UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

pmmunro · 18th Aug 2004, 1:05 pm

The Avometer Model 8 was introduced in May 1951. The first version can be recognised externally by the black, knurled screw terminals. The ranges are in the sequence 1, 2.5, 10. The movement is based on alnico block magnets (unpainted) and the shunts and multipliers are a variety of wound wire resistances and component resistors.

The Model 8 Mark II was first produced in 1956 and had the then new red and black socket terminals. The movement used shorter alcomax magnet blocks, painted with red crackle paint, but was very similar the the 'Mark I' internally. Soon after introduction, a 500V (DC) range was added.

The Model 8 Mark III came around 1964 and is basically a Mark II with a fuse to protect the resistance ranges. More component resistors were used.

The Model 9 Mark II is very similar to the Model 8 Mark II but has internation symbol switch legends and ranges in the sequence 1-3-10.

Some people believe the Model 8 Mk III and 9 Mk II to be the best Avometers ever made - they prefer these to the later Model 8, Mks V, VI and 7.

The Model 8 Mark IV (c. 1968) is substantially redesigned internally and the function switches are in the form of printed paxolin discs and phosphor bronze contact fingers. The discs can be subject to silver migration problems. The case changed from bakelite to moulded thermoplastic but lack of design experience with this material resulted in stress around the case screw holes which are very often the site of serious cracks. The magnet material again changed and is painted in light blue crackle paint. The main objective of the redesign was to reduce production time to restore manufacturing profitability as otherwise Jules Thorn threatened to end Avometer production.

The Model 8 Mark 5 was a complete redesign around a centre-pole movement which was developed from a Taylor Instruments patent. Internal wiring uses flexible printed circuits which are quite fragile and need great care in handling and soldering - use a low-temperature bit. The design was in close conformity to a NATO specification which include severe vibration and drop testing. Despite this, there have been reports of movement bearing bridges breaking under only moderate forces. These meters are usually event more accurate than the previous models if they have not been mistreated. (All Avometers in good condition are usually well within the published figures). The Model 8 Mk V also replaced the Model 9. Ranges are in the sequence 1-3-10. The front panel is still bakelite (to resist soldering iron burns) and the case is a much improved thermplastic moulding. Push-button switch 'boots' are often perished but can be bought from AVO (Megger) at Dover or a repair agent. The overload cut-out on these meters is easily adjusted and is very effective when correctly set.

The Model 8 Mark VI (c. 1980) uses the same basic movement as the Mk V but is completely redesign internally to use a pair of rigid pcbs for fast assembly. This required re-arrangement of the range switches whic now partly mirror each other so that the current ranges are on the outside of both switches. The 2500 V ranges were deleted to avoid safety regulation problems and to save the cost of high-value precision resistors.

The Model 8 Mk 7 is the current production model - basically a Mk VI with additional fusing.

NATO variants were made for Model 8 MKs I to III mostly with 1-3-10 range sequences, potted components to withstand tropical use and external supplementary screening cans around the case.

The 'best' version to acquire is partly a question of personal taste as condition is probably more important than other considerations.

If acquiring an Avometer with a faulty movement, do not be tempted to change scaleplates from one movement to another. Up to MK IV they were individually hand calibrated and later the were matched to the movement using 12 basic scaleshapes. (Look for a figure at the bottom centre of the scaleplate).

Up to Mk IV, the last figues of the serial number, at the lower right edge of the scaleplate are the month and yaer of manufacture. From Mk V the date is on a self adhesive serial number label on the right hand centre of the case, repeated inside the battery compartment.

Apologies for any slight inaccuracies of memory concerning exact dates.

18th Aug 2004, 1:05 pm	#8
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,812	Re: AVO 8 Variants (Mk1,Mk2,Mk3,Mk4...etc) The Avometer Model 8 was introduced in May 1951. The first version can be recognised externally by the black, knurled screw terminals. The ranges are in the sequence 1, 2.5, 10. The movement is based on alnico block magnets (unpainted) and the shunts and multipliers are a variety of wound wire resistances and component resistors. The Model 8 Mark II was first produced in 1956 and had the then new red and black socket terminals. The movement used shorter alcomax magnet blocks, painted with red crackle paint, but was very similar the the 'Mark I' internally. Soon after introduction, a 500V (DC) range was added. The Model 8 Mark III came around 1964 and is basically a Mark II with a fuse to protect the resistance ranges. More component resistors were used. The Model 9 Mark II is very similar to the Model 8 Mark II but has internation symbol switch legends and ranges in the sequence 1-3-10. Some people believe the Model 8 Mk III and 9 Mk II to be the best Avometers ever made - they prefer these to the later Model 8, Mks V, VI and 7. The Model 8 Mark IV (c. 1968) is substantially redesigned internally and the function switches are in the form of printed paxolin discs and phosphor bronze contact fingers. The discs can be subject to silver migration problems. The case changed from bakelite to moulded thermoplastic but lack of design experience with this material resulted in stress around the case screw holes which are very often the site of serious cracks. The magnet material again changed and is painted in light blue crackle paint. The main objective of the redesign was to reduce production time to restore manufacturing profitability as otherwise Jules Thorn threatened to end Avometer production. The Model 8 Mark 5 was a complete redesign around a centre-pole movement which was developed from a Taylor Instruments patent. Internal wiring uses flexible printed circuits which are quite fragile and need great care in handling and soldering - use a low-temperature bit. The design was in close conformity to a NATO specification which include severe vibration and drop testing. Despite this, there have been reports of movement bearing bridges breaking under only moderate forces. These meters are usually event more accurate than the previous models if they have not been mistreated. (All Avometers in good condition are usually well within the published figures). The Model 8 Mk V also replaced the Model 9. Ranges are in the sequence 1-3-10. The front panel is still bakelite (to resist soldering iron burns) and the case is a much improved thermplastic moulding. Push-button switch 'boots' are often perished but can be bought from AVO (Megger) at Dover or a repair agent. The overload cut-out on these meters is easily adjusted and is very effective when correctly set. The Model 8 Mark VI (c. 1980) uses the same basic movement as the Mk V but is completely redesign internally to use a pair of rigid pcbs for fast assembly. This required re-arrangement of the range switches whic now partly mirror each other so that the current ranges are on the outside of both switches. The 2500 V ranges were deleted to avoid safety regulation problems and to save the cost of high-value precision resistors. The Model 8 Mk 7 is the current production model - basically a Mk VI with additional fusing. NATO variants were made for Model 8 MKs I to III mostly with 1-3-10 range sequences, potted components to withstand tropical use and external supplementary screening cans around the case. The 'best' version to acquire is partly a question of personal taste as condition is probably more important than other considerations. If acquiring an Avometer with a faulty movement, do not be tempted to change scaleplates from one movement to another. Up to MK IV they were individually hand calibrated and later the were matched to the movement using 12 basic scaleshapes. (Look for a figure at the bottom centre of the scaleplate). Up to Mk IV, the last figues of the serial number, at the lower right edge of the scaleplate are the month and yaer of manufacture. From Mk V the date is on a self adhesive serial number label on the right hand centre of the case, repeated inside the battery compartment. Apologies for any slight inaccuracies of memory concerning exact dates.