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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 4th Feb 2012, 12:02 am   #1
Phil G4SPZ
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Default A tale of two Avometers

I offer another test-gear restoration which I hope qualifies as a 'success story'.

My interest in restoring vintage Avometers was sparked a couple of years ago when I acquired my first Model 7 Mk 1 as a very cheap eBay purchase, only to find that it had a faulty movement. An appeal on the Forum produced a Model 7 Mk II, kindly donated by Mike Fulton, in damaged condition and missing its transformer but with a good moving-coil movement. A movement transplant was planned, but it has taken me over a year to summon up the necessary courage to tackle the job, during which time I have restored a few other Avometers and gained some confidence in servicing these fine instruments.

Before starting work, I wired up the new movement to the old instrument with short test leads, and was pleased to see that the 'new' meter movement gave approximately correct readings. Unfortunately, its mirror had dropped out and broken in transit. The original mirror is very thin, less than 1mm in thickness, and is held in place by an aluminium plate bolted in position behind the slot in the scale plate. In an effort to replicate a mirror (I discounted the notion of trying to obtain a proper replacement) I experimented with various ‘shiny’ materials, including kitchen foil and ‘chrome’ Sellotape, until I spotted a new aluminium foil pie dish in the kitchen. As Mrs G4SPZ was away for the weekend and would not notice the loss, I had no qualms about cutting the base out of said pie dish. The material itself was too flexible, so I glued it to a piece of thin Plasticard using spray adhesive, and when dry fitted the replacement ‘mirror’ to the movement. The result certainly isn’t as good as a mirror scale, but is visually acceptable as a replacement until a suitable part comes to hand... any offers?

Transplanting the movement from the Mk II into the Mk 1 Avo model 7 panel proved relatively straightforward. Following guidance on the Forum and published elsewhere, I tripped the cutout, unsoldered the two leads from the movement, and removed the two bolts holding the 1uF capacitor to the front panel, enabling the capacitor to be swung out of the way of the scale plate. Using a large-bladed screwdriver with its shaft wrapped in corrugated cardboard to prevent the steel coming into contact with the magnet, I then removed the two fixing screws and carefully lifted the movement out of the donor meter. Care is required to maintain a clean working environment throughout this process to prevent dust and bits of ferrous material from contaminating the movement, which unlike more modern instruments, is completely open internally. I remembered to clean the inside of the dial glass of the recipient meter carefully with spectacle lens cleaner before fitting the new movement.

There are numerous detail differences between the Mk 1 and Mk II Model 7 Avometers, but the moving coil meter itself is basically the same 1mA FSD movement, giving the instrument its characteristic 1,000 ohms-per-volt sensitivity (but only with the ‘divide-by-2 button pressed!) If you compare the circuit diagrams for the two marks, the only differences are in the “P.F.” sockets and the use of a different rectifier in the Mk II, although the instruments look very different internally mainly because of the design of the multiplier resistors. Over the years, different magnet styles were used, and the donor movement was much more modern than the original, having red-coloured Alnico magnets. It was found necessary to re-fit the thick spacer washers between the movement and the threaded inserts in the front panel of the Mk 1 instrument, and to use the shorter Mk 1 machine screws to secure the movement to the panel. Thankfully, the cutout operating pin and the zeroing peg aligned nicely with the new movement. Once the bolts were tightened and the new movement was wired in, testing began.

The first problem that became apparent was that none of the DC or AC voltage ranges above 10 volts were working. The cause was quickly traced to an open circuit 44,200 ohm wire-wound bobbin resistor on the top multiplier board, and I replaced this using a series combination of modern 39k and 4.7k resistors, selected on a DVM to give exactly 44,200 ohms. It was then noticed that the meter read about 5% low on all voltage and current ranges, and I assumed that this was due to a slight difference in sensitivity between the new and old movements. I was unwilling to start removing turns from the meter’s series bobbin resistor, so I experimented with adjusting the magnetic shunt. This takes the form of a small slotted plate secured by a single screw and locking washer on top of the magnet assembly, and which can be moved towards and away from the moving coil pole piece to adjust the strength of the magnetic field and hence trim the meter’s basic sensitivity. I don’t have access to an accurate constant current source of 1.000 milliamp, so decided instead to adjust the meter to read exactly 10 volts FSD on the 10 volt DC range when being fed with 10.000 volts DC as measured by my DVM. A small outward adjustment of the magnetic shunt was found necessary, and once this had been done the remaining ranges were checked and found to be extremely accurate.

Having reassembled the meter, it was time to fit batteries and try out the resistance ranges. 1.5 volt and 9 volt supplies are used in the Model 7, and I made no attempt to make replica batteries, using a single “C” cell and a PP3 respectively. The original 1.5V zinc-carbon cell available in the late 1930s would have had far higher internal resistance than the alkaline "C" cell I had available, and it was found necessary to insert a 12 ohm resistor in series with the "C" cell's positive terminal to permit accurate zeroing on the 10k and 100k ranges. Previously, I have used a single “C” cell holder; this time I didn’t have one to hand, so I soldered wires directly to the cell. A cheap ‘pound shop’ PP3 battery made an adequate replacement for the two 4.5 volt flat batteries in series that the Model 7 battery compartment was designed to accommodate.

The remaining work was cosmetic. I used a cream-coloured wax crayon (which fortuitously matched the original colour) to fill in the engraved legends. Avo’s own Servicing Information recommends using light mineral oil, applied using a small paintbrush, to enhance the appearance of the crackle-finish Bakelite front panel, but I used silicone dashboard spray, applied by spraying some into a small receptacle and then brushing into the Bakelite using a cut-down ½” paint brush. The results are very satisfying, see thumbnail - the Model 7 is on the left.

The meter on the right is a relatively modern Avometer Model 40 Mk II which I acquired more recently, again very cheaply, via eBay. On arrival, it was absolutely filthy (why do I always forget to take those ‘before’ photos?) but thankfully working on all ranges. Most of the muck and grime came off with lukewarm water, detergent and a toothbrush, but virtually all the engraved legends got washed off in the process. I used a white wax crayon to fill-in the engraving - white seemed to look better alongside the more modern red and black terminals - and the more observant of you will have noticed that the black plastic terminal cover is missing at present.

The Avometer Model 40, and its military derivatives the Air Ministry Model D and the Admiralty Pattern Model 47A, were designed as electricians’ instruments, incorporating a 6mA FSD movement and giving a basic sensitivity of just 167 ohms per volt, insufficient for radio work. The Model 7, on the other hand, provides 500 and 1,000 ohms-per-volt sensitivity, which is adequate for the majority of vintage radio work, and in addition incorporates resistance ranges up to 1 Megohm from internal batteries, as well as audio power (dB) and capacitance ranges which make it a very versatile and almost indestructible meter capable of virtually all vintage radio servicing duties. ‘Traditional’ Avometers of all types also offer AC current ranges with a very low voltage drop, which are rarely found in other multimeters and which can be extremely useful on the test bench as an adjunct to a simple lamp limiter.
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Old 4th Feb 2012, 12:32 am   #2
pmmunro
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Default Re: A tale of two Avometers

Phil,

It sounds as though you got a lot more than a working Model 7 out of this experience.

Be careful though, satisfaction and Avometers are known to be addictive!

PMM
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Old 4th Feb 2012, 12:49 am   #3
Phil G4SPZ
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Default Re: A tale of two Avometers

You're quite right, Peter, although the Forum is partly to blame for offering so much support and information to the budding Avo collector and restorer!

I have had to confess to becoming an Avo enthusiast, albeit in modest proportions - I have around eight, several in daily use but more are on their way - and as my wife acknowledges, Avometers take up less space than vintage radios. The sheer quality of construction of the classic Avometer, especially the Bakelite mouldings, makes them collectible for that aspect alone. I have also worked on other vintage Avo test gear including a two-panel valve tester, Electronic Testmeter and All Wave Oscillator, and the same Avo build quality is apparent throughout.

Phil
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Old 4th Feb 2012, 9:05 am   #4
G4XWDJim
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Default Re: A tale of two Avometers

Excellent write up Phil. Having done a very small amount of work on one AVO recently I appreciate the difficulties encountered and the nerve required to start delving deeply into such a delicate and fragile area.

Holding your breath all the time no doubt!

Very well done and thanks for the posting.

Jim
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Old 4th Feb 2012, 2:30 pm   #5
Chris Parry
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Default Re: A tale of two Avometers

The Avo7 can be very useful on old radios because a lot of the service information was written around them. For example, the DST100 EMER and the Trader sheet for the Philips 735A both call up an Avo7 explicitly for the voltage measurements. Chris
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Old 4th Feb 2012, 3:34 pm   #6
John M0GLN
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Default Re: A tale of two Avometers

Quote:
Originally Posted by Phil G4SPZ View Post
The Avometer Model 40, and its military derivatives the Air Ministry Model D and the Admiralty Pattern Model 47A, were designed as electricians’ instruments, incorporating a 6mA FSD movement and giving a basic sensitivity of just 167 ohms per volt, insufficient for radio work.
This is generally accepted as correct, its something that I've always thought was logical, and I was quite suprised when I downloaded the 'GEC Radio service bulletin No 94' for the GEC Model BC4652 and BC4652L 5 valve superhet receiver and saw all the voltages listed had been measured with a 167Ω/V meter, even the 60v screen supply to a KTW61, they had all been measured on the 1200V range to keep the loading down.

It was a bit of a coincidence that I bought the GEC radio and an AVO Model 40 from the Lowton Swapmeet on the same day.

John

PS, this post is not meant to be critical of your excellent work and write up.

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Old 4th Feb 2012, 4:27 pm   #7
Phil G4SPZ
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Default Re: A tale of two Avometers

Jim, thanks for your kind comments. Chris and John raise an interesting point, and taking John's example where the screen grid feed resistor R10 is 56k, the connection of a 167 ohms-per-volt multimeter set on the 1,200 volt range imposes a load of 200k across R10 and drops it, effectively, to 43.75k - they were probably using a 34- or 36-range Avometer or its later derivative the Model 40. No doubt this causes a change in the circuit conditions, but probably not enough to upset the working of the stage. The biggest problem would have been trying to read, accurately, 60 volts on a meter scale measuring 1,200 volts FSD! The Model 40 would have been fine for HT and LT supply measurements, it's only screen voltages and others such as AVC with a high impedance source where meter loading becomes an issue.

Service sheets are most likely to quote the use of a Model 7, as these were launched in the mid-1930s when setmakers were bringing out new models every year.

Some years ago, Practical Wireless printed a very good series of articles entitled "Are The Voltages Correct?" which make useful reading as it deals with both valve and transistor circuitry. I have a copy of the reprint, and when I last read it, I managed to get the right answers to all the worked examples! The important thing to remember is that most vintage test equipment only needs to be as accurate as the sets on which it is used.
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