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Old 7th Apr 2018, 4:44 pm   #1
TonyDuell
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Default DC AVOmeter

Last week I bought a DC (only) AVOmeter. It looks to be the '13 range' model, there is no divide-by-2 button. It has a cable cleat in the middle of the bottom of the panel but the normal screw terminals in the bottom corners, which look to be original. It would have used a cuboidal 1.5V cell which fits into a wooden box at the rear of the metal case, but a D-cell holder has been very untidily wired to the connections.

There is what I think is a serial number -- 5994 -- on the bottom right of the scale.

Apart from the battery-bodge, other problem are a totally missing handle, some stripped threads in the bakelite panel that take the screws that anchor the case, and the rheostat seems to be open-cicuit (I think the wiper is not making contact with the resistance wire).

Can anyone tell me more about it, and can I get a circuit diagram anywhere?
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Old 7th Apr 2018, 8:39 pm   #2
The Philpott
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Default Re: DC AVOmeter

First stop is richards radios but you've probably already been there..? Don't know off-hand if there is a circuit diagram on there.

Just occasionally these particular stripped threads can be defeated by fitting a slightly longer screw (if this can be done without hitting anything or getting too close to live parts inside- & you might need to deepen the hole a little) Those early meters are vulnerable to this- brass inserts were phased in later on.

I wonder if those inserts could be retro-fitted from a later scrap meter.. you'd have to be accurate with the drilling..

Dave
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Old 9th Apr 2018, 12:45 pm   #3
pmmunro
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Default Re: DC AVOmeter

Please post a photograph of two and send me a private message with an email address. I should be able to grrtirm a circuit diagram and possibly some more information.

PMM
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Old 13th Apr 2018, 9:15 am   #4
TonyDuell
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Default Re: DC AVOmeter

Here are a few photos of the instrument. I think they are pretty much self-explanatory, but of course ask about anything
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Old 13th Apr 2018, 9:38 am   #5
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Default Re: DC AVOmeter

That's very nice indeed, no corrosion from the cell, and a good scale plate. You might even successfully repair the corner of the facia..?, (it would have to be right on the corner, wouldn't it!?)

Those wooden cell boxes are really well made, given their utilitarian purpose..I have a 'D' from 1942 that has an aluminium back-box and (mahogany? teak?) cell box, very similar.

I have recorded a (reasonably standardised) set of measurements for the leather handle, you should find them on this 'site easily. (Tack shop the best place?) I think you probably have the domed rivets which would make a new one easier to slide on.

Dave
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Old 13th Apr 2018, 9:43 am   #6
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Default Re: DC AVOmeter

Now then here's a thing- i think i recognise someone's signature-

I have a slightly later meter that arrived with an identical (white with black helical) wire soldered in to the battery box, complete with tab of red PVC tape- it seems we may have acquired units that have been through the same pair of hands!

Check the resistance of the fuse if it is intact; i believe it to be critical to the calibration of the meter. Valuable info!

Dave

Last edited by The Philpott; 13th Apr 2018 at 9:48 am.
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Old 13th Apr 2018, 4:12 pm   #7
TonyDuell
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Default Re: DC AVOmeter

I thought it looked quite nice too...

The fuse must be good. I have tried connecting a PP3 battery to the AVO on the 12V range, and put my (not formally calibrated, cheap) DMM across the battery at the same time. The DMM read 8.69V. The AVO 8.7V as near as I could see. Not bad for what must be quite an old instrument.

Alas there was no spare fuse stored inside the battery compartment.

I will measure its resistance sometime. Is this not something that is now generally known, though?

As for 'The Philpott' having a meter from the same person, I got this one from a dealer at the local antique fair who does house clearances (and buys things from other sources, of course). He knows I like old electrical stuff, and offered me this AVO and a metal-clad MEM switch-fuse for £20 the pair. I am not sure what the AVO is worth, but I don't think I paid too much.
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Old 15th Apr 2018, 5:42 pm   #8
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Default Re: DC AVOmeter

Tony, Dave,

I have been doing a little bit of research on the 13-range Avometer ("Model 2") which may be of interest to you both.

You're quite right Dave, the fuse is calibrated and has a critical value of 4.0 ohms. At one time replacements were available from ACWEECO (AVO) on an exchange basis but as this is no longer possible, a replacement with a suitable fusing value, probably about 150mA, and the correct resistance is quite a challenge. A fast acting 5 x 20mm 160mA fuse has a resistance of 5.349 ohms, so the wire from one of these might be a starting point.

I didn't have a circuit diagram for the 13-range meter; the one I had was for the very rare 22-range type with the divide-by-two switch. The 22-range diagram was redrawn in the AVO drawing office as issue 2 on the 29th June 1961. I can't imagine why as production seems to have ceased in 1939. I've used this diagram and a 13-range meter to produce the attached circuit diagram. Please look it over and see if you can find any mistakes. I've done a set of calculations which are summarised in the simplified current and voltage range diagrams which are intended to verify the component values. The circuit functions are very simple, so I hope I've got them right. If this set of diagrams is correct, maybe we could do the same for the resistance ranges.

Tony,

The serial number of your meter is a very late one for this type of meter. The screw terminals look as if they may have been factory fitted, either when the meter was made or as a modification for a customer. The terminals on the 22-range meter (4th attachment) are very like yours but that instrument does not have the original style of terminal for fixed leads. The 13-range Avometer was still being listed as a footnote to ACWEECO advertisements up to the late 1930s but it seems unlikely that sales volumes were high, probably not enough to be considered viable nowadays. If my speculation is correct, it makes your meter quite a significant one because it's unlikely there are many of exactly the same construction.

PMM
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File Type: pdf Current Ranges - Simplified-Model.pdf (115.6 KB, 56 views)
File Type: pdf Voltage Ranges Simplified-Model.pdf (105.1 KB, 59 views)
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Old 15th Apr 2018, 6:01 pm   #9
TonyDuell
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Default Re: DC AVOmeter

Thanks for that, it is very interesting to me.

I am sure the terminals are original on my instrument. There is no evidence of any soldering inside (other than that wire in the battery compartment). Under the cable cleat are 2 metal bushes which I assume were the connections for the captive test leads more commonly found on this instrument. One shows continuity to the terminal on the same side, the other is unconnected (and as far as I can see has never been connected to anything).

Thanks for the circuit diagram, I will compare it to my instrument when I get a round tuit.

I no nothing about the history of my meter though. I would like to find out more if anyone has any comments of course.
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Old 15th Apr 2018, 9:36 pm   #10
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Default Re: DC AVOmeter

Good grief, i didn't spot the duplicated terminals; they were hiding in plain sight..

I'll have a look at those pdf's for interest, thanks pmm.
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Old 17th Apr 2018, 11:52 am   #11
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Default Re: DC AVOmeter

Quote:
Originally Posted by The Philpott View Post
First stop is richards radios but you've probably already been there..? Don't know off-hand if there is a circuit diagram on there.

Just occasionally these particular stripped threads can be defeated by fitting a slightly longer screw (if this can be done without hitting anything or getting too close to live parts inside- & you might need to deepen the hole a little) Those early meters are vulnerable to this- brass inserts were phased in later on.

I wonder if those inserts could be retro-fitted from a later scrap meter.. you'd have to be accurate with the drilling..

Dave
Could the threads be fixed by filling the holes with epoxy then drill and retap?

If originality not important, retap to next metric size up and replace fixings. (I've fixed quite a few diecast boxes with stripped/missing screws by tapping to M4 from original 4BA or M3.5.)

Bodgit wise, a few strands of 0.2mm wire (from xx/0.2 flexible hookup wire) can be effectively run down the side of the hole, or some slivers of cooking foil......
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Old 17th Apr 2018, 1:25 pm   #12
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Default Re: DC AVOmeter

Oh speaking of which, low to medium strength loctite has sufficient gap filling properties, and the next time you have to remove the loctited screw, there is better than half a chance that there will be a tough blue or red threaded 'insert' in the hole waiting for the screw to be refitted.
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Old 17th Apr 2018, 2:50 pm   #13
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Default Re: DC AVOmeter

At the time when the first of these meters was made, phenolic resin (Bakelite) was quite a new material and its properties would not have been well known. It was probably thought that direct drilling and tapping would be satisfactory and, given the thick sections of the moulding there was some justification for this.

The front panel design was sketched out personally by Donald MacAdie and he may well have come to know the material through his work as the manager of the Post Office telephone factory in Holloway. I'm speculating that the Post Office was interested in the material for telephone casings and other parts.

One possible solution to the stripped thread problem would be to fit M3.5 brass inserts such as these http://www.theinsertcompany.com/bras...or_plastic.php but that would detract from the originality of the 4 BA screws originally fitted. It would also be an expensive solution and the 4.7 mm pilot drill is a bit big for comfort. Since the screws will not be remove and refitted often, an epoxy resin, possibly filled with Bakelite dust, could be used as a filler and drilled and tapped 4 BA when fully set. There are "plastic metal" epoxy resins available but these would set much harder than the original panel with the risk of causing the tapping drill to wander off centre.

The casing holes could be used as a drilling guide, but the cord gasket around the front panel would have to be in place to give the correct panel height.

PMM
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Old 17th Apr 2018, 5:09 pm   #14
Leon Crampin
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Default Re: DC AVOmeter

A technique I've used to repair stripped threads in phenolic material is to use full strength Araldite as a threaded bush. The trick is to very thoroughly grease the existing screw.

Using well mixed and warmed epoxy, apply a thin layer with a cocktail stick to line the hole. Then select the longest possible screw of the correct thread, warm it and apply grease. Petroleum jelly works well, but work it in thoroughly. Insert the screw fully, ensuring it's perpendicular to the surface and leave to set.

The screw might need some persuasion to remove, but I've not had a failure. The result is a good, rather tight thread.

Leon.
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