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Old 7th May 2020, 4:24 pm   #1
vintage_8bit
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Default AVO VCM Mk 3.

AVO VCM MK3. Resistors.

Hello again.

I have a mk3 which has now been retired along with my self, from my company. Its been there probably from new and appears almost untouched inside.

It is reading around 20% down on current and mA/V readings.

I have checked the meter and its FSD is 30.1uA, with a coil resistance of 3223 ohm.
In preliminary investigations I have noticed that R35 10K is reading high at 11.54K. There are at least 2 other resistors that show signs of overheating.

Im working my way through all the resistors just now.

My question is....
Are there any corrections to the circuit diagram or parts list in the service manual that would be handy to know. I noticed, it looks like RV3 and RV5 are marked the wrong way round in my issue. So wondered if there are any more errors. The 2 burnt resistors I can see have lost the values on them. If parts list is ok, I can continue with confidence.
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Old 7th May 2020, 7:49 pm   #2
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Default Re: AVO VCM Mk 3.

If you use 10053 Ohm across the meter it will be within 0.5% of what it should be. Otherwise I'd leave it as it is.

I've never seen RV3 & RV5 exchanged in any schematic, RV5 being the Grid Volts potentiometer and RV3 being the mA/V potentiometer in all that I have.

There is one error that can be found in many of the schematics for the AVO Mk III and that is that R39 @ 750 Ohm is missing. It is positioned between R32 and the switch contact in the mA/V position on the Meter Switch so that R32 is shorted by the "inner" position on the slope switch but R39 is always connected. See the cutout from my corrected schematic. There might be Mk IIIs out there that is missing this resistor but all that I have repaired have had this resistor.

AVO had a lot of errors in their schematic diagrams and seems to have forgotten to add modifications in them when they changed details in their VCMs.
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Old 7th May 2020, 8:41 pm   #3
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Default Re: AVO VCM Mk 3.

R39 could of caught me out thankyou.

Ill check RV3 and 5 again, that might be my mistake?

I will be changing R35 as advised, again thankyou.
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Old 7th May 2020, 10:12 pm   #4
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Default Re: AVO VCM Mk 3.

Hi.

My Mk III is ex Rolls Royce from Bristol but suffered an o/c meter coil after several years of ownership. Fortunately, I was able to purchase a meter kit many years back which included a small amplifier using a 741 op amp and also a power supply for the amp. I transfered the moving coil assembly from the new meter to my original meter. It has been working well ever since the repair which was about 15 years ago. I can't recall where the kit came from and don't know if they're still available. I believe that meter coil problems do occur fairly often with Avo VCMs.

I really ought to give the unit a good service. I would like to pay particular attention to the thumbwheel switches as these seem to be quite stiff and would benefit from a stripdown and clean.
I'd like to add a B9D base to my Mk III for checking TV valves such as the PL508, PL509, PY500A etc. It looks to be fairly easy to do. I'll try to find a chassis mount B9D base, I only have PCB versions to hand at the present time. The Mk IV has this base fitted.

As a youngster, I always wanted a Mullard valve tester after seeing one in operation in a TV workshop that I gained early experience working in. I remember the Mullard tester had a large lever like a one arm bandit and a CRT type display to show the characteristics. That was back in around 1974. I think though the Avo VCM is more versatile and is a much better proposition. The trouble with the Mullard is mainly the cards, especially if missing though they can be replicated apparently.

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Old 18th May 2020, 1:07 pm   #5
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Default Re: AVO VCM Mk 3.

I had the option of a Mullard recently but not got the room here.

Back to the MkIII.

So after checking the meter I have gone through all the resistors and 1 cap, I changed any out of spec.
My question now is about the "Set mA/V" calibration. The grid volts cal appears spot on.
In the service manual Section 2(e) and with the grid circuit link removed, I performed the following.

I am reading the grid voltage with a
keithley 2000 DVM or a cheapy hand held DVM. With the meter switch to 2.5mA I am adjusting the the Neg grid volts controls to 105mV I am then setting meter switch to mA/V and nulling out the grid volts with the set mA/V control. The result should be 5. I'm getting 5, so good. If I do the same with the grid volts at 0.52V I get a result of 1, which is also right. However the spec also asks you to perform this with a grid volts of 52.5V. this would mean producing a +ve grid volts to null out? I am obviously doing something very wrong here, but what is it?

Just had another go at it.... Is it a typo? should it be 52.5mV. This would be very difficult to achieve on my tester as the grid volts Add control is on the edge of its track.
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Old 23rd May 2020, 5:13 pm   #6
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Default Re: AVO VCM Mk 3.

After checking all the resiotrs and meter, I have now been through the cal procedure.

Checking a charactorised EL84 using D.C. power supplies the mklll is reading low on current and mA/V. I have tried a 6L6 with similar low results. I have made some comparasons as follows.

VCM 163. 55mA, mA/V, 12.8.
DC bench supplies. 53mA, mA/V 14.
Mklll. 35mA, mA/V 10.2

I have re checked the grid voltage (Link removed) and checked the meter again and its linearity. The current reading ties up with my multimeter using the anode link (X.5).
Any thoughts on further checks I could make.
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Old 23rd May 2020, 6:27 pm   #7
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Default Re: AVO VCM Mk 3.

What are the voltages measured on the EL84 under test on the VCM163 and the Mk III (heater, anode, screen and grid)?
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Old 23rd May 2020, 7:17 pm   #8
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Default Re: AVO VCM Mk 3.

Will do. Just D.C. measurements?. AC for heater of course.
Just thought, anode will be an AC measurement as well?

Last edited by vintage_8bit; 23rd May 2020 at 7:33 pm. Reason: Afterthought.
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Old 23rd May 2020, 11:03 pm   #9
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Default Re: AVO VCM Mk 3.

Thanks for the quick reply Martin.

First the MkIII. EL84 fitted. 35mA

aV 267V ac.
g2 146V ac / 120V dc
g1 3.98 ac / -8.22 dc. . mA/V possition and nulled, 10.22 ac / -14.57 dc.
h 6.30V ac.

Now the 163. EL84 fitted. 55mA
aV 197V ac. 200mV ac.
g2 159V ac / 129V dc
g1 4.24 ac / -3.92 dc. .
h 6.39V ac.

Quite a difference with the D.C. grid volts?
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Old 23rd May 2020, 11:49 pm   #10
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Default Re: AVO VCM Mk 3.

Measure both AC & DC voltages so that you can see how close they are between the testers (or how far off).

The correct Grid voltage in the Mk III can only be measured with the valve removed and the "SET mA/V calibration" link open so you only measure the negative period used for grid voltage and not the extra negative period that is added for the use of leakage/shorts tests.

Also make sure that the meter you use can measure half wave rectified voltages properly on the DC and AC ranges.

If you have an oscilloscope (connected to an isolating transformer so you accidentally don't form shorting ground loops) you can check what the voltages looks like with the valve under test - there is a slight chance that testing an EL84 results in it oscillating (more so if you measure the voltages present with long wires to the instrument), which will result in strange readings. This is why using a triode as a "calibration" standardized valve is a better option as they are harder to get to oscillate while being tested.

When you get correct DC-readings for both testers you can use those with DC power supplies to see how the EL84 performs under those conditions to try to rule out if any of the testers shows incorrect readings at those voltages, or if there might be something else that is incorrect. Don't forget that the EL84 might oscillate with just DC-voltages present, so short wires and possibly grid stopper resistors and capacitors close to the socket and other means to hinder oscillations might be necessary there too.
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Old 24th May 2020, 4:02 pm   #11
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Default Re: AVO VCM Mk 3.

I have checked both my DVM's they seem to measure correctly when set to DC on a half wave rectified voltage (.45) There is a discrepancy when looking at the same waveform on ac...... With 200V RMS and a silicon diode I get 103V on my keithley 2000 and 111V on my cheap Digitek handheld DVM. I used this for my measurements in post 9 so I should perform the tests again with the Keithley.

Going back again through the cal procedure, I have noticed that RV6 has to be set to max to hit the set mark. I will investigate this as I assume this is not desirable.

In post 6 you will see the values I obtained from just using bench supplies on the EL84. As you mentioned oscillation can happen, when I first tried this I had trouble with it oscillating. The cure was nice short wires. You can see the results were similar to the 163.
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Old 26th May 2020, 10:58 pm   #12
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Default Re: AVO VCM Mk 3.

I haven't rechecked my voltages comparisons between the 163 and MkIII yet due to the inconsistencies between my two meters.

I am confident with the EL84 measurements I made with the DC bench supplies though, with the HT at 250V bias at -7.3V results were aI = 53mA, mA/V = 14. This matched the 163 results close enough I thought?, see post 6.

I coudent see any signs of oscillation on the MkIII using an isolated scope. Please see attached, pictures of the grid waveform with and without the link in.

What I have tried since though is isolating the grid circuit to the valve using the thumb wheel switches and applying the 7.3V bias (as per the AVO data book for the EL84) with a DC supply. It was nice to see the anode current now the same as the D.C. bench tests, and the 163 at 55mA. Not sure what's next yet as cal procedure for bias appears to work. Do the waveforms look correct?............. I have just found similar waveforms in Martins "AVO VCMs Circuit Comparisons" literature.
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Last edited by vintage_8bit; 26th May 2020 at 11:09 pm. Reason: Further reserch
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Old 27th May 2020, 7:42 am   #13
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Default Re: AVO VCM Mk 3.

Can you use the scope on the VCM163 and check the grid volts there too? If possible also check the anode and screen voltages with the scop on both the VCM163 and the Mk III so you can compare them.

I can't really see how many volts the grid voltage is on the scope screen but the waveform looks right.
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Old 27th May 2020, 5:03 pm   #14
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Default Re: AVO VCM Mk 3.

Martin, find attached 4 scope images as you requested.

Photo filename prefixed 1 is anode & grid of the MKIII.

Photo filename prefixed 2 is screen & grid of the MKIII.

3 & 4 are the same points but on the 163.

The scale for channel 1 has been adjusted to give 100V / div. The scale is correct for channel 2 at 5V / div.
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Old 27th May 2020, 5:12 pm   #15
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Default Re: AVO VCM Mk 3.

One more question: What roller selector settings did you use on the Mk III and the VCM163?

The screen voltage on the Mk III looks a little bit strange in between the measurement portions, it slightly dips and then rise somewhat instead of being a straight line.
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Old 27th May 2020, 5:51 pm   #16
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Default Re: AVO VCM Mk 3.

MKIII 041 230 605

163 041 230 807

I did at one point connect screen and anode together by setting the thumb wheels both to anode and so eliminating the screen diode. I got the same results.
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Old 27th May 2020, 6:57 pm   #17
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Default Re: AVO VCM Mk 3.

Apart from the voltages being somewhat lower in your Mk III compared to the 163 in your scope photos I can't see any reason for the anode current to measure lower unless the resistors in the measurement circuits are wrong - have you measured them all and checked their resistance compared to the BOM?

You mentioned some resistors that looked overheated, could there be any switches that have some contact resistance that might affect the current flowing in the circuit?
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Old 27th May 2020, 8:36 pm   #18
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Default Re: AVO VCM Mk 3.

I have been through all the resistors but I think I will go through them again. I did change quite a few. R32 was the wrong value a 7.5k had been fitted should be 6.8K.
I was encouraged with the current up at 55mA with the external DC bias. I will report back.
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Old 27th May 2020, 9:36 pm   #19
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Default Re: AVO VCM Mk 3.

Also check the wiring so nothing has come loose or is in the wrong place, if you changed any of the wiring or if AVO had it wrong to start with, Iíve seen a few errors and also wires shorting in AVO VCMs.

You could also check the EL84 at other operating points to see if it looks good with other anode currents, a more negative grid voltage should result in lower anode current and then you can change anode current range and see how it works then.
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