AVO 36 worth restoring? - UK Vintage Radio Repair and Restoration Discussion Forum

Radio_Dave · 11th Mar 2007, 2:55 pm

I've had this AVO for a while now but it doesn't work. The meter movement seems ok but some of the ranges have problems.

I've had a look inside and it's a mass of little bobbins with resistance wire wound onto them. It's obvious that some of these have burnt out.

Should I attempt to repair the damage or leave alone?

It's never going to be a meter that I'll use in anger but it just bugs me that it doesn't work

.

If it's worth restoring, I only want to do it by re-winding those bobbins.

Any advise?

Thanks
David

pmmunro · 11th Mar 2007, 3:34 pm

David,

The 36-range Universal Avometer was in production from late 1934 until sometime in 1939. It took over from the short-lived 34-range Universal meter, whose instruction plates were used up on some of the early 36-range units, causing some confusion.

In 1936, in time for Radiolympia, it was joined by the Model 7 which could claim 1000 ohms per volt - although this is with the divide-by-two button pressed.

Before deciding what you want to do, it would be best to check the large windings on the SRBP slab at the bottom of the meter assembly. These are, of course, the multipliers for the high voltage ranges and they use very fine wire which is vulnerable to damage. I have calculated that there is about a kilometre of wire on the larger of these. This gauge of insulated resistance wire is hard to obtain now and is expensive.

The smaller bobbins are too small to contain great lengths of wire but could still be demanding to rewind.

Another consideration is the state of the scale plate. The paint is applied directly over nickel plating which is polished to act as the anti-parallax mirror. This does not make for the best paint adhesion and if the storage condition have been adverse, the paint can flake off, in extreme cases almost completely.

You have checked the movement(3ma fsd 20 ohms with swamp - normally shunted to 6ma by the divide-by-two switch); these can go open-circuit due to corrosion at the soldered joints at the hairspring collets. The magnets usually hold their magnetism quite well. There is a small adjustment range provided by the magnetic shunt clamped over the horse-shoe magnet and they can be remagnetised if necessary.

It is also worth checking the fuse screwed into the front panel. These are calibrated as they are part of the moving coil circuit and the resistance value is critical.

I would very much encourage you to restore this meter and I can help with a circuit diagram and some experience if you wish. ON NO ACCOUNT destroy it - PLEASE. Some day old Avometers will get all the respect they deserve, although I hope that wealthy collectors never take an interest.

SPCH would be very interested to know the serial number for his survey.

Best regards,

Peter M. Munro

11th Mar 2007, 2:55 pm	#1
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	AVO 36 worth restoring? I've had this AVO for a while now but it doesn't work. The meter movement seems ok but some of the ranges have problems. I've had a look inside and it's a mass of little bobbins with resistance wire wound onto them. It's obvious that some of these have burnt out. Should I attempt to repair the damage or leave alone? It's never going to be a meter that I'll use in anger but it just bugs me that it doesn't work . If it's worth restoring, I only want to do it by re-winding those bobbins. Any advise? Thanks David

11th Mar 2007, 3:34 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO 36 worth restoring, if so how? David, The 36-range Universal Avometer was in production from late 1934 until sometime in 1939. It took over from the short-lived 34-range Universal meter, whose instruction plates were used up on some of the early 36-range units, causing some confusion. In 1936, in time for Radiolympia, it was joined by the Model 7 which could claim 1000 ohms per volt - although this is with the divide-by-two button pressed. Before deciding what you want to do, it would be best to check the large windings on the SRBP slab at the bottom of the meter assembly. These are, of course, the multipliers for the high voltage ranges and they use very fine wire which is vulnerable to damage. I have calculated that there is about a kilometre of wire on the larger of these. This gauge of insulated resistance wire is hard to obtain now and is expensive. The smaller bobbins are too small to contain great lengths of wire but could still be demanding to rewind. Another consideration is the state of the scale plate. The paint is applied directly over nickel plating which is polished to act as the anti-parallax mirror. This does not make for the best paint adhesion and if the storage condition have been adverse, the paint can flake off, in extreme cases almost completely. You have checked the movement(3ma fsd 20 ohms with swamp - normally shunted to 6ma by the divide-by-two switch); these can go open-circuit due to corrosion at the soldered joints at the hairspring collets. The magnets usually hold their magnetism quite well. There is a small adjustment range provided by the magnetic shunt clamped over the horse-shoe magnet and they can be remagnetised if necessary. It is also worth checking the fuse screwed into the front panel. These are calibrated as they are part of the moving coil circuit and the resistance value is critical. I would very much encourage you to restore this meter and I can help with a circuit diagram and some experience if you wish. ON NO ACCOUNT destroy it - PLEASE. Some day old Avometers will get all the respect they deserve, although I hope that wealthy collectors never take an interest. SPCH would be very interested to know the serial number for his survey. Best regards, Peter M. Munro