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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

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Old 9th Jul 2019, 6:57 pm   #2201
The Philpott
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Default Re: AVO Multimeter survey

Yes 17754-74 would be July 1934; on examples from the 1930's the decade is assumed, thus my early smooth side DC Avominor 1034-93 originates from Sept. 1933.

The figures for production between Sept. 1933 and Sept. 1936 are surprising- around 40,000 units. This was a popular device! From Sept. 1936 to June 1941 another 22,000 were produced.

In the 1930's (certainly on full size avometers anyway) sometimes the serial number and date codes are reversed, which can cause confusion.

Dave
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Old 12th Jul 2019, 11:26 am   #2202
micheal
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Default Re: AVO Multimeter survey

DC Avominor serial number 95965-1058 this must have been one of the last to be produced, i'm surprised Avo were still making these as late as 1958.
Had to rewind the 60volt and one of the 300volt bobbins.
The later ones had the strengthening skirt removed so making the case the same as the very early one.

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Old 12th Jul 2019, 1:16 pm   #2203
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Default Re: AVO Multimeter survey

I'm also very surprised they were making the DC version in '58, i would have thought the Universal AC version would have pushed it out sooner, primarily by better accuracy with the larger scale (the glass is also of a sensible thickness..)

The use of smooth-side mouldings for the cases suggests to me there could have been a stockpile of the early mouldings which were dusted off and used when the meter was nearing the end of it's production..? I assume that the later moulding was introduced due to breakages of the early one.

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Old 13th Jul 2019, 11:46 am   #2204
micheal
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Default Re: AVO Multimeter survey

I think the strengthening skirt was added because the cell was prised out with a screw driver and the thin case could not stand the strain and broke.

Early case's had 6v,120v and 300v markings where the later versions had 6v,60v and 300v. and a thicker overall case, so probably a new moulding.

Mike.
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Old 15th Jul 2019, 8:01 am   #2205
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Default Re: AVO Multimeter survey

Quote:
Originally Posted by AndyGilham View Post
A new ebay purchase
Avometer Model D (10S/10610) - Serial Number 19110-345 (March 1945)

Looks to have a missing Terminal Knob, so i will 3D print a replacement. Other than that, it looks in good condition. I will upload some photographs once its delivered.

Regards
Andy

As promised, please find attached some photos of the Moel D.
The instrument front panel is missing a terminal, but generally its in good shape.
The first thing i noticed internally was the resistors. It seems at some point one of the current shunts on the shunt board had burnt out (low current, 0.015A??), and so replaced by 2 series resistors.

Additionally, one of the shunts on the resistance board has been wrapped in a solder resist tape. I assume this was due to the windings becoming loose.

Andy
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Old 15th Jul 2019, 11:17 am   #2206
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Default Re: AVO Multimeter survey

Hi Andy, the translucent tape is often in evidence on the 150v winding pictured- not sure why.

I think that the scorched lower left winding replaced by resistors is probably the 7.5vAC winding. That's a typical place for the lowest ACV range to be.

Dave
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Old 15th Jul 2019, 11:24 am   #2207
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Default Re: AVO Multimeter survey

Ouch- just noticed the safety cut-out's contact leg has been arc damaged in photo 4- the big carbon contacts are aligned to open after the inboard gold-silver contacts, which (you hope) protects the latter.

Dave
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Old 15th Jul 2019, 5:50 pm   #2208
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Default Re: AVO Multimeter survey

Model D- I can confirm that the burned out winding in pic 4 should be around 283 ohms..So the repairer did their sums right, even if they did rush it.

Dave
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Old 15th Jul 2019, 10:50 pm   #2209
AndyGilham
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Default Re: AVO Multimeter survey

Thanks for the observation and the update on the resistors. It’s a nice assembly, but does not look as robust as the model 7... which is strange, being a ministry variant
Andy
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Old 16th Jul 2019, 8:58 am   #2210
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Default Re: AVO Multimeter survey

my model D from August 1945 has a wire wound resistor on a glass tube rather than the pink ones.
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Old 16th Jul 2019, 12:41 pm   #2211
AndyGilham
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Richard,
I do wonder if mine had been modified, since the resistors are supported either end by a very flimsy board and the entire assembly is just held in place by the connecting wires.
A very weak assembly.

Andy
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Old 16th Jul 2019, 3:23 pm   #2212
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Good point, i hadn't spotted that- it is unlike the (late) '300v/750v' resistor packs i have seen, which consist of pairs of similar resistors either side of a much thicker paxolin board, (the connecting wires go through the board) and the whole thing has a brass bracket attached which enables it to be retro-fitted to any Model D using the fixing holes already present in the meter. What you have there may be an intermediate arrangement before the better ones were introduced. I think i recall that your meter is one of those that went back to the factory for a refurb indicated by the ACW mark on the scale plate..? The internals may have been changed then.

If you wanted the interior to look more period correct for year 1944/45 like Richard's (but NOT work on 300v/750 ranges) i have an open circuit glass tube type resistor somewhere that i have no use for.

Dave
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Old 16th Jul 2019, 6:53 pm   #2213
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Default Re: AVO Multimeter survey

Here is the later high voltage resistor pack for the Model D, the brass mounting frame can be seen below it. Perhaps the one Andy has was one that somebody prototyped and the factory then improved on.

The second picture is a side view of an early Model 7- the only thing which doesn't look too robust is the wallace & gromit style temperature compensator- but it can actually be seen to work when you breathe on it!

Dave
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Old 17th Jul 2019, 7:16 pm   #2214
AndyGilham
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Default Re: AVO Multimeter survey

I think this where my opinion of it not being robust stems from, this floating resistor arrangement. It just moves about within the recess.
It's always intriguing seeing these small variations in build! I am currently up to 9 variants of the model 7 (mark 1) resistance board..!
Think I'll leave it as it is...... Seen another model D for sale with glass tube!

Andy
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Old 18th Jul 2019, 3:42 pm   #2215
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Andy, i couldn't find the Model D glass tube i thought i had (on account of the fact it never existed..) but have sent you one from a Model 40 series for academic interest. Have a look at the thickness of the wire where it has pulled away from the tube- it is 1 thousandth of an inch thick, (25 micrometres)- a human hair is 3 thou. They managed to make these things to a resistance tolerance of around 0.2-0.3%...good even by modern standards.

When they made the slab and bobbin type windings they were always up against capacitance and inductance, hence the odd (usually patented or pat.pending) ways they wound them. I had one small bobbin that showed 12kohms then 14kohms when the polarity of the DMM leads was reversed- although i think this may have been down to the effects of internal corrosion. A resistor that dreamed it was a diode!

Dave
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Old 18th Jul 2019, 7:13 pm   #2216
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Default Re: AVO Multimeter survey

The glass tube form of resistor was the subject of two Patents assigned to Harold Edward Bayliss both applied for on March the 9th 1942 and accepted on the 17th December 1943. This would set the date at which these components could have been intriduced commercially.

The first patent concerns embedding the resistance medium in a coating on the tube as the resistor is wound to improve the mechanical stability of the wire. The second concerns the use of pressed out tabs in the clamp bands to provide an attachment point for the winding.

H E Bayliss took out these patents in his own name but later also had joint patents for winding machine accessories with ACWEECO, so may have been an employee of the company, at least at some period. He seems to have been a specialist in detail improvements in resistor manufacture.

In my experience, it is very rare to find a glass tube resistor of this type which is not open circuit, possibly due to stresses on the extremely fine wire used. In the long term, I do not think this type of resistor could be considered a success.

Concerning Andy's comments about the construction of the Type 'D' Avometer, I think it would be fair to think of those produced during the middle war years as having "Utility" construction due to shortages of specialist materials, such as the contact material for the cut-out. I don't think this is true of the corresponding Admiralty Pattern 47A & 48A; perhaps the Admiralty was less willing to take a pragmatic view than the Air Ministry.

PMM
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Old 19th Jul 2019, 6:50 pm   #2217
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I think an additional problem with the (glass tube) resistors is that there wasn't a durable way of fixing the clamp bands to the glass.

Remarkably 4 of the 6 which i have are serviceable...at the time of writing.
Dave
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Old 25th Jul 2019, 5:14 pm   #2218
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Default Re: AVO Multimeter survey

Two lovely goodies have come my way very recently.

First, something I've wanted for a very long time is the Power Factor and Wattage unit made to connect to the PF sockets of the Model 7 and 40. It's complete with the leads. I've just yet to try and see if it's working. There's not much them really in circuitry, so it should work. No serial number on this unit that I can see.

Also, with a very warm welcome indeed, I introduce into my collection a 20 range Avometer, other wise known as the Model 3. Serial No.13-928.
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Old 25th Jul 2019, 5:18 pm   #2219
Phil G4SPZ
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Default Re: AVO Multimeter survey

Nice!

I thought that the PF & Wattage Units were made in different models for the Model 40 and Model 7? I have the Model 7 variant.
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Old 25th Jul 2019, 5:30 pm   #2220
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Yes, you're quite right. Mine does indeed say it's for a Model 7.

Now, this means there's another one I need to collect!

I assume the difference then is the range to FSD.
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