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Old 15th Jul 2017, 4:42 pm   #1441
Phil G4SPZ
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Default Re: AVO Multimeter survey

A very enlightening forensic examination of the Q knob, thank you. I have a 1940s Model 7 (now retired from regular service) in which the point contact seems to need cleaning every time the meter is used. It upsets all the other ranges, too. I now wonder if there's a poor contact inside where the pointed tip fits inside the drilled end of the rod?
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Last edited by Phil G4SPZ; 15th Jul 2017 at 4:44 pm. Reason: Typo
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Old 15th Jul 2017, 5:53 pm   #1442
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Default Re: AVO Multimeter survey

I have recently acquired an ex army Test Set multi range No1 High sensitivity. Ser. Number 377 1266. I assume it was made in December 1966. Complete with case, leads, clips and probes. Pleased to say once fitted with batteries all ranges work well except the low ohms, they are reluctant to zero. Could it be the wipes need a bit of Vaseline and is it easy to apply. Fortunately I have alternative meters with working low ohms, but it would be nice to get this one all working 100%. Ted
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Old 15th Jul 2017, 8:41 pm   #1443
Phil G4SPZ
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Default Re: AVO Multimeter survey

Ted,

In my experience, absolute cleanliness of all the contacts and battery terminals in the circuit is essential if the lowest ohms range is to zero correctly. Try polishing the battery contacts and the main terminal sockets with a spot of mild but good quality abrasive such as Greygate 'Paste Polishing No 5' (also marketed as 'Bake-O-Bryte') which was originally specified by the GPO for cleaning telephone exchange jack plugs and sockets.

Take care not to apply any sideways pressure on the battery box contact 'spikes' as this can fracture the Bakelite mouldings on which they're mounted.
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Old 15th Jul 2017, 8:57 pm   #1444
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Default Re: AVO Multimeter survey

Ted.. iffy zeroing of the low ohms range alone, could be tarnishing of the central potentiometer shaft just below the movement. Waggling it from side to side may confirm this. It is possible to undo the screw at the rear without removal of the actual meter movement, but it needs tweezers to lift out the parts, which usually include two little copper washers either side of the wiper. i have used a tiny amount of vaseline on the actual 'wiper to winding' area on several occasions. The more significant tarnishing is probably found on the back of the bakelite knob itself, which has a brass shaft which must have continuity with the springy leaf which you will now see sitting across the hole in the front panel. Very many avo's from the sixties have this problem by now. Again, vaseline used to prevent the brass going dull in future. This problem is, i think, actually mentioned in an Ebay guide to buying Model 8's! It is an understandable problem given that the meter is quite sensitive.
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Old 15th Jul 2017, 9:07 pm   #1445
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Default Re: AVO Multimeter survey

....And yes, it could also be what Phil has detailed!

As regards the Q bodge on My Mod 7- to my slight surprise the intermittency is now solved- although it did occur to me that the Q shaft must ALSO have continuity with the intricate brass housing in which it sits in the front panel---but must be loose enough to rotate in said housing. It's a silly system if i can be so bold!

Before i reassembled i polished all possible contact areas of this housing and shaft just to make sure, and this combination of attacks has worked.
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Old 16th Jul 2017, 5:17 pm   #1446
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Default Re: AVO Multimeter survey

Error- i said the Model 47A and 48A did not have a Q pot- they do have a Q pot but it requires an external voltage source.

Q pot on Model D is a dummy, firmly glued in place. Most of them now sport marks from misguided attempts to rotate them with pliers!
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Old 24th Jul 2017, 8:49 pm   #1447
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Default Re: AVO Multimeter survey

Model 7- 769-2929 now assembled and working consistently- not before a couple more oddities found though:

-Strap held on by dome type rivets- one aluminium and the other brass. (ally one loose and had to be expanded from inside with a pin hammer with the dome hard up against a shaped dolly)

-Fixing screws for battery box were domed, so the lid could not sit properly.

- fixing holes at the top of the bakelite battery box are unnecessarily countersunk- as if the person working on it had never seen one being installed. I have left this alone and just restricted the torque when installing.



The magnet had weakened a bit and needed tweaking up. I decided to put the original cut-out points back in after filing off the arc flash stalagmites and stalactites. One of the power resistors on the voltage board looks like it's probably Siemens Halske, but is unmarked.

Pictured is battery compartment with D cell adapter and token PP3 to supply 9v. The D cell adapter is a very simple (period) one specifically designed for the job- it seems fragile but as soon as it is slid into place it's perfectly ok as it gets it's rigidity from the compartment it sits in.

Finished!... But- do i trust the condenser and leave it in place?
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Old 24th Jul 2017, 10:27 pm   #1448
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Default Re: AVO Multimeter survey

Have you checked the calibration on the uF ranges? I confess to being remiss in this regard myself. I've never replaced the large capacitor, and I do wonder if many Model 7s were ever used to measure capacitance 'in anger'. The capacitor will therefore probably be as good as new-old-stock.
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Old 25th Jul 2017, 5:47 pm   #1449
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Default Re: AVO Multimeter survey

I haven't checked it Phil, but that's quite a good point about it probably having little or no use. I will leave it till the weekend and take it to a certain ex. Marconi engineer who knows far more than me about Farads,Henries,Coulombs etc.
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Old 1st Aug 2017, 9:47 pm   #1450
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Default Re: AVO Multimeter survey

HR Avominor 548-1140.

I had no intention of buying this however there was a surprising lack of interest, followed by the reserve being dropped by two-thirds. Rather an odd situation, but anyway there are some points of interest:

The box has had a hard life and is breaking up, but the standard leads and brief instruction leaflet were included. (The leaflet does not contain any surprises)

The wire from the swamp card had come loose, however on connecting up it was apparent that something was obstructing the movement very badly. Having removed it and failed to rectify by backing off the front jewel, the jamming cleared completely once the rear jewel had been loosened 1/4 turn. At this point i glued the glass back in as only 2 screws are used on this meter and it is very reliant on adhesive. The movement was significantly tail heavy and i counterbalanced it with a blob of paint further up (I really don't fancy trying to reduce the mass of a tail weight of this size!)

Voltage readings were far too high at this point, and damping was very poor, which made me think that either the 2.73k or 1k bobbins was o/c. They were however fine.. which then led me to the zero ohms winding- this is (unfortunately) critical to the voltage/current functions and mine had gone o/c in at least 3 places. Making an accurate assessment of the overall resistance was a problem but i settled on between 1200 and 1250 ohms, and combined 2 small resistors in parallel to get a reading of 1229 on the DMM.

At this point the keyhole surgery inevitably resulted in the 2.73k bobbin suffering brittle fractures of it's winding at the slightest touch, always a possibility as i was working right alongside it. Two resistors in parallel (15k and 3.3k) got used as a 'near enough' replacement.

The 1000v pillar had worked loose in the front panel and a small dob of superglue from the back is my usual cure for this, as from that position it does not spread over areas which you need to be conductive. The fumes must be allowed to escape though. The Z screw was stiff and got lightly oiled up at this stage.

Having cleaned up the bronze spring leaf i assembled and powered up with not much hope of success, however i was rewarded with a reading of 2v at a reference source of 2.05v. This was easily corrected by adjusting the magnet shunt. The damping, however, was sub-standard- the movement should have 3 oscillations (up-down-up) and this one had 6 or 7.

The penny drops- the manky (and irreplaceable) ohms zero winding (i assume) acts as an inductive damper, and replacing it with component resistors removes some of the damping. (Correct me if i am wrong here.)

Conspicuous by it's absence is a big fat Siemens 20Meg resistor as seen in the earlier meters; 5 meg ones are stacked in series instead, possibly an indication that Siemens Germany were the only actual manufacturing base and ACWEECO had run out of the larger ones. (I know Siemens had a presence in Britain before and after the war.. but do wonder exactly what occurred with stock and premises around the outbreak of war..)

The photo indicates scorching and cracking of the centre of the insulating sheet which is interesting since nothing looks baked inside the meter. Perhaps it was just left on load for a long time.

Overall, not a complete fix but it's certainly come out as more than just a paperweight- and it's good to know that the movement is satisfactory.
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Old 11th Aug 2017, 10:13 am   #1451
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Default Re: AVO Multimeter survey

Hi All,

I picked up another Model 9sx recently, Serial No 12775 11 71

I should point out at this stage, that I am not collecting AVO meters! Honest!

Anyway, I noticed quite a few differences between this one and my
1963 Model 9sx, so if anybody is interested in this information.

The newer 1971 Model
No Identity Stamp on the Front, moulding is there but blank (1963 one does).
Scale Plate does not have the db range (1963 one does).
Scale Plate says "Test Set Multi Range High Sensitivity" (1963 one says "Multimeter Type 12889)
Mirror positioning is different. Mirror between A/V & R (1963 one is right at the bottom)
Battery Compartment has Two Fuses and later type connector for the 15v like the Mk IV (1963 one same as Mk II)

Both are Test Set Multi Range No 1 High Sensitivity and have the same Military Case,
but only the older 1963 model actually says Pan-Climatic on the back.
(Is the 71 a PC or not? I assume it must be (does a 9s even exist?). It came with a 9sx Manual which I was told was original to the Meter.

1963 AVO is based at AVOCET House and is a Member of the Metal Industries Group.
1971 AVO is based at Dover and is a Member of the Thorn Group.

Not opened either of my 9sx to see if /how they differ internally as they both work flawlessly and I don't normally open a Meter that's working correctly.

The other odd thing is, as I understand it, the 9sx is considered to be a 9 Mk I, however, I have an older Model 9 Mk II (non Military) from 1967.

Ian
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Old 11th Aug 2017, 8:11 pm   #1452
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Default Re: AVO Multimeter survey

Could you post a few photographs please? These would help confirm the identity of your meter which seems to be a screened panclimatic version of the Model 9 Mark IV.

PMM
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Old 12th Aug 2017, 9:50 pm   #1453
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Default Re: AVO Multimeter survey

Hi PMM and all,

I have attached some Photo's.
First Two are the 1963 9sx, last Three are the 1971 9sx (potentially!).

Strictly speaking, the 1971 Model doesn't actually say 9sx anywhere, I only have the included Manual for one, that and the fact it is outwardly almost identical to the 9sx I already have. It also comes in the same NATO style case NATO No. 6625 99 105 7056

Does anybody have any information on the Test Set Multi-Range High Sensitivity Models?
I know it came in different versions, with the first No 1's being I believe a Model 9sx, and I've seen a No 3 which seemed to be a Model 8 Mk V or later.


Ian
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Old 22nd Aug 2017, 8:11 pm   #1454
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Default Re: AVO Multimeter survey

I picked up a Heavy Duty Avometer in an auction.

It appears to be an HD6 and has British Rail Signal Eng. Dept. stamped on it. Came with brown leather case, leads and a document saying it was last calibrated in 1985.

Label on the side says

FD No 0540810 HD6

If anyone knows more about it, especially the battery it takes and anything else of interest then I would appreciate it.
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Old 27th Aug 2017, 7:01 pm   #1455
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Default Re: AVO Multimeter survey

HRM High Resistance Avometer No. 593-748 (July 1948)
(As a guide to likely production numbers up to the advent of the Model 8,another is known to exist with No. 2000-450, although things are complicated a little by the screened and/or panclimatic versions seemingly having their own devoted series of numbers)

Photos follow, but it's interesting that all the knobs and terminals to the front of the meter are exclusive to this version, so replacing anything was impossible- it all had to be left alone, cleaned up or 'mended'. The terminals are an early version of the captive type used later on, but without the push-in voids on top. The threaded section is quite narrow and when extended they are rather tall; this had resulted, over the decades, in two of them being slightly bent and a third extremely bent, with a chunk having been forced out of the bakelite at the same time. Ouch!

The movement was rather out of balance, in respect of both tail weight and side weight, but this having been rectified the meter still requires a gentle twist to settle it's reading. (jewels or spring loaded pivots not quite right probably)

The back-box was very bowed inwards where the casing screws go through it, but i found that it's dimensions were originally correct and the bowing was due to the meter facia moulding itself being 80thou undersize, which distorted the back-box as the screws were tightened at the factory! I cut a ring from an old motor car inner tube which stretched nicely over the mating surface of the facia, then persuaded the facia into the back-box with the aid of a little red rubber grease to avoid it ruckling. I then pierced it to allow the casing screws to be re-inserted. This has exerted just a little pressure on the back-box which-one hopes- will relieve some stress with time... either that or it will crack.

Many of the switch contacts inside are evil (or impossible) to get to with the movement installed so i cleaned them all when balancing the movement.

The badly bent terminal had to be removed, the threaded section clamped in aluminium vice jaws, and then belted straight with application of a rubber mallet to the bakelite knob. It sounds brutal but it was a case of kill or cure, and on this occasion it worked.

The chip missing from the facia was rebuilt using a mix of araldite and black paint, details in next post along with photos.

There are about 12 of those fragile little bobbins inside this meter- any one of which could fail at any time, but at this point everything is working.
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Old 27th Aug 2017, 8:23 pm   #1456
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Default Re: AVO Multimeter survey

HRM 593-748

1. HV terminal after straightening, before repair to facia. One 'flat' remains on the facia which helped to prevent the terminal spinning after repair.

2. HV terminal repaired with 4 parts Araldite Rapid (5 minute) to 1 part Docrafts Artiste Acrylic Paint (water based) in Black. This formed a dubious looking sloppy emulsion which repeatedly became grey when mixed then turned black when allowed to settle. The 5 minute settling time of the Araldite Rapid became 5 days- although after 1 day it had become stable but slightly rubbery. Application by blunted cocktail stick.

3. Unusual Z assembly using a wheel and brass quadrant. Z setting is adjustable without a screwdriver, which might suggest an acknowledgement that zeroing was more sensitive than other AVO versions of the time.

4. Battery box containing 1.5v and 4.5v power packs. There is a void which can accomodate a spare 4.5v battery. Some of the screws seen in the photo are steel, and at least one of the battery leafs is retained by steel self tappers (they all look original)
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Old 27th Aug 2017, 8:37 pm   #1457
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Default Re: AVO Multimeter survey

HRM 593-748

1. Tall easy to use selectors. Press button range for Low Resistance, Infinity at FSD unlike other resistance ranges.

2. Tail weight supplemented with auto paint, Side weight (which was absent) now made of tiny fragments of un-cored solder glued on with auto paint.

3. Cleaned but rather heavily used frontage.

4. Scale showing usual fading of red graduations, quite bad on this meter.

No AC current ranges, DC current ranges only extend up to 1 Amp. Quite wide resistance range coverages, with 3.7 Ohm, 500 Ohm, and 50KOhm at centre scale.
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Old 2nd Sep 2017, 9:40 pm   #1458
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Default Re: AVO Multimeter survey

HRM 593-748--- postscript

Failure to zero properly (and failure to give steady consistent readings) failed to resolve by pulsing 6v at 100 pulses per minute into the 10v range...but was then remedied by loosening the rear jewel by 1/4 turn.

Slight smell of pipe tobacco, particularly when shunts are under load for a while...how come this smell doesn't seem to get stale..? I think it could be because it's in a cocktail with the other 'normal' smells.
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Old 4th Sep 2017, 11:59 am   #1459
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Default Re: AVO Multimeter survey

Hi
I have 4 x mk5's and 1 x mk6 to add to your collection
94631 8v/8/73 slow cutout reaction
58465 8v/3/75 no apparent needle damping
xxxxx xx/x/xx sorry label missing sorted, broken cutout replaced, seems fine now
57256 8v/3/75 movement replaced
and the mk6
2368 m90285 4/98 current work shop item
hope that helps your record
cheers
snowy
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Old 4th Sep 2017, 5:47 pm   #1460
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Default Re: AVO Multimeter survey

Back again, bought an AVO 7at the auto jumble sale No. 860-A-964. Please to say it works on all ranges except the Meg Ohms as I haven't got a 9 volt battery. I though of using a pp3 and using a piece of foam take up the space. I have used 3 X 3volt button cells for my others meter. As the interior, when viewed via the meter glass looks very dirty I would like to get the meter apart to clean its interior. I'm unable to get the aluminium case off/ from the front panel. I have removed all the screws, there is a gasket all around but I don't want tear it, any ideas how to get the meter apart will be much appreciated. Finally, the positive terminal 4mm ?? has been screwed off, are they still available or is there a second hand place where I can obtain one from. Ted, Oh and thank to others who offered advice about my ex WD avometer.
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