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Old 12th May 2017, 4:21 pm   #1
radiozero
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Default AVO VCM MKI & MKII: The D.R. Circuit

Hi. My ultimate quest is to ensure that when I test rectifiers, my MKI AVO VCM gives an accurate reading. I have drawn the circuit associated with rectifier and diode testing. I believe it is correct and has no errors.

I am puzzled by the position of C1 a reservoir capacitor, in the circuit. I see that if C1 was to be connected across only R LOAD and not a portion of R SHUNT as well as R LOAD as it is, all current flowing through the rectifier would have to pass through R SHUNT. And because the meter naturally averages out current, it would not matter what the waveform looked like. Of course, the meter is effectively measuring the load current by measuring the voltage across R SHUNT.

What I am wondering is, how can the meter possibly show the correct current flowing through the rectifier, when C1 is bypassing the meter circuit.

Can anyone please explain why C1 in the position it is in, is perfectly alright?

Circuit shown testing at 30mA current. Thanks.
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Old 12th May 2017, 5:34 pm   #2
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

Your circuit seems to be correct.

There is a simple description of the diode/rectifier testing in "1281 Testers Valve AVO EMER Y812 Part1 Technical Description" on page 11 (which I've included here, but I cut away the other information on that page).

The load resistor and shunt resistor for each range are chosen so that if the current on the knob flows through the diode/rectifier the needle will land in the middle of the GOOD scale, if less current flows it will deflect to the left and if more current flows it will deflect to the right.

Let's say you chose the 60mA range and 60ma flows through the rectifier, then the needle will land exactly in the middle of the good scale, if only 50mA flows through the rectifier the needle will land more to the left and if 70mA flows through the rectifier the needle will land more to the right.

The AVO valve tester weren't only designed to be used to measure the current as such but to show how good a tube is on the bad/good scale compared to the setting you choose.

Regarding the reservoir capacitor C1 it is correctly placed, just like in any half wave rectified power supply. Simplifying the circuit by removing the unused switch positions and adding up the resistors in series makes it a simpler layout that might be easier to understand. The load and shunt resistors also work as a voltage divider presenting less voltage across the shunt resistor than across the load resistors, presenting less voltage to the meter.
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Old 12th May 2017, 7:02 pm   #3
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

There is a difference between the two circuits. I attach a picture of the circuit that is on page 8 of "THE "AVO" VALVE CHARACTERISTIC METER WORKING INSTRUCTIONS THIRD EDITION" by AVO.

Notice that the meter is always across all the resistors that make up R SHUNT, (i.e R15, R16, R17, R18, R19, R20) plus R14 and that the current that flows through the rectifier flows through all of them when 1mA is selected, and only R20, when 120mA is selected.

So, which is the correct circuit? There appears to be a discrepancy.
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Old 12th May 2017, 7:13 pm   #4
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

There seems to always be discrepancies between the real AVO tester and the different schematics found in different documents, I come across this all the time when repairing AVO valve testers.

The only "correct" one will be the one that you can trace in your instrument, however that can differ to other testers of the same model due to updates at a later stage compared to when yours was manufactured.

AVO sometimes made updates to their instruction manuals but the new schematics often contained errors even when they were updated.

To my knowledge the most correct schematics is the one that I made for my friend Euan MacKenzie in his article in Radio Bygones no. 134 Christmas 2011 - that schematic was checked by Euan and Peter Holtham against their AVOs. The schematic is based on AVOs original but with updates from Euan and Peter when they traced their testers. Due to copyright reasons I can't publish it here but you can still buy back copies of the magazine I think.
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Old 12th May 2017, 7:36 pm   #5
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

Neither circuit is correct. They are both somewhat simplified. There will be details in terms of the make and break characteristics of the range switch which prevent the meter getting damaged whan the switch is turned.

The circuits won't be simply designed around testing rectifiers, they will have been also designed that various parts of them can be re-used for other measurements, and so that the switching for all the other modes is made as simple as possible overall.

It's just a valve tester. The circuit is ground-free as drawn so you can arbitrarily coose any convenient point as your reference. Then it becomes a single-phase rectifier with a smoothing capacitor, a load resistor to take some power, and a current shunt resistor to measure the current in that load. It could just as easily have measured the voltage across the load, and taken Ohms law into account. With the resistor shown in series with the meter in the EMER circuit, the resistors could have been partitioned in many ways and still yielded the same result. Avo probably chose one that best fitted in with the other modes.

It isn't a precision instrument. It relies too much on the ratio of rectified average to peak ratio of your mains waveform versus what it was calibrated with. With the ratio of the number of electronic loads versus the number of resistive loads on the mains today, the waveform is not as good as it used to be. Calibration accuracy is affected, but how accurate does it need to be to let you decide whether a valve is still worth using. Don't worry. It's an effective machine for its task.

David
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Old 12th May 2017, 10:31 pm   #6
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

The first circuit (a redraw I made) that I started the thread with, is copied from a circuit diagram I have. I attach it. I think it matches the redraw I made. But, is not it a mistake to have wired C1 in the position shown on the original diagram? Because C1 will be passing current, which is not being measured by the meter.

The circuit shows that the meter is measuring (when it's connected to points 5 or 6 on S1cb and S1da I think) whatever voltage is across R14-R20. So, current flowing in C1 is not being measured. Which of course, is flowing through the rectifier.
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Old 13th May 2017, 8:55 am   #7
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

True. Most of the AC component of the rectifier valve's current is flowing through C1. The DC component plus a little of the AC component flows through the resistors, and a little of this flows through the meter movement.

If you were to think of the -ve end of C1 as ground, then the rectifier valve thinks it's in a bog standard half wave circuit. It's fed from a mains transformer HT winding, its cathode drives a reservoir capacitor C1, and there is a resistive load taking DC from the reservoir. The resistive load happens to be switchable and there is a meter tapped part-way into it, but the valve seen a nice resistive load.

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Old 13th May 2017, 9:44 am   #8
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

I am taking the view that the diagram is in error, that C1 is in the wrong position in the circuit.

Of course, it's not a mistake in a strict sense that most of the current flows through C1, and the meter gets a small part of the current flowing through the rectifier, not least because a smaller part flows though the shunt resistors. I guess that is a circuit design that will work in a fashion. However, if C1 changes it's value and it draws more or less current, that will not be reflected in the current flowing through the meter. And it is because of that that I think C1 might/must be in the wrong position in the circuit. The circuit as drawn would be unreliable. That's my take.

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Old 13th May 2017, 10:08 am   #9
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

So where should the reservoir capacitor be connected then?

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Old 13th May 2017, 11:16 am   #10
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

I would have thought C1 should go across R LOAD as shown in this modified circuit.
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Old 13th May 2017, 11:24 am   #11
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

I see that I can get the Radio Bygones 134 issue from here:

https://radiophile.co.uk/product_inf...roducts_id=298
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Old 13th May 2017, 11:36 am   #12
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

Quote:
Originally Posted by radiozero View Post
I would have thought C1 should go across R LOAD as shown in this modified circuit.
R shunt plus meter is part of the load so C1 should be connected across that whole load to my way of thinking.

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Old 13th May 2017, 11:41 am   #13
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

Why not trace the circuit in your tester to see where the capacitor is connected?

That way you will know for sure where the capacitor is connected.

The capacitor should be connected so that it smoothes the voltage after the rectifier so that the meter sees a smoothed voltage in the circuit, not an AC voltage.
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Old 13th May 2017, 12:12 pm   #14
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

I downloaded issue No. 134 of Radio Bygones, which includes an article on the servicing and calibrating AVO MKI & MKII VCM. The circuit on page 6: "Figure 7. Circuit diagram of MKI and MKII. Drawn, with corrections, by Martin Forsberge."

The diagram shows C1 across both the shunt resistors and the load resistors, precisely as the diagram in my first posting on this thread. Therefore, it surely must be the case that C1 is in the correct position as per the published diagram I have. But I still have the same concerns though, that any current through C1 is not being measured by the meter.
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Old 13th May 2017, 2:11 pm   #15
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Default Re: AVO VCM MKI & MKII: The D.R. Circuit

There ought to be no DC current through C1.
The meter is only interested in DC current.
C1's job is to stop AC current going through the meter and giving it a bit of a shaking at 50Hz.

C1 does affect the reading though.

If C1 was very large, the current through the rectifier would becoe a large, short pulse. Rectifier voltage drop would be magnified and the mean output voltage would drop a little. The large, short pulse business ids the reason that many rectifier valves have a max reservoir capacity rating.

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