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Old 30th Sep 2018, 12:25 pm   #1741
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Default Re: AVO Multimeter survey

Hi Dave,

My mistake, you are completely correct, the others were Model 5's.
No excuse, I should have remembered, I bought one of the others!
At the time, I thought they were all the same Model.

I didn't spot yours was a Model 4, otherwise there may have been another Bidder!
I need to pay more attention.
The plus side, I noticed Richards Web Site is now updated with the Missing Model 4 Photo and a bit of a Facelift by the looks of it. Great Site.

Also, yours is now only the Second Model 4 in the Survey, making it the Rarest AVO
I guess!


Don,

Thanks, I look forward to seeing Photo's of your Mk IV. No hurry, thanks for letting us
know.
I guess this is the same Mk IV you are having Rf trouble with on the other Thread.


Ian
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Old 30th Sep 2018, 12:48 pm   #1742
NottsIan
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Default Re: AVO Multimeter survey

Hello, some more serial numbers.

I have an AVO 8 MKIII 68880-1067

Also an AVO 40 MKII 24052-B-164 (number on the dial) which came with a manual and a packing slip showing the serial number 2023-B-667-M. I've not had this meter very long so have no idea how long the meter and leather case have been together. It seems strange that what seems to be the earlier number has 5 digits but the later one only 4. Does anyone know the significance of the M?

Ian.
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Old 30th Sep 2018, 2:12 pm   #1743
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Default Re: AVO Multimeter survey

Here are the photos of the PO AVO8 mkiv.
It is the one with the problem with the RF, if indeed it is a problem
Don, m5aky
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Old 30th Sep 2018, 6:44 pm   #1744
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Avometer Model 4

Fault with 1,200v range/s identified and (partly) fixed! Required an Avo 8 and more fiddling. Phil's comment led me to focus on the main switches.

Set up Avo8 to monitor resistance across main terminals and jabbed the rear of the rotary AC selector ring near to the 1,200v area. Pressing the bakelite of the multi-contact ring caused slight fluctuation of the ohm reading, then removing one of it's fixing screws dramatically increased the ohm reading. An improvement! Once the AC HV contact screw had been removed and the wire sat on paper (as per picture) the 1,200v (DC) range worked properly.

Sintered Bakelite in the contact rings, Batman! I believe the rings might be detachable but have nothing of this sort to hand and for all i know the conductive sintering damage could include the facia surface. Too much intervention for an uncertain result, perhaps.

A fault to be aware of. Indicative of abuse, but not necessarily conclusive..? Occasionally bakelite does suffer with this deterioration, i am told.

Dave
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Old 30th Sep 2018, 8:28 pm   #1745
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Default Re: AVO Multimeter survey

Evening All

I stumbled on this and pass it on in case anyone is interested..............

https://cellpacksolutions.co.uk/prod...s/multimeters/
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Old 30th Sep 2018, 9:44 pm   #1746
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Default Re: AVO Multimeter survey

I've never heard of sintered Bakelite, Dave... this must be pretty rare. Are you describing some form of carbonisation due to arcing/burning across the surface of the Bakelite moulding, leading to insulation breakdown between adjacent switch contacts?
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Old 30th Sep 2018, 10:32 pm   #1747
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Default Re: AVO Multimeter survey

I'm still entering/updating my own spreadsheet containing my own collections. I'm up to entry 114 now. But I need to tidy it up a bit and create more pages splitting the models into groups. Then I can carry on with entering them into Jeremy's database.
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Old 30th Sep 2018, 11:43 pm   #1748
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Exactly Phil, i picked up this usage of 'sintering' from my Dad- ie tracking formed through the material. I can't see it but i (now) know it's there!

Dave
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Old 1st Oct 2018, 9:25 am   #1749
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Default Re: AVO Multimeter survey

Okay Dave. It could therefore be very difficult to repair. I wonder if you could try Meggering the resistance (using a high DC test voltage) between the 1,200 volt stud and the adjacent one? Then, if space permits, grinding or abrading away the conductive surface of the Bakelite where it surrounds the stud.

I can imagine your reticence in tackling this when the instrument is working on all other ranges!
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Old 1st Oct 2018, 10:08 am   #1750
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Model 4

..I did try and be clever by moving the AC 1,200v wire to the adjacent stud Phil, but this was affected too.

I was shown a bakelite 15 Amp plug recently which had failed in service (it was used to feed a domestic immersion heater.) The water had stopped heating and there was a faint smell of piddle in the airing cupboard. (The smell was the result of the failure, not the cause; we never did establish the cause beyond the plug becoming conductive)
The tracking in this case appeared to be through the material rather than across the surface..though it could have occurred on the surface and progressed from there i suppose.

In the case of the Avo, as there is no obvious visible surface corruption, i wonder whether there was an inconsistency in the material, or perhaps a foreign body or swarf found it's way into the moulding at manufacture. It is quite an early one, when all's said and done.

Dave
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Old 1st Oct 2018, 11:58 am   #1751
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Default Re: AVO Multimeter survey

Very interesting, Dave. I have known Bakelite pendant lampholders start to smell of fish when they degrade, but that is after thousands of hours exposed to the rising heat from a 100 Watt light bulb!

As you say, you'll probably never know what caused the fault, but the important thing is that you found the cause! Perhaps it will be best to put that particular Avo on display with a discreet label saying "do not use 1,200 volt range"...
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Old 1st Oct 2018, 6:54 pm   #1752
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Model 4

I suspect much or all of the ferrous swarf originated from one of the main screws securing the magnet to the concentrator, which was nickel plated steel, in poor condition. Swapped for a brass screw. Accuracy is encouraging,the loss of 1,200vAC/600vAC facility..ah well, no Avo of mine with a metal case goes near mains electricity unless it's a one-off careful test. 70v DC reads accurately on the 1,200vDC range so that huge wound shunt and it's half-mile of wire is OK-for the majority of it's long life i think it has been kept away from damp.
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Old 2nd Oct 2018, 12:37 pm   #1753
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I've been told that they were all flash-tested at 7kV in the factory, so you may be worrying unnecessarily, but I suppose they are 80 years old now so you are right to stay on the safe side!
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Old 2nd Oct 2018, 3:10 pm   #1754
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Default Re: AVO Multimeter survey

Might not be as old, but the other week when I needed to test a 2500 Volt PSU,
I grabbed the first Meter handy I knew could measure up that high!

Happened to be a 1960 Screened Model 7.

I'm still here and the Model 7 still works like a dream.

I would certainly be cautious with a very old Meter like the Model 4, but Hey,
it's all part of the Fun and it's great when they work to Spec!

Ian
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Old 2nd Oct 2018, 4:14 pm   #1755
The Philpott
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..I am certainly more confident with the ally cases once i have been inside and re-glued the insulating sheets- the tolerances are fairly tight in there should a wire come adrift!

In the interests of balance i should say that it appears i screwed up in one respect with the Model 4. I was unaware that these early models had teeny 4BA terminal knobs. Had i accepted the offer of a pair of early knobs instead of forging ahead with the belling-lee mod. the originality of the whole might well have been closer. Check first, drill second. Oops moment.

Dave
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Old 2nd Oct 2018, 6:35 pm   #1756
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Are you still collecting serial numbers as we have quite a collection of them.
Regards
Dave
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Old 2nd Oct 2018, 9:22 pm   #1757
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Hi Dave,

Yes, we are still building the Database of Serial Numbers, so any you have
are greatly appreciated.

Hopefully you will find that although the main purpose of this Thread is the
recording of Serial Numbers for Historical reasons, it is also a valuable source
of general AVO meter information along the way.

Thanks also to Nottslan and everybody else still contributing Serial Numbers
to this Thread.
It went quiet for a while.

I hope to get the next List out later on in the Month.

Ian
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Old 2nd Oct 2018, 9:30 pm   #1758
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Default Re: AVO Multimeter survey

Forgot to mention,

If we can have the Model and Mk Numbers as well, where known,
and if not known, a Photo would help other Members identify the said Model ect.

Any other interesting features if any are also quite useful.

With Regards to Model 8/9 Mk IV's, other information of interest are the Case Style,
Green or Black Plastic Colour or Mk V style, and whether the Serial is a Label on the
Side, or written in the Scaleplate.

Ian
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Old 3rd Oct 2018, 3:04 pm   #1759
The Philpott
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General observations on a 1934 vintage Avometer-

Pic 1- Further to comments about the aluminium back-box, being a pressing it's always going to be slightly tapered and a less than perfect fit, but in this case i had to carve out some material from the insulating sheets that i had only just glued in. Fortunately with the help of a blade Araldite gives up sufficiently easily to avoid damaging it's surroundings. It is entirely possible to take a lump out of the bakelite by forcing on a tight box, and this is exactly what a previous owner had done.

Pic 2- The vertical horseshoe magnet has a shunt consisting of two iron bars linked by a screw. In the picture 3/16" of movement towards the concentrator has (on a 10 volt test) reduced deflection of the needle from 85% to just a shade over 83%. A useful amount and more than i expected. The picture shows the shunt as close to the concentrator as it can possibly get on this particular meter- otherwise the (hidden) bar hits the cork blocks that define the min and max needle stops, and pushes them out of alignment. I assume that if one had a magnet that was lacking significant flux that could not be compensated for, removing the shunt altogether would be worth trying. Note of course that disturbing the shunt often liberates (or at least makes visible) ferrous swarf which has to be removed.

In the event of, say, a later Model D with horizontal horseshoe magnet which had too much flux, it might be feasible to fabricate (or salvage) a shunt of this type, as it is more effective than the swivel type fitted to these later models. Care would have to be taken to give clearance to the wirewound shunt chain, and also the radial leg at the rear of the movement that acts as a coarse 'needle zero' adjustment. A magnet that is slightly TOO powerful after 80 years sounds optimistic, but i have at least three instruments where this is the case.

Dave
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Old 10th Oct 2018, 10:50 pm   #1760
The Philpott
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The steampunk epidemic which i had hoped would have lost it's shine by now.......hasn't.

Avometers being the weapon of choice, there's not a lot we can do other than mop up any super cheap sorry looking articles at flea markets and car-boot sales, as to a certain extent the perpetrators will often appreciate that a decent looking meter without missing parts will be in demand for what it is, not what it can be made into.

Dave
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