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Old 26th Sep 2019, 12:16 pm   #1
Superscope
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Default AVO Model 7 Restoration

I'm just about to start the restoration (or is it Conservation? ) of a 1946 Model 7.


I don't need one, but out of sheer idle curiosity was there ever a Model 7 Service Manual?
I have never seen or even heard of one, but surely AVO must have produced one?

Anyone ever seen one?



This Model 7 came with my sorry looking 36 Range a while back and I assumed it would
be for spares only, but my philosophy is, if the Meter Movement is functional
the Meter can be saved regardless of it's general condition.
Actually, when I looked inside, the standard of workmanship was quite stunning.
Well worth saving.

The P Knob shaft is broken, there is a chip out of the Panel ( lower right ) and both Terminal
Knobs are missing their Bakelite covers.
Some Ranges works, other's don't.

Time to get cracking on resurrecting a Model 7 !
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Old 26th Sep 2019, 6:29 pm   #2
The Philpott
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Default Re: AVO Model 7 Restoration

Unusual for the P adjuster to have broken there, a smear of 3 in 1 on the replacement one where bakelite meets bakelite won't do any harm. Occasionally you get a meter of this vintage where the orifice is moulded/drilled on the skew which puts extra stress on 'P'. If the bakelite has swollen/distorted later on this can exert even more force and 'P' can seize up. Don't expect a donor 'P' knob to necessarily fit properly first time..

'Q' and 'R' don't seem to suffer from this- but can have issues of their own.

Dave
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Old 27th Sep 2019, 7:59 pm   #3
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Default Re: AVO Model 7 Restoration

Superscope,

Strangely there does not seem to have been a service manual for the Model 7. The key service manual for Avometers before the Model 8 Mk IV seems to have been for the Model 8; even the Model 40 was essentially a supplement to that.

There were however a series of "Recommended Spares Lists" for a range of meters which went some way to given full service information. The model 7 version is attached.

I hope this helps.

PMM
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Old 29th Sep 2019, 1:01 pm   #4
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Default Re: AVO Model 7 Restoration

Dave,

The P Knob Shaft had for sure been whacked whilst the Back's been off.
Because the Shaft has one of the Battery Spikes on it, it sticks up a bit, but it
is still unusual that this particular Spike to be broken off since it's in the Middle of the other Two.

I have tried to Pin and Glue it, but unfortunately that has failed miserably.
Works fine until the Brass Spike is screwed down, then the pressure is to much on the repair.

The Shaft rotates very freely in this one, which is a pity since it now needs to be replaced.

I will carry on and repair the rest of the Meter, but I will have to find a replacement P Knob from
somewhere before the Ohms section can be fixed.

Currently, all DC Ranges work but are around 5% High, but no AC Ranges work, which I guess might be to do
with not having any Batteries connected.


PMM

Thanks for uploading the Spares List, I've not seen that document before.

Not much new information in there, but I still found it quite interesting to read.

Interesting to see the mention of Six different Scaleplates for the Model 7.
Never really thought about the Model 7 in that regard before, but I assume the
same will be true of the Model 40 as well.

Ian
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Old 29th Sep 2019, 2:05 pm   #5
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Default Re: AVO Model 7 Restoration

Quote:
Originally Posted by Superscope View Post
Currently, all DC Ranges work but are around 5% High, but no AC Ranges work, which I guess might be to do
with not having any Batteries connected.
I thought that the batteries in AVOmeters were only for the "Ohms" ranges. I might have suspected that failure of the AC ranges would be down to a problem with the rectifier, but I know very little about the innards of AVOmeters , especially the Model 7.

Colin.
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Old 29th Sep 2019, 2:46 pm   #6
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Default Re: AVO Model 7 Restoration

I've got a 1955 Model 7 that I bought for 1 at a local junk auction maybe a couple of years ago and it's got a home made 'P' knob. I've never actually opened it up, so no idea what it's like on the other side of that knob. It worked ok and is only slightly inaccurate. It was disgustingly filthy when I got it and just needed a really good clean. It's showing a reading in the first picture below as it's in use at the moment monitoring the receive and transmit current on a radio transceiver under test. The last picture shows a much earlier Model 7 like yours with the little disc on the pointer/needle. This meter dates from 1941 and I've had it for decades and made it up with shunt and multiplier parts from several scrap ones of the same age that were throw outs from work back in the day. The remains of the scrap ones that I cannibalised for parts seem to have got lost over the years, so no spares anymore unfortunately.
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Old 29th Sep 2019, 2:50 pm   #7
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Default Re: AVO Model 7 Restoration

None of the "Traditional" Avometers, except the Model 7, needs a battery for the voltage and current ranges. The AC ranges of the Model 7 are an exception as the 1.5V cell provides a slight bias to the rectifier. See page 611 of the attachment which is an extract from "The Services Textbook of Radio, Volume 1". AC ranges will however work without the cell, but the low end readings will be inaccurate.

Since none of the AC ranges works, the most likely faults are a defective rectifier or the contacts of the cam operated switch being out of adjustment. Four germanium diodes, OA91 or OA96, for example will satisfactorily replace the original copper oxide instrument rectifier. The braided leads of the copper oxide rectifier should be considered integral to it. It is best not to unsolder them at the rectifier end as excessive heat can affect to copper oxide elements.

PMM
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Old 29th Sep 2019, 2:55 pm   #8
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Default Re: AVO Model 7 Restoration

Hi Colin,

Normally yes, the Batteries are only used for the Resistance Ranges on a Model 8.
The Model 7 is a bit of an oddity from what I understand in that it uses a DC Bias for the AC ranges.
I've never looked into this in any great detail, so I don't actually know yet if the
AC Ranges should work but inaccurately or don't work at all without a Battery.

Hopefully I will have time later to test a working Model 7 to see what should be normal behavior.
Unless somebody more knowledgeable beats me to it.
This will be the first Model 7 I've restored, so still plenty to learn.

Of course it could also be the Rectifier, Transformer or any number of Switches or Resistors,
but I haven't got that far yet, as I've been tied up trying to repair the broken Potentiometer.



Ian
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Old 29th Sep 2019, 3:15 pm   #9
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Default Re: AVO Model 7 Restoration

Thanks PMM,

Looks like our replies crossed.

Thanks for the update.

At least I don't have to muck about testing another Model 7.

I can understand why AVO designed the 7 that way, but it seems a
bit risky (back in the Day) that the AC Ranges will work when there
might be a Bias Fault effecting accuracy.

Something very few would even have noticed I think.
How inaccurate does the 7 get on low ranges without the Bias?
Something else to experiment with.



Ian
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Old 29th Sep 2019, 8:15 pm   #10
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Default Re: AVO Model 7 Restoration

- I can't experiment with my Model 7 with and without 1.5v cell as it's too early to have the bias for low voltage ACV. Out of interest i will compare it to one or two digital ones on the 10v AC range and see how good it is (I predict error of maybe 4 to 8% of the reading, but let's see.)

I too have tried gluing as per post No.4, but it never seems to work in these stressed applications.

I appreciate that people are restoring Model 40's and 7's, they are a bit vulnerable to stylised vandalism when advertised on the open market at the moment!

Dave
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Old 14th Oct 2019, 3:36 pm   #11
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Default Re: AVO Model 7 Restoration

So was that just a co-incidence?

Having never seen or heard about a Model 7 Service Manual even existing,
within Three weeks of asking one turns up on Ebay!

I think we can all now comfortably assume that a Service Manual
will probably exist (or at least existed) for the Model 40 as well.

Maybe the same Genie can magic up a Model 40 Service Manual during the next Three Weeks!
No harm in asking!

Hopefully, since the Manual fetched a decent price, it might encourage any other surviving examples to come to light.


Ian
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Old 14th Oct 2019, 5:08 pm   #12
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Default Re: AVO Model 7 Restoration

Ian,

When I mentioned the Model 40 manual higher up this thread, I should have made it clearer that I have a copy which I have scanned.

If you send me a private message I can email a copy to you if you can accept a 14 Mb attachment, otherwise I'll try to put it in a Dropbox file for you.

PMM
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Old 2nd Nov 2019, 1:06 pm   #13
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Default Re: AVO Model 7 Restoration

Not had a lot of time to work on the poor old Model 7 recently, but have finally
managed to come up with a reliable repair method of the broken Bakelite P Control Shaft.

Absolutely fine in principle, but not the greatest execution on this occasion.

There is only one thing worse than not having a Modellers Lathe for this type of work!
and that is having a Crap one where the Chuck continually comes lose and everything moves about!

Small Hole drilled down centre of surviving Shaft. Brass Insert screwed into Hole and
Araldite'd in place.

The Knob is back in the 7 and it works fine, although the alignment is far from
perfect on the Battery Spike, but the Battery Box Blade is wide enough to
accommodate this.

I know not many will want to go to this effort, but I had no access
to any suitable Spare parts, so needs must!


Ian
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Old 3rd Nov 2019, 6:46 pm   #14
The Philpott
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Default Re: AVO Model 7 Restoration

A noble effort. I have seen one or two that were that much out of line from new, because the locking pin had been drilled at the wrong angle.

Dave
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Old 17th Nov 2019, 11:46 am   #15
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Default Re: AVO Model 7 Restoration

Does anybody know if there is a Written specification for the
Temperature Compensation found on the early Model 7's ?

I have found the following after testing the one on my 1946 Vintage
Model 7.

The temperature Compensator was measured as 2.5 Ohms and this is in
addition to and in series with the Swamp and Movement.

The Compensator seems to have a Range of around 22 Degrees C.
I tested mine from 3.5 Deg C up to 26 Deg C.
3.5 deg C equates to the full 2.5 Ohms.
26 deg C equates to Zero Ohms.

So the question really is, at what point should the Compensator be set?

At what Temperature is it supposed to start reducing it's resistance from maximum and
at what Temperature should it reach Zero Ohms?



Ian
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Old 18th Nov 2019, 2:40 pm   #16
The Philpott
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Default Re: AVO Model 7 Restoration

My compensator came out at 2.4 ohms, very close to your 2.5 ohms. (I didn't use a very exotic meter to check this)

Best thing is to refer to the instruction plate on the early meter, which gives a clue that:

Full resistance is used @ 40F (4.5C)
Zero resistance is used @ 90F (32C)

Dave

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Old 18th Nov 2019, 11:02 pm   #17
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Default Re: AVO Model 7 Restoration

I have one(1952 I think) which I use on the occasional REALLY old radio that the service data specifies a 1000 opv meter. It makes the maths easier than trying to work out any differences caused by a DVM or AVO8 etc. It's actually pretty accurate.
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Old 19th Nov 2019, 6:03 am   #18
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Default Re: AVO Model 7 Restoration

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)
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Old 19th Nov 2019, 11:22 pm   #19
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Default Re: AVO Model 7 Restoration

Dave,

Thanks for your Compensator Data.
Very similar to mine.
I rounded up a bit, I think mine measured out at the high end of 2.4 Ohms around 2.46 as I remember, so they are pretty close.

By tweaking the Coil Mount the Max and Min can be adjusted both upwards or both downwards, hence I was interested to know where Avo would have set it to start with?

I had to remove the Coil to repair the Resistance Wire, hence it might not be exactly where
it should be, but it matches yours quite well.

When I get the old Girl back up and running, I will carry out some more detailed Temperature Checks.
The Model 7 was certainly a sophisticated piece of Kit at the time.

Does anybody know when the Temperature Compensator was dropped from the Model 7 ?


Tim,

Does your 1952 version have the Temperature Compensator do you know?

Ian
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Old 19th Nov 2019, 11:46 pm   #20
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Default Re: AVO Model 7 Restoration

The bi-metallic spiral temperature compensator seems to have been eliminated when the block magnet movement replaced the horseshoe type. This would imply that some characteristic of the block magnet design was less susceptible to thermal variations, but I don't know which characteristic had the dominant temperature coefficient.

The change in movement type was some time during 1947.

PMM
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