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Old 3rd Jun 2020, 6:11 pm   #21
vintage_8bit
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Default Re: AVO VCM Mk 3.

I suppose whats confusing for me here is that the only possition for the plastic etched dial on RV5 in my mind is, at fully anti clockwise the witness mark should line up with 0V's. In the service manual it talks about adjusting the skirt which I assumed is the plastic dial?
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Old 5th Jun 2020, 6:52 pm   #22
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Default Re: AVO VCM Mk 3.

I think Ive got the Grid voltage "Add" set correctly now by adjusting the skirt possition and RV4 Preset.

I have -52.5V at setting -101V, -7.87V at -15V and -2.62V at -5V. This means at the 0V position I have -130mV. I initially thought this would have to be 0V but this is only going to occurr when RV5 is fully A.C.W. Any comments welcome.

I still have the low anode current problem but the MkIII seems to be doing what it should?
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Old 6th Jun 2020, 9:55 am   #23
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Default Re: AVO VCM Mk 3.

I think you have your grid-volts potentiometer set up as best as it can be.

There are small difference between potentiometers to begin with and then over the years the wire might be move around on the track so that they change some and in some cases they get a little bit non-linear in places which might affect their tolerance against a scale.

I've also had problems with the zero position on a few AVO VCMs that I've calibrated, so I went with the best tolerance on those three points and let the owner know that low grid-voltage positions aren't exact (as most positions aren't anyway) and told them that it is as good as it can be. Replacing the potentiometer with a new one might not make it any better, just changing the voltages at those three points and possibly changing the zero position the other way, that you have to pass the zero position by a small amount until you get a negative grid voltage).

When it comes to low readings it might be due to lower voltages but also on something wrong on the measurement circuit, check the connection of the series resistors and also the 200 Ohm measurement resistor - check the wiring so they are connected correctly in the circuit, especially the 200 Ohm resistor so that you used the correct connections on it if you un-soldered them.
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Old 7th Jun 2020, 3:59 pm   #24
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Default Re: AVO VCM Mk 3.

Thanks again for your reply, much appreciated. It nearly went on the "Too hard to do shelf"
With the switches all set for anode current, I have rechecked continuity from the H.T. transformer 0V's through the over current relay coil to the cathode socket on the B9 base on the top panel.
Also the anode current path from top panel B9 socket via R36 (201 ohm) to the common of the anode volts switch and on to the 250V winding. I also rechecked connections to R28 (249.0k ohm) and meter.
R28 seems to be doing its job as the anode current reported by the MKIII is the same as the current going through the anode link, x.5. I followed the path in a similar way for the screen circuit.

What I have noticed is the poor waveform on the incoming mains. See attached. Comparing Martins photographs in the excellent circuit comparisons document and mine in post 14, the tops are flattened more. Martin also mentioned the strange -ve going part of my screen waveform. The 163 VCM seems to work ok on the same supply though, as far as anode current is concerned.
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Old 7th Jun 2020, 9:00 pm   #25
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Default Re: AVO VCM Mk 3.

Are you sure that mains waveform is not the 'scope at its limit? Try setting the volts/div. to next higher setting and see if that fixes the shape. Otherwise it looks quite clean.
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Old 7th Jun 2020, 9:24 pm   #26
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Default Re: AVO VCM Mk 3.

I first noticed it on the output of my variac thats fed from an isolation transformer. The voltage was low enough to be well inside the scope limits. I cant say I ever paid much attention to the mains waveshape before. Does it ever look good?
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Old 7th Jun 2020, 10:31 pm   #27
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Default Re: AVO VCM Mk 3.

These waveforms on the mains voltage is perfectly normal these days with equipment drawing power on the peaks and also power line communications distorting the sinusoidal waveform. It varies over the days/weeks/months/year depending on when power is drawn and the communication is the heaviest (what technique is used depends on which country you live in).

Power Factor Correction (PFC) and other filtering techniques tries to remedy part of this but it is a fact of the power we use.

I made some experiments with both motor-generators (dynamotors) and DC/AC-inverters with perfect 50Hz sinusoidal waveform and usually there are minimal effects on the VCMs but if you have really poor AC mains voltage due to heavy electrical machines in the surroundings (elevators or big motors that start and stop a lot) you will see a small effect on the measurements. Likewise if you have DC/AC-inverters for solar panels or a very strong interference from radio frequencies nearby and you use an AVO VCM163 you might see the mA/V-meter rise a little bit even when there is no measurement being made, switching off for instance solar panel inverters will sometimes make a few percent difference in the measurement if they are within the frequency range picked up by the amplifier.
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Old 2nd Jul 2020, 10:11 pm   #28
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Default Re: AVO VCM Mk 3.

I think I now have the tester as good as I can make it. I made two school boy errors which made things look worse than they were.

When I characterised the EL84 I was measuring the screen & anode current together.

My VCM 163, I then wrongly assumed was correct because it was reading the same anode current as my characterised valve.

Now the VCM 163 is calibrated and I have re run the DC tests on the EL84, the two results compare.

There still seems to be a large difference in anode current with some valves measured, but I can live with that for now. The thing is, I am not sure how much difference between these testers is normal. See below the differences in anode current between my 2 testers. Worse case just under 10%. The mA/V results between the two testers are very similar.

Valve-------------163--------MKIII

PEN 220--------10.0mA----10.2mA----- +2%
EL84------------48.0Ma----46.0Ma----- -4.2%
VR7/MH4-------10.5m-----11.0mA----- +4.8%
ECC82 a side---14.0mA----13.0mA----- -7.1%
--------b side---16.0mA----14.5mA----- -9.4%
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Old 3rd Jul 2020, 1:13 pm   #29
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Default Re: AVO VCM Mk 3.

Normally there should be no differences, if they both are properly calibrated and all controls work identical, and you test on identical voltages, worn grid-volts or mA/V potentiometers can make a small difference.

If you test those valves on regulated DC-powersupplies, what measurements do you get then? Are the DC-measurements more like your VCM163 or like your Mk III? That should tell you which one of your testers is best compared to DC-measurements.

How much do you have to move the grid-volts control or the mA/V control to get identical readings? If it is just a small amount, say one line or two then it is probably the potentiometers or the setting of the scale that is the reason for the differences you see.
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Old 3rd Jul 2020, 6:42 pm   #30
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Default Re: AVO VCM Mk 3.

A little more work required then.

I will run the DC test on the ECC82 as that gave the biggest error and report back.

Perhaps just an observation but Martin noticed earlier in thread that the anode and sceen voltages were down compared with the 163. I can see these drop around a couple persent more than the 163 voltages do when I change from no anode current to 50mA. So in total perhaps a 4 percent drop. ?
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Old 5th Jul 2020, 10:05 pm   #31
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Default Re: AVO VCM Mk 3.

I have re run an ECC82, EL84, and a MH4 on the bench with D.C. supplies and have the values for these. Before I go further can someone confirm that on the ammended circuit the slope switch "Inner and outer" are as on drawing. Despite the switch appearing to work correctly, mine shorts out R32 in the "Outer" position. The two possitions on the AVO origional drawing are drawn the opposite. I previously found a wiring error here and a wrong resistor value fitted now fixed.
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Old 8th Jul 2020, 2:33 pm   #32
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Default Re: AVO VCM Mk 3.

I re run the cal procedure again. I am quite happy that the grid volts are accurate and repeatable from 2.0 V onwards. Below that the pot is not so good. I could not detect any hysteresis on the grid or mA/V pots. I will be changing or cleaning the grid pot.
So find attached results between all 3 methods. Perhaps introducing the 163 into the mix wasn’t a good idea. The biggest error is on the mA/V results for the ECC82. I did a full plot for the valves and the ECC82 bias point looks more curved than the other 2. These are being operated in a more linear area, would any discrepancies therefore not cause a greater error in the mA/V measurements? The errors in anode current I think I can live with for now.
The meter has now developed a sticky point now in the rest position. I did check the anode current calibration again because of this and it’s still spot on, so I don’t think this is causing any problems elsewhere. I had early on already removed a flake of debris from the movement so looks like I will be out with the microscope again!
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File Type: docx results between all 3 methods.docx (12.7 KB, 28 views)
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Old 9th Jul 2020, 4:06 pm   #33
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Default Re: AVO VCM Mk 3.

I was not happy with the Mullard ECC83 results so I tried a new Mazda example. The current was lower on this valve, and in line with the printed results in the AVO data manual of 10.5mA. The two sides were also very well matched. When plotted the slopes appeared more linear as well. This time the mA/V results were closer.

D.C. conditions, 9.6 & 9.7mA / 2.4 & 2.4 mA/V.

MKIII, 10.5 & 10.5mA / 2.3 & 2.2 mA/V.

163, 10.9 & 10.8mA / 2.1 & 2.1 mA/V.

I will live with these differences for now, but comments always welcome. I feel a bit of a new boy to all this as I’ve only recently taken up playing with valves in the last 4 or 5 years. Previously to this it was back in the late 70’s. So thanks for your patience.
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