Avo 7X - needle won't zero - UK Vintage Radio Repair and Restoration Discussion Forum

LCTorana · 29th Apr 2014, 1:57 pm

I've just acquired an Avo 7X Panclimatic in nice cosmetic order, but the needle won't rest at zero even at the minimum setting of the case screw.

The needle rests at circa 10% FSD.

On DC voltage ranges, the needle *will* swing back to zero if a small negative voltage is applied.

Thoughts?

pmmunro · 29th Apr 2014, 2:49 pm

Provided that the movement of the pointer is free throughout the scale, including the part below the rest point of the pointer, you should be able to adjust the range of the front adjuster screw be resetting the rear hairspring adjuster.

If this offset is a recent development and it is not due to an overload, one of the adjuster lock nuts may have come loose.

If for any reason, you have to take the movement off the front panel, you need to be aware that a non-magnetic screwdriver must be used and that the cut-out will have to be adjusted when the movement is put back.

I'm assuming that you are using your meter on its back, i.e. with the scale horizontal - as it should be. The moving coil should be balanced so that there is no more than 1% movement from zero with the body vertical and the scale zero line either horizontal or vertical.

PMM

LCTorana · 30th Apr 2014, 2:00 pm

Thanks PMM,

Still looking into it.

I have a horrible feeling the problem is down to me touching the magnet with a screwdriver. If so, mea culpa but I'm not giving up on it.

The problem is now greatly reduced. I ***can*** now zero the meter with the front screw, and the needle goes up 1.5% when I stand the meter up.

The pointer is a little sticky. When I try a 4.5V battery on the 10V scale, the first reading is 4.3V, I disconnect and it swings to 0.02V and settles back to 0.00 after a few seconds, a second try giver the true 4.7V reading.

Readings in FSD territory are much better.
Both a fresh 9V battery on the 10V scale, and a 4.5V battery on the 5V scale give accurate readings and return to zero more swiftly.

Hope I'm not being a pain. Appreciate the help.

pmmunro · 30th Apr 2014, 2:53 pm

No problem LCTorana. I'm glad you're getting there.

It could be that the pivots of the moving coil of your meter are a bit sticky. You could try washing them out with a little isopropyl alcohol applied with a small artists paint brush in the first instance. I would give this a try with the movement fully assembled. Let the fluid dry off before you assess the results. Make sure that no sticky contaminant is left on the hairsprings - you don't want the coils sticking together.

One correspondent to this forum tried applying an intermittent pulse to the meter for a day or two - he connected it to one of the direction indicator lamps of his car. I seem to remember it freed of the movement of his meter to some degree.

With the movement back on the panel, you will have to adjust the cut-out. The procedure depends on the age of your meter. I think yours will look like the one in the attached image, except the contacts 8 & 9 will be parallel. If your mater has the later cut-out with die-cast parts, let me know and I'll try to find the procedure for that.

To adjust the cut-out, slacken screw 21 and post 15 to allow you to rotate the table 22. The AVO "Recommended Spares List" for the Model 7 Mk I & II states that "The application of a sudden forward overload of 20:1 should cause it to trip every time, an overload of 4:1 should not". It sounds a bit brutal but that what they state.

If you feel up to it, I suggest that you dismantle and clean the cut-out table and the stem of the button, part 6. The screws 16 are shouldered and I have found it is best to put them back where they came from although, in principle, they should all be the same size. Note the setting of the spring 18 before you start.

Give the table, including the tubular part inside, and the stem of the knob a good polish with brass polish and clean thoroughly with alcohol (methylated spirit in the UK) then isopropyl alcohol to remove any remaining abrasive particles. Cotton buds (cotton tips) are useful here. Check there are no burrs on the bell crank 13 and the lever 20 and remove any with a very fine stone. Be especially carefully with the synthetic sapphire pallet stone in the bell crank 13. They can come detached and you don't want to lose it.

Its a delicate job with parts you can't afford to lose or damage, but at the age of your meter, most cutout tables could do with a clean to make the parts move freely.

While the movement is off the panel, give the three pot. tracks a good clean with isopropyl alcohol and a cotton bud.

PMM

LCTorana · 30th Apr 2014, 3:44 pm

Thanks PMM for the trouble you're taking.

One question entirely escapes me - why would you presume the cutout needs adjusting? Using my ham-fisted logic, I would have thought that if the needle gives the correct reading then the cutout is making electrical contact.

Mind you, when I opened the parcel for the first time, the cutout was popped...

pmmunro · 30th Apr 2014, 4:14 pm

The Avometer cutout is essentially in two parts linked by a wire pushrod passing through a guide bracket screwed to the front panel.

The movement part has two operating modes. The simpler of the two is an end stop trip at each end of the moving coil's limit of travel. The coil frame will contact one of the two bell cranks situated below the moving coil and these will pivot downwards to bear on the phosphor bronze leaf spring fitted below the movement. The spring then applies pressure to the pushrod.

The other mechanism is an inertia trip which comes into action if the moving coil accelerates too rapidly. The pointer is pivoted independently from the moving coil but held in its mormal relative position to the moving coil by a spring wire. On acceleration, the moving coil will lag and displace a lightweight bell crank whose edge will engage the teeth on the aluminium disc pivotted on the magnet's concentrator. The serrated disc has a slot at the bottom whose edges are adjacent to the pair of cranks below the moving coil and when it pivots it too will exert a force on the leaf spring and hence the pushrod.

The wire pushrod acts on an eye at its lower end to pivot the crank on the cutout table, releasing the lever and the plunger of the cutout.

When the movement is removed from the panel, the clearance between the leaf spring and the pushrod will in all likelihood change and its is this change which makes adjustment necessary.

PMM

LCTorana · 1st May 2014, 1:45 am

Ahh! The penny drops.

Funny, I've been using an Avometer model 7 for over 20 years and have always thought of the cutout as an electrical device; a sort of circuit-breaker.

Now I get it - it must also be mechanically linked to the pointer if it limits acceleration.

I'll have a close look at the cutout, and use the isopropyl alcohol, but my courage might fail me beyond that.

One more trivial fault with this machine - to get the DC ranges to work, the AC switch needs to be turned to a point midway between the "DC" position and the white line before POWER DECIBELS. A point between the detents. Could this be just a knob misalignment?

29th Apr 2014, 1:57 pm	#1
LCTorana Triode Join Date: Mar 2009 Location: Melbourne, Australia Posts: 17	Avo 7X - needle won't zero I've just acquired an Avo 7X Panclimatic in nice cosmetic order, but the needle won't rest at zero even at the minimum setting of the case screw. The needle rests at circa 10% FSD. On DC voltage ranges, the needle *will* swing back to zero if a small negative voltage is applied. Thoughts?

29th Apr 2014, 2:49 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avo 7X - needle won't zero Provided that the movement of the pointer is free throughout the scale, including the part below the rest point of the pointer, you should be able to adjust the range of the front adjuster screw be resetting the rear hairspring adjuster. If this offset is a recent development and it is not due to an overload, one of the adjuster lock nuts may have come loose. If for any reason, you have to take the movement off the front panel, you need to be aware that a non-magnetic screwdriver must be used and that the cut-out will have to be adjusted when the movement is put back. I'm assuming that you are using your meter on its back, i.e. with the scale horizontal - as it should be. The moving coil should be balanced so that there is no more than 1% movement from zero with the body vertical and the scale zero line either horizontal or vertical. PMM

30th Apr 2014, 2:00 pm	#3
LCTorana Triode Join Date: Mar 2009 Location: Melbourne, Australia Posts: 17	Re: Avo 7X - needle won't zero Thanks PMM, Still looking into it. I have a horrible feeling the problem is down to me touching the magnet with a screwdriver. If so, mea culpa but I'm not giving up on it. The problem is now greatly reduced. I ***can*** now zero the meter with the front screw, and the needle goes up 1.5% when I stand the meter up. The pointer is a little sticky. When I try a 4.5V battery on the 10V scale, the first reading is 4.3V, I disconnect and it swings to 0.02V and settles back to 0.00 after a few seconds, a second try giver the true 4.7V reading. Readings in FSD territory are much better. Both a fresh 9V battery on the 10V scale, and a 4.5V battery on the 5V scale give accurate readings and return to zero more swiftly. Hope I'm not being a pain. Appreciate the help.

30th Apr 2014, 3:44 pm	#5
LCTorana Triode Join Date: Mar 2009 Location: Melbourne, Australia Posts: 17	Re: Avo 7X - needle won't zero Thanks PMM for the trouble you're taking. One question entirely escapes me - why would you presume the cutout needs adjusting? Using my ham-fisted logic, I would have thought that if the needle gives the correct reading then the cutout is making electrical contact. Mind you, when I opened the parcel for the first time, the cutout was popped...

30th Apr 2014, 4:14 pm	#6
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avo 7X - needle won't zero The Avometer cutout is essentially in two parts linked by a wire pushrod passing through a guide bracket screwed to the front panel. The movement part has two operating modes. The simpler of the two is an end stop trip at each end of the moving coil's limit of travel. The coil frame will contact one of the two bell cranks situated below the moving coil and these will pivot downwards to bear on the phosphor bronze leaf spring fitted below the movement. The spring then applies pressure to the pushrod. The other mechanism is an inertia trip which comes into action if the moving coil accelerates too rapidly. The pointer is pivoted independently from the moving coil but held in its mormal relative position to the moving coil by a spring wire. On acceleration, the moving coil will lag and displace a lightweight bell crank whose edge will engage the teeth on the aluminium disc pivotted on the magnet's concentrator. The serrated disc has a slot at the bottom whose edges are adjacent to the pair of cranks below the moving coil and when it pivots it too will exert a force on the leaf spring and hence the pushrod. The wire pushrod acts on an eye at its lower end to pivot the crank on the cutout table, releasing the lever and the plunger of the cutout. When the movement is removed from the panel, the clearance between the leaf spring and the pushrod will in all likelihood change and its is this change which makes adjustment necessary. PMM

1st May 2014, 1:45 am	#7
LCTorana Triode Join Date: Mar 2009 Location: Melbourne, Australia Posts: 17	Re: Avo 7X - needle won't zero Ahh! The penny drops. Funny, I've been using an Avometer model 7 for over 20 years and have always thought of the cutout as an electrical device; a sort of circuit-breaker. Now I get it - it must also be mechanically linked to the pointer if it limits acceleration. I'll have a close look at the cutout, and use the isopropyl alcohol, but my courage might fail me beyond that. One more trivial fault with this machine - to get the DC ranges to work, the AC switch needs to be turned to a point midway between the "DC" position and the white line before POWER DECIBELS. A point between the detents. Could this be just a knob misalignment?