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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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Thread Tools |
24th Feb 2020, 2:00 am | #1 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Avometer Model 7
Hi folks,
I've just bought a Model 7 for spares, the battery cover in particular that I've been trying to get hold of for quite a while now but also possibly the meter movement to replace a sometimes slightly sticky one I've got in my Model 7 I repaired last year. However, having taken delivery I've discovered that it was made in April 1944 and it could be fully functional! Can I bring myself to cannibalise it? I'm very reluctant to do so! It has clearly suffered a serious drop because the front has a crack, the bakelite back is broken at the battery opening and the battery box was badly glued having been in pieces! I've dismantled the badly glued parts with copious amounts of acetone and superglued everything back together as best as I could. The glass is also cracked in 3 places on the right hand side so before reassembly and further testing I'd like to try to replace it. The serial number is 7444-14412. I've attached a few photos below. I do have a couple of questions: 1) Is there a recommended procedure for removing the movement which seems quite complex having a bimetallic strip attached to it. 2) Has anyone got a spare glass or suggestion about where to get a piece? Regards, David |
24th Feb 2020, 1:31 pm | #2 |
Octode
Join Date: May 2017
Location: St Austell, Cornwall, UK.
Posts: 1,018
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Re: Avometer Model 7
Hi David,
You will need to remove the Temperature Compensator (bimetallic strip) first, in order to gain access to one the the Meter Movement retaining Screws. The Temperature Compensator Board is held in by Two Screws and also includes the Swamp Resister if Memory serves me right.. I would recommend un-soldering the Compensator Board first being careful not to loose the Screws, Nuts and Washers which can drop and get lost easily. Once the Board is out, the Movement can be un-soldered and it's Two retaining Screws can be removed and the Movement taken out. Of course, follow the normal precautions about touching the Magnets with any Ferrous Tools! It is not as complicated as it might look at first. I find these early Model 7's fascinating. Since this is an early Model 7 (Relatively), it is definitely worth saving. This was the Thread I started a while back regarding my Model 7 of a similar age to yours: https://www.vintage-radio.net/forum/...d.php?t=160057 Ian |
24th Feb 2020, 1:42 pm | #3 |
Octode
Join Date: May 2017
Location: St Austell, Cornwall, UK.
Posts: 1,018
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Re: Avometer Model 7
Forgot the mention, regarding the Glass, they do come up on Ebay occasionally, but tend
to be expensive when one considers the poor Prices Model 7's normally achieve. You might be able to find a Glass supplier who could knock one up for you, but it should be Instrument Glass which I think is around 3.5mm and that is not always easy to find. I have successfully used old Greenhouse Glass on larger Meters, and you could possably get away with that at a push. Although, I try to keep repairs as accurate as possible when it comes to AVO's. Ian |
24th Feb 2020, 1:51 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Worksop, Nottinghamshire, UK.
Posts: 5,553
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Re: Avometer Model 7
It is the right time of year to get a bit of scrap glass from a gardener who is fixing a greenhouse.
It is nice and windy so with a 12 volt drill and a diamond wheel running from a battery in a strong side wind cut a bit of glass the right size and glue it into the front panel. |
24th Feb 2020, 2:04 pm | #5 |
Dekatron
Join Date: Jul 2007
Location: Penrith, Cumbria, UK.
Posts: 3,687
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Re: Avometer Model 7
I believe the glass is held in with Stockholm tar. I have an AVO 8 that needed a new glass; original was cracked. I took the old piece to a glazier in Penrith and he found a piece of glass the same thickness and cut me a new one. Eight quid.
__________________
Regds, Russell W. B. G4YLI. |
24th Feb 2020, 2:21 pm | #6 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Re: Avometer Model 7
Thanks for that Ian (and other contributors). I really enjoyed reading your restoration account, particularly the information about the rectifier which I have wondered about with my other Model 7. This forum really is a fantastic resource of knowledgeable and experienced people!
David |
24th Feb 2020, 4:33 pm | #7 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,106
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Re: Avometer Model 7
..If you go a glazier take the bare meter with you to trial fit the new glass- it's easy to get it marginally oversize and get left with a tricky trimming job (But in my experience if you raise the issue straight away they will simply cut a new piece without further charge.)
I did some work on a Model 7 from year 1939, which i summarised on the Avometer Survey Thread, page 72 et. seq. I think i got the movement out without disturbing the bimetallic coil, but i wouldn't recommend it. Dave |
24th Feb 2020, 5:56 pm | #8 |
Heptode
Join Date: Nov 2009
Location: Scarborough, North Yorkshire, UK.
Posts: 510
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Re: Avometer Model 7
The glass from an old picture frame is thin and might be OK as a replacement. Ted
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24th Feb 2020, 10:47 pm | #9 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Re: Avometer Model 7
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12th Mar 2020, 6:42 pm | #10 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
|
Re: Avometer Model 7
Well the glass from a photo frame was successfully cut to size and shape. I used a standard glass cutter for the straight lines and nibbled bits off where necessary then I got my Black and Decker Dremmel with a diamond cutting wheel for the curved cut. I went right through the glass on one side of the curve and then twisted the curved section and it snapped free.
I haven't glued the glass into the meter, I've just used the four fixings. |
12th Mar 2020, 7:26 pm | #11 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Re: Avometer Model 7
I have repaired the case and battery box with "SupaFix" which I got from Halfords. Its a kit intended for repairs which uses super glue and one of two different types of catalyst in the form of either black or grey powder, which both contain calcium oxide. When mixed with a drop of super glue the mix goes rock hard in seconds and can be used to fill cracks and holes. The black powder I've been told is for hard plastics and the grey powder is for soft plastics. The instructions don't tell you this though.
After reassembling the back and battery box I made a replica Exide B23 battery for the 1.5V cell and tested the two lower resistance ranges. These both worked! I then tested the DC voltage and current ranges. The lowest of each worked but nothing higher so I checked the shunts. The 4500 ohm resistance was open circuit on the top resistance board (voltage) and the end of a wire had come away from a soldered joint on the bottom resistance board (current). I took loads of photos and then removed the boards one at a time. Resoldering one wire restored the current ranges. The 4.5k resistor on the top board was open circuit and I couldn't see the break so I made up the value with two resistors in parallel. I then found that the 44.2k was open circuit! I must have done some damage while soldering. Anyway I had measured this with my Avo DMM at 43.9k so I made this value up with two more resistors and put it back together. Thank goodness I'd taken lots of photos because I had to use them for re-assembly! Having got it all back together all the DC ranges and resistance ranges are now working and appear to be very accurate! David |
12th Mar 2020, 7:35 pm | #12 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Re: Avometer Model 7
I forgot to mention that I disguised the new resistors by wrapping them in masking tape and coating it with walnut coloured varnish. I tested a 3.9k resistor and a 15k resistor on the 1M ohm range and it was spot on as can be seen in the last two photos.
David |
12th Mar 2020, 10:57 pm | #13 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,106
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Re: Avometer Model 7
Saved! That glued repair should hold- looks like clean cracks.
The irony is that those who make these things into table lamps would have rejected this one on the basis that it was too much effort. Dave |
13th Mar 2020, 11:21 am | #14 |
Hexode
Join Date: Oct 2007
Location: Ossett, West Yorkshire, UK.
Posts: 406
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Re: Avometer Model 7
They would! Incidentally, I managed to get the meter out without disturbing the temperature compensator too. A very thin, good quality screwdriver was required but it saved a lot of messing around! All of the info on the forum was really useful! At one point when reassembling I thought I'd have a look in my 1951 Model 7 to check a couple of connections. I'd forgotten that the top resistor board is completely different so I rechecked to the photos I'd taken instead.
David |