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Old 17th Jul 2023, 7:34 pm   #3201
The Philpott
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Default Re: AVO Multimeter survey

Someone made a good quality repro label Jeremy, i can't remember who it was. (and i'm unsure whether the Model 2 label would be the same as the later ones)
Mine is too damaged to scan unfortunately.

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Old 17th Jul 2023, 8:31 pm   #3202
pmmunro
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Default Re: AVO Multimeter survey

Jeremy,

Congratulations on your find. It's good to know that another really old meter has found an appreciative home.

There is some anomoly about the serial number though. I agree that 1627 should be a very early Model 2, and one which would not have had an externally accessible fuse. The first attachment is a close up view of the movement of Serial Number 2234 which clearly has an internal fuse - just a piece of wire soldered across two (slightly) thicker wires. It looks very much to be an "adjust on test" type. Serial No. 2020 is also a Rochester Row production instrument with cross hatched front panel and internal fuse - Model 2A. Your meter, with its smooth front panel sounds more like a Model 2B made at Douglas Street. Is it possible that the serial number on your scale plate could be 7627?

The nature of the screw-in fuses used on Avometers has been a question which has been concerning me for many years. (These were used on later Models 2 to Model 5, early Air Ministry type 'D' and on Type 'E' meters, the 'D' and 'E' fuses having these letters moulded into the top cap). I found several of these fuses with a short length of wire between the terminals and had, until now, believed that that was how they were made. The problem was I could not see how a wire could be calibrated to give the required 4 ohms nor how it could be thin enough to protect such a delicate movement, at least for its time.

I had a look at some intact screw-fuses earlier today an I think I have found at least part of the answer; these fuses don't us a wire at all! The fuse element is what appears to be a thin film of gold paint or some other thin gold film, please see attachments 2 & 3. The end view would suggest that the gold film might have been painted on. It was then scraped of the surface of the substrate to give the required 4 ohms. I have to wonder how the fusing current was estimated; possibly this could only be assessed by some tests. Perhaps they weren't too concerned about the actual fusing value as long as the fuse acted quickly.

It might be possible to make a substitute fuse using silver loaded paint which has a resistivity of 0,01 to 0,03 ohms per square at a typical thickness, but I calculate that the element would have to be about 0.2 mm wide - not very easy to achieve. Gold would be about 0.3 mm,but I don't know of a readily available source of conductive gold paint.

PMM
Attached Thumbnails
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Name:	Rear of Movement  & Fuse - Close up.jpg
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Name:	Avometer Fuse Photograph 1.jpg
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ID:	281497   Click image for larger version

Name:	Avometer Fuse Photograph End View.jpg
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Size:	40.4 KB
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Old 17th Jul 2023, 9:48 pm   #3203
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Default Re: AVO Multimeter survey

The image Peter has provided matches up with a couple of intact Type E fuses that i have- in that case they are around 11 to 13 ohms.

Certainly looks like gold or gold loaded material that's scraped back progressively to give the required result. From the factory a (compressed cardboard?) black tube was slid over the lower part of the fuse and (stockholm tar?) used to seal it in place, seemingly applied through a small hole in it's side. If i remember right a factory tamper-logo was stamped into this black substance.

When i reworked 'E' fuses using tiny resistors i had to dispense with the aforementioned tube.

Dave
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Old 17th Jul 2023, 10:15 pm   #3204
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Default Re: AVO Multimeter survey

Aitor produced a good replica of the Battery Label a while back.
It's a shame that this Work is so easily lost and forgotten.


https://www.vintage-radio.net/forum/...=152127&page=5


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Old 18th Jul 2023, 7:23 am   #3205
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Default Re: AVO Multimeter survey

Nice find, Jeremy! As regards the fuse, I have an early 36-range with a fuse which was broken, and I soldered a tiny 3.9 ohm resistor in place in the fuseholder. The resulting accuracy pleasantly surprised me! Perhaps I was lucky, as I have read elsewhere that the fuse resistance was selected on test to match the rest of the instrument. If you have access to some appropriate resistance wire, that would make a more original solution.
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Old 18th Jul 2023, 9:34 am   #3206
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Default Re: AVO Multimeter survey

Ian,

I have to apologise for not remembering the description you gave from post 106 in the thread "Avo 36-range restoration" which started in December 2018 and ran for a year.

I had either forgotten or failed, at the time, to read about your experiments with gold leaf as a fuse element which showed how the original fuses were made or a technique very similar. I find that an impressive bit of work so thank you for reminding us of it.

PMM
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Old 19th Jul 2023, 2:30 pm   #3207
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Hi Peter,

Absolutely No apology needed.
It did highlight a problem though.

I have found that, even if you know the information you want has been discussed before,
the Search facility on the Forum doesn't always seem to work.

Take these recent Thread Posts for example.
I wanted to put a Link to the Battery cover Label Aitor produced, but I couldn't remember
the Thread Title since it was a few years ago.
No matter how I phrased the search, I could not find that Thread.

I only found it in the end because I remembered Aitor had started it and I did a Member Search for
his Threads instead. Hence I was able to find the Link.

Interestingly, now that I know the exact Thread Title, my Searches still don't pull it up, even when I
tried a few minutes ago.
Not sure if it's some quirk with my PC, login or what ever.

Getting back to the Fuses, I have successfully repaired most of mine now, and although I
can't guarantee the Rupture value of the Element, (as indeed you can't for a commercial offering)
it is for sure going to be better than a 4 Ohm Resistor or Manganin wire.

I did a fair bit of research, but I can't for the life of me work out how Avo would have
achieved anything close to 60 mA which is what I would expect for a 6mA Movement.

80mA is extremely difficult to achieve, but probably possible, but the Fuse Element would
be significantly narrower than what I have observed on Surviving Fuses.

My work would suggest Avo Fuses were probably around 120mA. The Movements are quite
robust to be fair, although I wouldn't want to be testing them out in that respect.


Ian
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Old 19th Jul 2023, 7:22 pm   #3208
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Default Re: AVO Multimeter survey

I have found the forum index rather limited too, neither returning the correct information nor eliminating a great deal which seems not to have any relevance. I assume that this is a limitation of the design of the system on which the forum runs and not in any way of the moderators.

Concerning the AVO fuses, I would think the fusing value which you have found of around 120mA is probably about right for early DC and Universal meters. The service manuals for later meters with mechanical cut-outs specified a "must trip" current as high as 20 times the range value so that a 120 mA fuse would give sufficient protection for a 6 mA movement.

That would suggest that it might be significant that the mechanical, end stop and acceleration, cut-out was introduced in the Model 7 which, with three times this sensitivity would have needed a 40 mA fuse, probably just too demanding. It would be very interesting to know if this was in fact the motivation for designing the mechanical cut-out.

PMM
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Old 19th Jul 2023, 9:19 pm   #3209
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Default Re: AVO Multimeter survey

Having once managed successfully to repair an acceleration trip mechanism from a late Model 7, I am an admirer of this ingenious device and the people who designed it. That said, I have never understood why Avo abandoned the acceleration trip when the Model 8 was introduced, with its far more sensitive movement. Presumably the reduced operating time that the acceleration trip contributed to the cut-out’s operation was deemed to offer no greater protection. Odd too that they continued to fit the (by now redundant) thin alloy ratchet trip-wheel to Model 8 movements.
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Last edited by Phil G4SPZ; 19th Jul 2023 at 9:20 pm. Reason: Clarification
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Old 22nd Jul 2023, 9:38 pm   #3210
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Default Re: AVO Multimeter survey

Quote:
Originally Posted by pmmunro View Post

That would suggest that it might be significant that the mechanical, end stop and acceleration, cut-out was introduced in the Model 7 which, with three times this sensitivity would have needed a 40 mA fuse, probably just too demanding. It would be very interesting to know if this was in fact the motivation for designing the mechanical cut-out.

PMM
That's what I was thinking too.


Quote:
Originally Posted by Phil G4SPZ View Post
I have read elsewhere that the fuse resistance was selected on test to match the rest of the instrument. .
Hi Phil,

Don't know if that might be related to my comments a couple of years ago, that my
Model 5 was more happy at 4.8 Ohms, but Avo for sure did not calibrate the Fuses
individually for each Avometer.

Fuses were available as consumables, and were definitely available in boxes of Two, and maybe in other quantities as well.
So they were generic.

What I think probably happened was, they set about designing a Fuse for a particular
Rupture Current, (which is still a mystery at the moment, but likely around 120mA) and
the resulting resistance happened to be around 4 Ohms. Maybe they adjusted the resistance to
the nearest integer to help with manufacture, who knows now unfortunately.

I suspect, they then designed the Meter around the Resistance that the final agreed Fuse exhibited.
That's all speculation of course.



Ian
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Old 24th Jul 2023, 7:09 pm   #3211
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Default Re: AVO Multimeter survey

I have bought a Heavy Duty Avometer, Mk5 I think. The number on the front is 6*** - 956. It may work but needs cleaning to free-up the meter without sticking. It is also marked EE 1914 on the front, possibly a company reference.
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Old 25th Jul 2023, 9:49 am   #3212
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Default Re: AVO Multimeter survey

Relating to my previous message, the glass needs to be stuck back in, what sort of glue is best? The original fixing seems to be a black tar-like substance. Thanks.
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Old 25th Jul 2023, 11:46 am   #3213
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Default Re: AVO Multimeter survey

I use black silicone sealant.
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Old 25th Jul 2023, 12:42 pm   #3214
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Default Re: AVO Multimeter survey

I use Araldite sparingly, but if the glass ever broke again I'd have a major job getting the bits out. It squidges around to the front sometimes, and has to be trimmed away when it's half-cured, no later.

i think sealant as mentioned is arguably preferable, but these days I'd lean towards something silicone free to avoid getting that slippery deposit everywhere. 'Old school' sealant gives off acetic acid fumes during curing, which I don't like the thought of.

Dave
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Old 26th Jul 2023, 11:13 am   #3215
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Default Re: AVO Multimeter survey

Putty mixed with a little black oil based paint to make dark grey, cleans up well with turps and looks very similar to original. can always replaced if glass gets broken again

Mike
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Old 16th Aug 2023, 8:03 pm   #3216
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Hi All!

New member here and recent acolyte having just 'discovered' AVO meters and this is my first post. I have a couple of Model 8, Mk IIIs here that I'd like to add to the list:

4XXXX.766
1XXXX.265

I also have a Model 8 Mk IV but can't find a serial. Maybe this means the scale plate was replaced? However, these are all working perfectly apart from 1 slightly flaky / 100 pot on a Mk III.

I also have a very broken MkIII that has ceased to function correctly on all ranges although the transformer section appears to be good and all the resistances I can test look correct. The needle does move but just not necessarily to the right places and the glass is broken. It's a bit of a puzzle but I will persist. Strangely, the magnets are painted teal rather than red but the scale plate looks correct so I'm uncertain as to whether maybe a dodgy meter was installed for some reason? For completeness I shall include the number here so at least there's a record:

3XXXX.166

Thank you,
Kieron
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Old 20th Aug 2023, 12:03 pm   #3217
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Default Re: AVO Multimeter survey

I'm guessing that we can't really add that to the list if the serials are obscured?
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Old 20th Aug 2023, 3:48 pm   #3218
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Ah! Are we not obscuring them any more? I probably missed a post along the way
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Old 20th Aug 2023, 5:17 pm   #3219
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Default Re: AVO Multimeter survey

It's your choice whether to give the serial number in full (my preference) or with some part obscured. The obscured numbers can be used if the provenence of the meter is disputable.

The magnets of Model 8 (and other Avometers), were unpainted or painted according to the magnetic material used. The general pattern is that the Mark I is unpainted alnico, the Mark II red painted Alcomax and shorter magnet blocks, Mark III is improved Alcomax and painted blue and the Mark IV is unpainted. Please do not take this as definitive information. It will take some more research to coordinate the details, but this is an interim general guide.

The serial number of the Model 8 Mk IV was on a self adhesive label on the left hand side of the meter, sometimes reprated inside the battery box.

PMM
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Old 20th Aug 2023, 6:48 pm   #3220
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Here are the full serials:

M8 Mk I 59786-C-457 (shorted movement coil)

M8 Mk III 10983.265
M8 Mk III 34322.166 (dropped, badly damaged)
M8 Mk III 43688.766

Had a good hunt inside and out on the M8 MK IV and no sign of a serial.

Kieron
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