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Old 20th Nov 2019, 12:30 am   #21
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Default Re: AVO Model 7 Restoration

PMM,

Thanks, that's useful to know.

Was the Model 7 launched with the Compensator do you know?
or did it come some time later?

Did any other Models have one?


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Old 20th Nov 2019, 3:50 pm   #22
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Default Re: AVO Model 7 Restoration

Quote:
Originally Posted by TonyDuell View Post
Silly suggestion (?) Could you not shunt a DMM (normally 10M input resistance) with a suitable resistor to make it the equivalent of a 1000 ohms per volt meter? If, say you would have used a 100V range on the AVO 7, connect a 100k resistor in parallel with the DMM (it will be near enough for this sort of measurement I think)
Yes probably, but then I wouldn’t have the pleasure of using( and looking at) an AVO7.
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Old 20th Nov 2019, 5:34 pm   #23
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Thatís what I told my wife, as to why I needed ANOTHER meter. Please donít give me away!
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Old 21st Nov 2019, 2:34 pm   #24
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Default Re: AVO Model 7 Restoration

Ian,

Temperature compensation was advertised as one of the characteristics of the Model 7 in the advertisements which announced its introduction so it seems certain that it was used from the beginning until the movement design changed.

It was patented, GB 476682, application date 12th May 1936, Complete Specification accepted 13th December 1937. The Model 7 was introduced in August 1936.

The complete specification states:

"The chief object of the invention is to provide means for compensating for changes in ambient temperature and to provide some correction for overheating due to misuse of the meter, whereby the accuracy of the reading may be retained within the limits of temperature compensation possible with the device".

I have never seen another Avo instrument which uses this form of compensator. This could be because the Model 7 was the most sensitive "full-size" Avometer until the Model HR was introduced around 1948, but the HR had a block magnet movement which perhaps did not need temperature compensation. The first HR which I obtained did however have temperature compensation using a thermistor. It had been fitted with a Model 8 Mk III movement in what seemed to have been a factory repair.

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Old 29th Nov 2019, 9:00 am   #25
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Default Re: AVO Model 7 Restoration

Hi Ian. Not sure about T. C thingy.
Prompted by this thread I had another look at my meter, which might even be 1951.
The blurb on the back of the meter mentions the fact the battery is required to bias the AC rectifier(which I didnít know until I read it here) but as far as I remember no mention of any temperature compensation.
I really must take some photos.
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Old 15th Jan 2020, 12:41 am   #26
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Default Re: AVO Model 7 Restoration

The AC Range Fault on my 1946 Model 7 turned out
to be a Faulty Copper Oxide Bridge Rectifier!

In order to preserve as much of the originality
as possible, I thought I would look at trying to repair the
Rectifier.

Thought this might be of interest to anybody concerned with keeping
things as original as possible.

I dismantled the Rectifier Body (Single Centre Brass Nut & Bolt),
removed the Copper Oxide Disks and re-packed it with small Signal Diodes.

I was having trouble finding OA95's and I noticed on another
Thread mention of the BAT 49.
The BAT49's seems to be another good replacement option.

After Reassembly and refitting to the Model 7, all the AC Ranges work
well.




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Old 15th Jan 2020, 12:14 pm   #27
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Default Re: AVO Model 7 Restoration

Great Job!! congratulations!!!

Thanks for posting it!
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Old 15th Jan 2020, 4:40 pm   #28
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Default Re: AVO Model 7 Restoration

Thanks Ian- I didn't realise quite how well those CuO rectifiers were suited to re-stuffing.

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Old 15th Jan 2020, 8:02 pm   #29
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Default Re: AVO Model 7 Restoration

Dave,

The Rectifier dismantles very easily.

It's a bit cramped in there and some grinding is required to the
Central piece to allow the Diode Leads to join each other. This
is all internal though, so invisible from the outside.

However, before re-stuffing, it is worth re-seating the Disks
and Springs first, as I was actually able to repair one with a
Short Circuit.
A second faulty one which I couldn't repair as I had no spares, meant having to re-fill with the BAR 49's.

In fact, you can experiment with the Disks outside of the
Rectifier Body. They are simply Sandwiched together under
pressure from the Spring.

There is the Spring, which then sits against a copper Disk thus spreading
the load, which in turn sits against another Copper Disk with a
Copper Oxide layer on it.
It's all actually very simple.

The Copper Oxide Layer by the looks of it, can shift around with time
and the Copper Disks can end up touching each other and shorting out.
This looks to be quite easy to fix.
If the Oxide layer is damaged, then it gets a bit more tricky.

But everybody, don't throw any old Rectifiers out!
There are salvageable spares inside.
You can throw them in my direction if you really need shot of them!


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Old 15th Jan 2020, 11:53 pm   #30
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Default Re: AVO Model 7 Restoration



"You can throw them in my direction if you really need shot of them!"

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Old 16th Jan 2020, 11:41 am   #31
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Default Re: AVO Model 7 Restoration

I should also mention, that there are at least Three different sizes of
Copper Oxide Rectifiers that I have found. There are probably more!

The example shown previously being repaired was 17mm in diameter
if memory serves me right. I can't find the piece of paper I wrote the
dimension down on at the moment.

A Model 40 I have of the exact same age has a larger Diameter, although
I haven't measured this one yet as it is still in the Meter.

But there are much smaller ones, which might pose more of a problem
with re-packing.



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Old 23rd Apr 2020, 7:43 pm   #32
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Default Re: AVO Model 7 Restoration

Well, if nothing else, the current situation is providing a fair bit of
spare time.
Finally got fed up with looking at the Model 7 sitting there waiting to be finished.

Had to repair two open circuit Windings in addition to the prevoius issues, so had to give in and replace
with Modern Resistors! Hate doing that, but there is no other way really these days.

Got the Two Pack Resin out and re-molded both Dome Terminal Covers. Fortunately the Brass Parts were still present.
Also took the opportunity to use left over Resin to make a small repair to the Chip on the Front Panel.

Tested AC & DC Volts, Amps and Ohms.
All pretty much now within Spec.
This Meter is now 74 and a Half Years old.

Photos show the before and after for the 36 Range and Model 7 which I
bought together as spares in 2018.
Now both restored and working.

Will they ever get used in Anger?
For sure they will !



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Old 24th Apr 2020, 12:08 am   #33
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Default Re: AVO Model 7 Restoration

Congratulations!! That is a very good job!!
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Old 24th Apr 2020, 9:28 am   #34
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Default Re: AVO Model 7 Restoration

Outstanding, what sort of two-pack resin did you use Ian?

I couldn't believe the spot-on accuracy of the 1000/10,000 ohm ranges on my pre-war 34-range yesterday. No histrionics after a year idle, just minor tweaking of P & R.

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Old 24th Apr 2020, 11:07 am   #35
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Default Re: AVO Model 7 Restoration

Dave,

I used Jesmonite, AC100 Solution and Black Pigment.
Result is a Matt Black that you can polish up to a Shine if desired.


Problem is 2.5 Kg is the smallest quantity of Base Power you can buy, and
it's quite expensive if you only have a small Job to do.

The other problem is, that it allegedly only has a Six Month Shelf Life.
It took me at least Six Months before I even got round to try it out!

However, that said, my Jesmonite is now over a year old and still appears to work just fine.
I think the Setting Times may decrease with age.

The Dury is still out at the moment as to whether this is suitable for Terminal Covers though.

I have concerns that if the Meter gets hot, the Brass Terminal Screws will expand and crack the
Jesmonite Covers. Time will tell.

The big chunk missing on the Lower corner of the front Panel repaired well.
I took a Mould from another undamaged 7.

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