AVO model 7 panel screws

[Silicon]

I am trying to separate the front bakelite panel from the metal case of an AVO model 7 Mk1 meter.

I have removed the 8 countersunk screws around the edge of the case but it is difficult to get the parts to separate. It might be corrosion.

Are there any more screws that I need to remove?

Also, are there any modifications or improvements I can make to the circuit such as adding a fuse to an input terminal or reverse parallel silicon diodes across the meter movement?

[pmmunro]

Silicon,

The case, if it's a metal one, can take a really strong grip of the front panel. There should be 8 screws, one of which is a deeply recessed seal screw, holding the case in place, two screws in each side. There might be three screws on the side with the seal screw.

You describe your Model 7 as a Mark one. As this was not an official designation, it could refer to several variants from between 1936 and 1956. The serial number would help determine the characteristics of your meter.

To release the front panel from the case you need to apply some force between the top lip of the back case and the flange of the front panel but care must be taken not to crack the flange or damage the edge of the metal case. Only a broad lever such as a 100mm (4") painter's scraper - one with a reasonably thick blade, should used. This will spread the load sufficiently to limit the risk of damage. You could also try gentle heat from a hair dryer, probably not a heat gun, as this would damage the paint.

The Model 7 has a cut-out which is designed to operate on excessive pointer acceleration as well as on contact with an end stop. The movement is therefore already quite well protected and you may find that yours already has a single diode across the movement so that the movement has some response to AC applied with the meter set for DC. Without this there is a risk the an AC overload while set to DC would simply overheat the moving coil with no pointer movement to operate the cutout.

Adding a fuse to the test leads could improve operator safety, but is unlikely to be able to protect the movement. Earlier Avometers did rely on fuse protection of the movement, but these fuses had to be calibrated (to 4 ohms) to maintain the accuracy of the meter. (They're could be bought on factory exchange for 1/9d plus postage, but I don't think the offer still stands)!

An 80mA fuse, the lowest value that Littelfuse list in their EN 60127 5 x 20mm range, is going to give only limited protection and with a typical resistance of over 28 ohms would require a complete redesign of the movement circuit. It would also be too slow to operate at any current which would give useful protection to the movement; the cut-out would operate before the fuse melted. Note that the resistance is typical, not calibrated, and could vary considerably about the nominal value.

PMM

[Brian R Pateman]

The issue with fused leads is an interesting one. They are not to protect the meter from damage.

They are intended to protect the operator from the potentially catastrophic consequences of something going wrong when connected to a high energy source - industrial switchboards with high prospective fault current for example.

Have a look on Youtube and you'll find plenty of videos showing the consequences.

PMM's point about the resistance of fuses is a good one and often forgotten and a poorly designed protection circuit can seriously upset meter performance.

For most things that we, as hobbyists, use meters for shouldn't require protected test leads.

[Silicon]

I was thinking in terms of a high current HRC fuse which should have little effect on the meter.

I have now taken the back off and found too much dust for reliable operation.
There are 'tin whiskers' about 6mm long on the meter magnet.

The needle sticks above half scale so dust may be in the magnet gap.

The wiper of the 'Q' potentiometer is too short. It does not reach the resistance element even when the button is lifted. I don't see how it could ever have worked.

There are no signs of burnt resistors but a thorough check is required.

'10S/1' and the government property arrow are engraved in the bakelite above the dial. I assume it is an RAF reference number.

[Phil G4SPZ]

I could probably let you have a replacement wiper for your "Q" pot from my small stock of scrap Avometers. Drop me a PM if interested. Depending on the age of your Model 7, it may have a horseshoe magnet or a block magnet.

Removing magnetic debris from the air gap is best carried out with the movement removed from the panel. Take great care throughout. Trip the cutout, unsolder the appropriate connecting wires then use a screwdriver with its blade sheathed with cardboard or similar to remove the movement bolts and prevent the blade from being attracted to the magnet and potentially weakening it. Once out, inspect the movement under a bright light and use a small paintbrush to tease out the debris. A thin jeweller's screwdriver may help detach magnetic debris.

If you blow very gently on the pointer, you should ultimately be able to get it to move across its full range without physically touching it. The pointer is not physically attached to the moving coil; it is driven by a spring-loaded actuator fixed to the coil, which provides the acceleration cut-out function that PMM referred to. The hair spring is almost invisibly fine.

Good luck!