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Old 26th May 2020, 10:03 am   #1
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Default AVO VCM MK3. Meter needle not moving when switched on.

I now have a new problem to sort out. When I switch on the MK3 the needle on the meter does not rise to the SET position in fact it does not rise at all. I checked the heater, anode volts which are good but when moving on to set the neg grid volts I do not get the expected volts so I tomorrow I will have to check out that part of the circuit.
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Old 26th May 2020, 10:32 am   #2
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

I would start by carefully checking the meter movement itself.

These are notorious for going OC

I have a MK 3 here waiting for a meter transplant (if I ever get the time)

Hopefully its something less drastic !

Cheers

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Old 26th May 2020, 10:46 am   #3
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

That's been sorted:-

https://www.vintage-radio.net/forum/...d.php?t=166489
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Old 30th May 2020, 4:36 pm   #4
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Not having any luck finding what the problem is.
I did decide to go through the procedure of testing a valve and as I expected could not complete the procedure. When I did the heater check it did not test as short as it should do and when switched to test position on the circuit selector the readings on the meter climbed much higher than they should. Also I noticed that when I moved the neg grid control it was not affecting the meter needle movement as much as one would expect. Will go through the circuit again.
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Old 30th May 2020, 4:42 pm   #5
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

I'm not familiar with this tester, but you're going to have to study the circuit diagram and find out how the meter is connected when doing the heater continuity check. There'll be some kind of series circuit with the heater, meter, voltage supply and possibly a few resistors. Start at the voltage supply and work towards the meter checking voltages.
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Old 30th May 2020, 6:26 pm   #6
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

It seems like R19, the 2.96 MOhm resistor (made up of a pair of 1.48 MOhm resistors) is not connected in the circuit.
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Old 31st May 2020, 12:50 pm   #7
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Continuing to study the circuit and locate where everything is.
I did attempt to adjust the neg grid volts and found there were no DC voltages at any of the settings so will have to looking that.
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Old 31st May 2020, 1:02 pm   #8
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Should I be getting a reading of 125 volts at RV6?
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Old 31st May 2020, 1:18 pm   #9
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

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Originally Posted by HBWOODY View Post
Should I be getting a reading of 125 volts at RV6?
Yes, if you look at the schematic RV6 is connected to the 125V winding.

What voltage do you have between the anode(s) of V1 and the junction between R3, R4 & R5?
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Old 31st May 2020, 2:39 pm   #10
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Voltage between the anodes 117v
at junction between R3, R4 & R5 95v
That is with the link open.
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Old 31st May 2020, 2:48 pm   #11
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

I meant between A (Anodes of V1) and COMMON (common side of R3, R4 & R5) in the included circuit diagram.

Measure with link open and closed so you get both measurements, and measure AC and DC voltages.
Attached Thumbnails
Click image for larger version

Name:	Anodes-Common.jpg
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Last edited by Dekatron; 31st May 2020 at 2:50 pm. Reason: Added measurement instruction.
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Old 31st May 2020, 6:06 pm   #12
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Thank you for getting the thumbnail together for me, hopefully these are the measurement you require:-
LINK OPEN. 58 V DC & 47 V AC
LINK CLOSED. 117 V DC & 42 V AC
I am trying to trace from the LINK to the 125volt tap on the transformer.
The positioning of the power supply panel is not good.
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Old 31st May 2020, 6:13 pm   #13
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

If you get those voltages there and no indication on SET AC or any of the CHECK tests it is probably the 2.96 MOhm resistor that is open circuit or broken or the wire in that circuit that has broken off somewhere.
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Old 31st May 2020, 7:35 pm   #14
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

I will do a thorough search in that area when I have time.
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Old 1st Jun 2020, 10:17 am   #15
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

"Should I be getting a reading of 125 volts at RV6?"
I should have phrased the above question better. I should have said that I am NOT getting the expected 125volts at RV6 and I am struggling to find the correct tap on the transformer to check continuity between the tap and RV6.
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Old 1st Jun 2020, 10:24 am   #16
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

There should be continuity between RV6 and ALL the taps on that winding of the transformer. The 125V (LK) tap will have the lowest resistance to RV6.
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Old 1st Jun 2020, 10:41 am   #17
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

You can also trace it to the Anode & Screen volts switches as I think the wire from the circuit board connects to them, at least it looks that way in old photos I have of my Mk III.
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Old 1st Jun 2020, 11:03 am   #18
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Quote:
Originally Posted by HBWOODY View Post
Should I be getting a reading of 125 volts at RV6?
Normally the voltage at the input to RV6 won't be what is shown on the schematic, eg: 125 volts, it should be approx. 137.5 volts, that's because the actual voltage supplied to the anode and screen of the valve under test will be 1.1 times higher than DC voltage indicated on the anode and screen voltage selector switches, applying 1.1 times that voltage applies to all these types of Avo testers going back to the first two panel testers.

In the modified ones (silicon rectifiers fitted) the feed to RV6 is taken from the 150 volt tap according to the schematic.

If that's any help.

Lawrence.

Last edited by ms660; 1st Jun 2020 at 11:24 am. Reason: Extra info
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Old 1st Jun 2020, 11:06 am   #19
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

Quote:
Originally Posted by HBWOODY View Post
"Should I be getting a reading of 125 volts at RV6?"
I should have phrased the above question better. I should have said that I am NOT getting the expected 125volts at RV6 and I am struggling to find the correct tap on the transformer to check continuity between the tap and RV6.
Does that mean you're measuring zero volts at that point?

If not, what voltage are you measuring?
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Old 1st Jun 2020, 11:28 am   #20
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Default Re: AVO VCM MK3. Meter needle not moving when switched on.

The reading is 15.7 volts ac with the LINK open measured from the transformer side of the LINK.
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