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Old 3rd Jun 2020, 1:18 pm   #1
Chrispy57
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Default Avo 2 Panel Valve Tester - Meter sticky

Inspired by an earlier thread on here I decided to finally check out the Avo 2 Panel Valve Tester that I bought on EBay a few years ago and have dry stored since. Again, acknowledging the very helpfull documentation that Forum members have uploaded on earlier threads, I have cleaned it up, switches, potentiometers sockets etc then worked through the calibration checks.

The issue that I'm looking for further advice on is the visible surface contamination of the metalwork in the meter movement, that I suspect is causing a "stickiness" in the action of the pointer. The attached photos show this flaky surface problem (when zoomed), and the sample photos of meter sensitivity checking, obtained after a slight tapping or wobbling of the panel if necessary to free the pointer, suggest that it can be pretty accurate when not sticking.

Does anyone have experience of treating this problem successfully that they can share?

Cheers
Chris
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Old 3rd Jun 2020, 2:27 pm   #2
The Philpott
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

Hi Chris, i would say that in 70% or more sticky movements there is indeed one or more ferrous flakes obstructing the moving coil. Because this debris isn't solidly fixed, merely held by magnetism, a tap can sometimes be enough for the flake to 'lean over' a little and allow the needle to pass. Most of these tiny, tiny, flakes seem to me to be shaped like a fragment of slate- very thin with jagged edges. If they happen to be edge one to your line of sight, they can be invisible even with x10 jeweller's loupe!

There are different preferred methods of tweaking out debris, ranging from blu-tack on a piece of solder or a cocktail stick, to a piece of post-it note to deliver the necessary grab to pull stuff out of the magnetic influence. Blu-tack is more effective if it's been worked in the hand for a while.

Not all debris is visible from the rear of the movement of course- several times i have had to pull Avo movements out of the instrument to hold them up to the light.

Just occasionally i have remedied a sticking needle by either loosening or tightening one of the jewel pivots by 1/8th turn or so. Very much trial and error, and not (entirely) without risk. Try looking for debris first! If the sticking is always in one distinct place i would say it's unlikely to be pivot problems.

Dave
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Old 3rd Jun 2020, 8:40 pm   #3
Chrispy57
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

Hi Dave,
thanks for kindly sharing your experience with Avo movement stickiness - I struggled for an adequate description, but I do like "ferrous flakes"! No Blu-Tack but I have tried your second suggestion of post-it note strips gently around the coil and blowing a few times and it may have had a positive effect. The panel was face down at the time and I was exercising the movement with an external current-limited PSU when it suddenly began moving smoothly again, so fingers crossed I may have got away with it this time.

While I was in the general area I decided to make up a small strip-board with pairs of 1N4007 and BAT23 diodes and a 100uF capacitor, as per the wisdom of Karsten Simon, and replaced a 0.050uF wax capacitor that read 293nF on my newly received replacement Duratool DMM that arrive from CPC this afternoon. A couple of 100nF 400V caps in series gave a nice 50.5nF combination. Together with adjusting the mains tapping from 240V to 250V I can now achieve a 5% calibration at 100V in the screen voltage test.

Now that I have violated the authenticity of the tester to protect the meter, I am toying with the idea of fitting a switched, fused, IEC socket to the rear of the case to protect me rather than jamming the old mains cable grommet back in situ in an un-earthed metal case. Apologies to the Purists!

I hope to have a look inside the valve panel tomorrow and make some more progress, but thanks again for your help this afternoon.


Cheers
Chris
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Old 3rd Jun 2020, 8:45 pm   #4
Superscope
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

Hi Chris,

I think the first thing to check out is what type of Stickiness your Movement has.

If it is sticky in certain places and not in others, then I would follow Dave's advice.
This type of Stickiness is invariably due to Ferrous contamination in the Air Gap.

However, if the Stickiness is right across the Range of the Movement or most of the
Range, you almost certainly have a Tight, Dirty or Damaged Jewel or Pivot.

Hopefully Tight in which case loosening it will solved the problem.

or Dirty, more complex but mostly solvable.

For the Jewel, Avo recommended using a small sharp piece of Orange Wood.
I use a Cocktail Stick soaked in Alcohol.

The Pivots unfortunately are extremely difficult to clean and I haven't mastered that one yet,
but I'm working on it!.

If you have a damaged Jewel or, Pivot, you probably out of Luck without specialist help





Ian
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Old 4th Jun 2020, 8:35 pm   #5
Chrispy57
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

Hi Ian,
thanks for your advice on my meter problem, which has increased my level of confidence that it may well have been the ferrous contamination issue, that has now resolved - for the present.

As I have observed the meter movement action with physical movement of the tester, it does seem that there is very little damping of the meter, much less than an equivalent age of Avo 7 that I have. I don't know if this is characteristic of these meters, or indicative of another slight issue with mine, but I think it's probably good enough for me to be able to use, once I have checked over the valve panel.

Thanks for your assistance.

Cheers
Chris
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Old 4th Jun 2020, 9:41 pm   #6
Electronpusher0
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

When you say there is little damping are you testing the meter in or out of circuit?
Out of circuit it is totally undamped.
When in circuit in the tester the meter is shunted by resistors and the damping is better.

Peter
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File Type: pdf AVO 2 panel valve tester meter circuit.pdf (76.8 KB, 48 views)
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Old 5th Jun 2020, 11:44 am   #7
Chrispy57
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Default Re: Avo 2 Panel Valve Tester - Meter sticky

Hi Peter,
thanks for your advice, and the circuit which makes for much easier understanding than the Avo schematic. I was talking about an unpowered tester, with switches in random positions - so meter out of circuit. It looks OK when approaching and settling on a reading, but seemed excessively "floaty" around zero when moving the tester, making me cautious about handling the base unit.

Cheers
Chris
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