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Old 29th Aug 2019, 9:27 am   #101
pmmunro
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Default Re: Avo 36 range restoration

It is likely that ACWEECO still accepted older fuse-protected meters for repair at least into the 1950s so it is possible that a procedure document for replacing fuse elements could be in the archived records.

If Andy Gilham would be good enough to research this it might solve this chronic problem.

The movements of these meters are quite robust and, based on the setting of the automatic cut-outs of later meters, they should be able to withstand a ten times overload, suggesting a fuse rating of around 60mA might be appropriate.

From their datasheet for 5 x 20mm fuses (IEC60127), Littlefuse give a nominal resistance of 7.4 ohms for a 63mA fuse. Extrapolating that would give a wire length of the order of 10mm for an Avometer fuse if the same gauge and material of wire were used and a fuse of this type would be fast enough acting to give reasonable protection.

https://www.littelfuse.com/~/media/e...asheet.pdf.pdf

One very significant difficulty is how to solder the wire in place while maintaining the critical 4 ohm resistance. The value could be adjusted by starting with a higher resistance and reducing the value by thinning the wire, but given its fineness and the techniques available at the time it would not be easy.

The fact that ACWEECO encouraged customers to return failed fuses for repair or exchange suggests that the company did not consider fuse repair to be a procedure that could easily done by most customers.

All the Avometer fuses I have seen are siimple wire links soldered in place so I don't think the bobbin element would be "official". The coiling of the wire would alter its characteristic as a fuse and to be effective such elements would have to have been batch tested.

PMM
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Old 29th Aug 2019, 9:49 am   #102
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Default Re: Avo 36 range restoration

Excellent Photo Aitor,

Exactly what was needed for the record!

This clearly shows that AVO did indeed use a coating
on the Fuse Body and then one assumes Hand Trimmed it to 4 Ohms.

I am guessing they experimented with the Current Capacity required to protect the
Meter Movement and then designed the Meter Circuitry around the resulting Resistance
which obviously turned out to be 4 Ohms.

Interestingly, the coating that survives on mine is not so distinctly shaped like yours.
Mine is still full width, but has a lot of small Holes and scratches.
No idea if that is 80 years of damage or part of the original calibration process.

I am now wondering if the Fuse was originally Gilded with Gold Leaf and not Brass?


Ian
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Old 29th Aug 2019, 10:15 am   #103
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Default Re: Avo 36 range restoration

Would a modern fusible resistor work as a replacement?
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Old 29th Aug 2019, 10:38 am   #104
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Refugee,

I did look into that, but it appears the resistances are far to high for
the Current Ratings required.

I based the 100mA on Resisters I could find today with resistances under 4 Ohm,
but I agree with PMM that 60mA would be a more likely accurate figure.
The resistance of which go right up to around 8 Ohms or more.

For me, an historically accurate a repair would be preferred where practical and possible.

I suddenly thought after seeing Aitor's Photo this Morning, that maybe that Brass Coating is actually Gold Leaf!

It would be easy to apply to the Plastic Fuse Body, and probably easy to calibrate to 4 Ohms using a scrapper.
Gilding was a widely available skill in the 1920's/30's.

I am taking my existing Fuse to Friends this Morning who are retired Antique Dealers & Restorers for their opinion on what is left of the Coating.

Depending on what I am told later, I might experiment with some Gold Leaf.
It could turn out, that restoring an AVO Fuse to originality might be a lot easier than anybody imagined. Fingers crossed.

Ian
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Old 29th Aug 2019, 1:27 pm   #105
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Default Re: Avo 36 range restoration

Please, let us know about that gold-fuse theory

Thanks
 
Old 5th Sep 2019, 11:05 am   #106
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Default Re: Avo 36 range restoration

Here is an update with the Fuse.

Spoke to my retired Friends and the conclusion as to the actual Material used
for the Fuse is in-conclusive!
It certainly could be Gold, but it is by no means a certainty.

I was given a sheet of Gold Leaf to try though.

In principle it Works!

Got it to 4 Ohms quite easily, however it still doesn't work as a Fuse. At least not at the moment for the Current we are interested in.
I still need to work on this.
The Track needs to be brought to a much finer point to give the required Resistance and low current Capacity.

At least the Fuse looks more original than having 4 inches of Manganin Wire wrapped round it.

My 36 Range does have a Gold Fuse now though! (Just don't know it's Rating)

Watch this space

Ian
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Old 11th Sep 2019, 1:16 pm   #107
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Default Re: Avo 36 range restoration

The latest fuse update for anybody interested.

Yes Gold Leaf can be made to work as a viable Fuse.

I carried out a few Tests on a Jig I made up and I got
a calibrated 4 Ohm Test Fuse to blow at 120mA.

I got a slightly un-calibrated Test Fuse (ie 8 Ohm) to blow at a very
useful 80mA.

So I have no doubt, a Calibrated 4 Ohm Fuse can be made to blow at
around 80mA with a bit more effort to get the Fusible section of the Gold
a bit narrower.
Although, I'm happy with 120mA.

The next step is to try some Copper Leaf and compare results.



Ian
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Old 11th Sep 2019, 1:35 pm   #108
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Default Re: Avo 36 range restoration

Thanks very much! Very interesting information
 
Old 3rd Oct 2019, 3:39 pm   #109
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Default Re: Avo 36 range restoration

Another variation of the Battery theme for the earlier AVO Models!

Standard D Battery Holder. (or any Size you want really).
Remove Positive Stud and lead.
Take a Brass M4 Screw, and thread from the inside of the Holder through the Hole left when the Stud was removed.
Find a spare Brass Eyelet from your AVO Spares and Solder to Negative lead.
Fit and finish with an M4 Brass Knurled Nut.

And the Battery doesn't flop around in the Bay.
Wicked!

Ian
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Old 3rd Oct 2019, 8:12 pm   #110
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Default Re: Avo 36 range restoration

Neat and effective.
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Old 12th Oct 2019, 6:42 pm   #111
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Default Re: Avo 36 range restoration

The finishing touches to the 36 Range.

Another Battery version!

I actually came up with the idea of the previous Battery adaptor whilst
researching this period replica Battery.

A Duracell AA is stuffed in the Back as shown.
This is my first Prototype.
I hope to improve this AA adaptor on the next version with the AA Battery inserted
from the Bottom.

I quite liked this Exide Battery, a bit different to the usual Ever Ready, and
found the Scan for this on the Battery Thread on this Forum for those who
don't know about it.



Ian
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Old 12th Oct 2019, 10:33 pm   #112
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Default Re: Avo 36 range restoration

I saved the cardboard shell from an ancient Ever Ready with a similar thing in mind (I wouldn't re-stuff it with a Duracell though..)

Dave
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Old 14th Oct 2019, 3:13 pm   #113
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Default Re: Avo 36 range restoration

Dave,

Was that a veiled criticism of the Poor Old Duracel?

The Poor old Duracel was taking quite a battering on another Forum Thread recently:
https://www.vintage-radio.net/forum/...d.php?t=151115


I only have one Original R1662, but it's in pretty good Shape, it's clean and not Leaked at all so far, so I will be keeping it for reference and won't be dismantling it.

There are some good Scans available on this Forum for various Battery's including the R1662.

The Exide B32 was very slightly smaller than the Equivalent Ever Ready.

I used a Block of Pine Wood to make the Body, but I can't recommend Pine much, as I generally have trouble Machining small parts from Pine without it splitting.



Ian
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Old 14th Oct 2019, 4:22 pm   #114
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Default Re: Avo 36 range restoration

Final update regarding the Original AVOmeter Fuses.

I've gone as far as I want with this for the Time being, and genuine
24ct Gold Leaf seems to be the DIY way to go.

I can't say for sure which Metal or Alloy ACWEECO used originally,
although I found Gold was the easiest to work with and I suspect
due to the lack of corrosion seen on surviving Fuses, that Gold is
the most likely candidate.

I have tried the following Materials available today in Leaf form:

24ct Gold Leaf.
Pure Copper Leaf.
Dutch Gold Leaf. (also known as Schlag Metal or imitation Gold Leaf which is 85%Copper/15%Zinc or there about).

I couldn't get the Copper or Schlag Metal up to the correct Resistance.
The nearest I got was 2 Ohms with the Schlag Metal, but at 500mA it
still wouldn't blow!
The Copper was simply to conductive to give any useful resistance.

To get these Two Materials up to spec, some form of Etching Technique
would probably be required.
Would AWEECO have been involved in Etching I wonder, back then?

At least with the Gold, I could reliably achieve the 4 Ohms (or there about),
although my 36 Range was better calibrated at 4.8 Ohms which is where I left it.
I have since improved the Gilding Technique I used, so I can probably
improve on the 120mA, but for now, my estimate, based on my tests my Fuse
will be around 120mA.



Ian
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Old 9th Dec 2019, 9:07 pm   #115
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Default Re: Avo 36 range restoration

Does anybody have any surviving original Test Leads for the
36 Range Meter?
Or any of the early Avo Meters to be honest!

I would be interested to see exactly what they looked like.

I am guessing they would have had Cotton Covered Cable,
but I would be interested to see the Connector Style (Both Ends) as well
if any survive.



Ian
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Old 12th Dec 2019, 9:40 am   #116
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I think I have a photo somewhere. I will try to find it today.
 
Old 13th Dec 2019, 3:50 pm   #117
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i found this one. model 2, not 34 or 36 ranges, but perhaps can help a little bit.
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Old 13th Dec 2019, 11:32 pm   #118
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Default Re: Avo 36 range restoration

The construction of the cables Aitor features look very much like the heavy duty leads supplied with the Avominor AM testmeter Type E. I was working on one of these whose ends had corroded, and although there was no tin plating on the copper i found 3 layers of insulation:

Layer 1- Cotton tightly wound onto the multi-core copper. This could be to protect the copper from Layer 2, which is rubber. Layer 3 in cross-hatch woven (i don't know the correct term) cotton. Coloured red and black when new, usually faded to beige and black by now.

These Type E leads differ from Avominor Type HR leads, which have wander plugs at either end, and the copper wire is tinned. There are no cotton layers.

I have no ancient leads for the Big Avometer unfortunately.

Dave
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