AVO 7 Mk 1 repair. - UK Vintage Radio Repair and Restoration Discussion Forum

Leon Crampin · 27th May 2012, 3:48 pm

I posted details of an AVO 7 in the database recently (Ser. 71145 - 24432) and undertook to report back on the repair of a number of faults - probably best done here.

The sticking movement was due to foreign material in the moving coil air gap, some of which was solder splash. Peter's advice to use a very soft brush to move the coil assembly was most helpful.

The whiskering on the moving coil terminations and balance posts was largely removed with a fine paint brush dipped in alcohol. This has the appearance of flux corrosion, although I understand flux free solder is advised.

With the movement removed, I found the serial number of the instrument written in pencil on the inside of the front panel, verifying that the movement is original.

I did not attempt to re-balance the movement as it provides a very marginal advantage in use for a lot of work. The remaining problem was a degree of stiction in the pivots (the end float was OK) which I have overcome in a rather crude fashion.

The proper approach is to remove and polish the pivots and to clean out the jewel recesses but I decided on a more unconventional remedy:

This meter has spent the last couple of days in the boot of my car set to the 10V DC range and connected to the right hand indicator circuit. It has thus seen quite a few excursions to over full scale and back. In conjunction with the car vibration, this has very largely overcome the stiction (sorry, Peter).

Finally, I found the cut out platform to be immovable (shellac?) for resetting, but solved the problem completely by adjusting slightly the position of the movement on its mounting holes - move the movement towards the bottom of the case for a more sensitive setting. This could be done within the size of the original fixing holes.

Why, on almost every AVO I've seen are the movement fixing screw heads damaged? This job was completed by refacing them with a fine file and very carefully cleaning them before replacing.

Another fine old instrument made usable - it's interesting to see how the damping varies with the use of div2 and other settings which affect the movement shunting.

Leon.

greenstar · 1st Jun 2012, 11:50 am

Leon, thanks for this. To my mind a tricky repair, and am impressed by the use of your indicators! It inspired me to look at my own Avo 7, which was not working on the DC volt ranges. A fairly simple fault to trace, as I found the first section of the upper wirewound shunt open circuit, and have temporarily inserted a 470 ohm resistor, and DC volts are working. It should be 450, which appears not a preferred value. Looks like a 10k resistor in parallel will do the trick.
On this meter two of the shunt sections have already been replaced by carbon film resistors.
All else appears good, including the movement fixing screws! I'm glad I had a go at this, what at first looked like a bird's nest turned out to be quite approachable.
Thanks Leon!
Tony

greenstar · 1st Jun 2012, 1:16 pm

Ha, that was premature. I have two voltage ranges, 100v and above not working yet. I've posted a request in case anyone has a spare shunt, as mine appears largely fried. The rest of it looks fine - ohms and amps are working.

1st Jun 2012, 1:34 pm

AVO did put a line of grease in the back of the case to trap any particles, a brilliant solution to the problem. It is so good and simple our (the works) mil spec. laptops have a bit of double sided sticky tape in the lid where the display goes (own trumpet time, it was I who suggested it). You can learn an awful lot from 'how it was done' (properly).

27th May 2012, 3:48 pm	#1
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	AVO 7 Mk 1 repair. I posted details of an AVO 7 in the database recently (Ser. 71145 - 24432) and undertook to report back on the repair of a number of faults - probably best done here. The sticking movement was due to foreign material in the moving coil air gap, some of which was solder splash. Peter's advice to use a very soft brush to move the coil assembly was most helpful. The whiskering on the moving coil terminations and balance posts was largely removed with a fine paint brush dipped in alcohol. This has the appearance of flux corrosion, although I understand flux free solder is advised. With the movement removed, I found the serial number of the instrument written in pencil on the inside of the front panel, verifying that the movement is original. I did not attempt to re-balance the movement as it provides a very marginal advantage in use for a lot of work. The remaining problem was a degree of stiction in the pivots (the end float was OK) which I have overcome in a rather crude fashion. The proper approach is to remove and polish the pivots and to clean out the jewel recesses but I decided on a more unconventional remedy: This meter has spent the last couple of days in the boot of my car set to the 10V DC range and connected to the right hand indicator circuit. It has thus seen quite a few excursions to over full scale and back. In conjunction with the car vibration, this has very largely overcome the stiction (sorry, Peter). Finally, I found the cut out platform to be immovable (shellac?) for resetting, but solved the problem completely by adjusting slightly the position of the movement on its mounting holes - move the movement towards the bottom of the case for a more sensitive setting. This could be done within the size of the original fixing holes. Why, on almost every AVO I've seen are the movement fixing screw heads damaged? This job was completed by refacing them with a fine file and very carefully cleaning them before replacing. Another fine old instrument made usable - it's interesting to see how the damping varies with the use of div2 and other settings which affect the movement shunting. Leon.

1st Jun 2012, 11:50 am	#2
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,684	Re: AVO 7 Mk 1 repair. Leon, thanks for this. To my mind a tricky repair, and am impressed by the use of your indicators! It inspired me to look at my own Avo 7, which was not working on the DC volt ranges. A fairly simple fault to trace, as I found the first section of the upper wirewound shunt open circuit, and have temporarily inserted a 470 ohm resistor, and DC volts are working. It should be 450, which appears not a preferred value. Looks like a 10k resistor in parallel will do the trick. On this meter two of the shunt sections have already been replaced by carbon film resistors. All else appears good, including the movement fixing screws! I'm glad I had a go at this, what at first looked like a bird's nest turned out to be quite approachable. Thanks Leon! Tony

1st Jun 2012, 1:16 pm	#3
greenstar Octode Join Date: Jul 2011 Location: Leicester, Leics. UK. Posts: 1,684	Re: AVO 7 Mk 1 repair. Ha, that was premature. I have two voltage ranges, 100v and above not working yet. I've posted a request in case anyone has a spare shunt, as mine appears largely fried. The rest of it looks fine - ohms and amps are working.

1st Jun 2012, 1:34 pm	#4
Guest Guest Posts: n/a	Re: AVO 7 Mk 1 repair. AVO did put a line of grease in the back of the case to trap any particles, a brilliant solution to the problem. It is so good and simple our (the works) mil spec. laptops have a bit of double sided sticky tape in the lid where the display goes (own trumpet time, it was I who suggested it). You can learn an awful lot from 'how it was done' (properly).