a connected EL34 gives a reading of 40mV on the meter . Today I put this adapter in my AVO MK4 and a good EL34 tube on top . With 400V anodevoltage
300 V on the screen and finally -25 VDC on grid nr1 , the AVO showed 28 mA
BUT THE MULTIMETER only 19mA . I checked this several times and with different multimeters but got the same result ! Why do You think the AVO
shows such a high current ? I´d be very grateful for any comment and/or advice here . Can I calibrate my tubetester ? I have the servicemanual somewhere . Stewen

[Dekatron]

What reading do you get on your dmm if you remove your adapter and plug the EL34 into the valve panel socket, and then open the anode link and connect your digital multimeter to the two electrodes in the anode link and set it to say 100mA DC?

If you get half the reading on your dmm compared to what the AVO shows everything is correct. The AVO VCM's work with half wave rectified AC voltage, earlier models used the valve as the rectifier and newer models have silicon diodes in the circuit as rectifiers.

So since you are making measurements on a half wave rectified voltage you will see half the current, the meter scale is then shown with twice that value as that is what it would show if the AVO VCM's would have used DC voltages. This is only the relationship between AC RMS voltage and DC Mean voltages.

I wonder if it should have shown 38mA on the AVO meter and 19mA on your dmm and not 28mA on your AVO meter? I have assumed that you are using a digital multimeter for your measurements set to DC mA.

[Stewen1]

Thanks for answer ! Well , I did this test with opened link and the same settings as before
and got 27mA ( earlier 28mA ) on the 100mA scale on my AVO Mk4 and 16,1 mA on my
Meterman dmm . Hmm....... I must admit that I can´t adjust my AVO completely to
the 230VAC (or so ) . The knob for that (Set) is in the max clockwise position, max to the right ! So
there is something I have to do inside the AVO , change a resistor which might have drifted in value ....... ? What would You recommend ? I have printed out Your excellent papers , Martin , and will study them including the servicemanual to learn more about my AVO . I really want to understand how its built and how its working . At our radiomuseum here in Gothenburg
we have one or two of this AVO MK4 so I can also compare my readings to these .

[Dekatron]

I am not familiar with the Meterman dmm's, do you have a link to the manual or do you know if it is a TRUE RMS meter?

Earlier I had some Beckman RMS3010 & RMS3030 which where supposed to be TRUE RMS meters but they were way off compared to my Fluke 87-III & IV and my Solartron meter. Even cheap DMM's from Biltema (a company supplying car parts and other "home" materials) worked better than my 30+ year old Beckman multimeters.

If you have access to a Fluke 87 you could compare the measurements.

You could also put a silicon diode in series with a power resistor of some 5-10K and 20W and use it as a check to see what current you read on the AVO meter compared to your DMM. By selecting different Anode voltages, starting at the lowest, you can check each range and see if they are within tolerance and show half the value on your dmm.

Comparing to other AVO's is good as long as you know that they are calibrated and give correct readings!

[Stewen1]

Well , I can make a unit of one powerresistor and one diode but where shall I connect this unit ? On top of the AVO ? With anodebridge "broken up" ?
Another question ( I realize think understanding of my AVO and the supposed calibration will take some real effort for me ...... so of course I´m very happy for Your advice and
comments. Today , I skrewed off both sideplates and the rear plate in order to see better and measure inside. The cap 8uF/350V should be at the circuit selector switch beside the two diodes , (if I look at the picture in the servicemanual) but in my AVO theres just a small cap there and a larger (bipol. it seems) is near the instrument fastened with a type of black rubberband . The diodes measure OK m but I should perhaps change the caps The small cap soldered with one end to the connection of the diodes is an ERO-TANTAL 10uF/25V I´m sure someone has changed this cap
earlier . It sure doesn´t look like an original part . When I measure around this cap I found that it is connected across the diodes . So its supposed to slow down the meter
I guess. But what is the purpose of the other , high voltage cap ?

A third question : The set adjustment didn´t manage to get the needle to the mark so
I controlled the fuse and it was in the 240-250 VAC position . Measured in the walloutlet
and got 229,5 VAC . Moved the fuse to 210-230 position and that made the needle a
bit closer to the mark ..... but 1 or 2 mm remained . What am I supposed to do ?
The trimpot , RV6 is in max end position .... What is wrong ... do you think ?
Stewen

[Dekatron]

Hi,

The anode bridge should be closed and the resistor and diode should be connected at the valve socket on the pins that are anode and cathode. You can read more about this in the Service manual.

The large 8uF capacitor is used in the rectifier test to smooth the voltage, there is also a set of power resistors that emulate the load in rectifier testing and the voltage across the rectifier is changed depending on what current range you select. The large 8uF capacitor is a polarised electrolytic capacitor but you can replace it with an 8uF non-polarised polypropylene capacitor, that way it will not leak and degrade over the years.

The small 8uF capacitor across the diode is a non-polarised type and you can also use a polypropylene capacitor here. You can read more about meter protection here: https://www.vintage-radio.net/forum/...ad.php?t=55782 where Simon karsten used an extra pair of Schottky diodes and also a 100uF capacitor across the meter, the 100uF capacitor consists of two small surface mounted capacitors so it won't take up as much space as a normal polypropylene capacitor does. You should remove the tantalum capacitor as it is not a good choice in this position, either use a non polarised electrolytic capacitor or a polypropylene type here.

Before you start to replace any other components you need to have your AVO VCM meter checked for what FSD current it needs to fully deflect the meter and also what internal resistance it has. This can be done by removing it and then putting it in a test circuit, just make sure so that the test circuit can't deliver too much current to the meter so you don't destroy it!

When you know the FSD current and internal resistance it is time to decide whether to have the meter sent for repairs or if it can be adjusted to work in the tester. A meter with up to some 36-38uA FSD current can be used in some circumstances but it migh benefit from being properly refurbished and re-magnetized.

If your meter has an FSD current of 30uA and an internal resistance of close to 3250 Ohm then you need to check all components to make sure that they are within tolerance. You need to replace those that are out of tolerance and then you need to calibrate the tester properly, for this you need to follow the calibration instructions in the Service manual.

The important thing when calibrating the tester is to not touch or change any of the potentiometers or scales to begin with, but just make measurements to try to understand what needs to be calibrated or adjusted. You shouldn't need to adjust the scales on either the grid voltage control or on the gm control, if you use the correct values for teh components surrounding them - if necessary it is much easier to adjust the surrounding components by some small amount to get the scales to agree than adjusting the scales themselves. If you need to adjust the scales it is better done by removing the whole controls from the front panel and then hooking the up with longer wires to the tester, this way you have better access to their internals and locking screws.

All components should have the values as they are stated in the bill of materials, you might have to adjust the "calibration" resistor some to get a correct value. This "calibration" resistor not only makes sure that you can set the needle to the calibration mark but the current flowing through it is also used for the insulation testing where the resistance is measured. So it serves several purposes and it shouldn't just be changed randomly to get the needle on the calibration mark as the insulation tests can be off then.

In your case I think that the meter is off by some amount for FSD current and/or that the calibration resistor is out of tolerance.

[Stewen1]

Thanks Martin ! Is the "calibration-resistor" the one parallell with the meter (10K) ?

Maximum neg. grid volt is : 101 V , right ? The manual says I shall be able to measure
0,52x that value ..... about 52 V . But where can I measure that ? The servicemanual
is difficult to understand , I think . Or I´m too stupid Stewen

[Dekatron]

The calibration resistor is R19, at 2.96 MOhm, it is made up from two selected resistors in series with values close to 1.48 MOhm each - these two resistors are selected to make sure that the needle lands on the calibration line when properly calibrated.

Paragraph (e) on page 6 in the Service manual states that the grid voltage should be measured with a standardised Electronic Testmeter (Valve voltmeter or a DMM today) ... between grid and cathode, with the grid link open and the meter switch set to 100mA.

Be sure to follow these instructions in the Service Manual as you will otherwise get incorrect readings.

[Stewen1]

OK , thanks ! Well , I did this and got 56V or so , so I turned VR4 a little to get 52,5V ,
but that made the set adjustment , needle ge to the left from the red line . So , I guess
I have to measure all resistors or at least most of them . Started with this measurement
and some resistors could be tested without desolder one end ( just turn the actual switch
back or forth ...... but several resistors can´t be tested without desoldering one end .....
...... I´m hesitating a bit there ...... but there´s no other choice ..... right ? I´ll also,
before going futher with this , will take out the meter and make that test-set up that you have recommended . Thanks so far ..... Stewen

[Dekatron]

If you are very careful you can use a thin piece of plastic and push it in on both sides of the rotary switch contacts to measure resistors on the switches, but other resistors are connected in the circuit in a way that it is impossible to measure them without unsoldering one side. If you have another AVO MK IV at hand you can always make comparisons with that tester without removing components in any of them, you can probably get a good idea on what needs to be fixed by doing comparisons but you probably can't catch all problems that way.

There is also a chance that the relay contacts or any other switch contacts have corroded over the years so that they need a good clean to lower the contact resistance to get the tester in perfect condition. So take a look atthose while you have the tester opened up.

[Stewen1]

Thanks! Good advice! I'll do that.

Stewen

[Dekatron]

I forgot to write that it is sometimes quite easy to desolder just the wiper connection on a switch, or where that connection ends at the other end, and then use that one as one of the electrodes for the measurement. This as the wiper is the common connection on the switch - but sometimes it is hard to find or reach that connection.