AVO Multimeter survey

[The Philpott]

I believe the glass bobbin comprises:

60 kohm energised on 300v range (ie 300v minus 150v)
180 kohm also energised on 750v range (ie 750v minus 300v)

I have one which is intact, but as Peter says they're mostly long since gone o/c.

Dave

[stickfly]

OK gents, Many thanks for your replies.
I'm not sure of the bobbin value as I've not had the time to trace the resistor, but will do so tomorrow.
I'll test the glass resistor again tomorrow but I definately had resistance readings this afternoon. I didn't know what readings to expect but tested to see if I got anything at all, due to the state of the resistor & the fact that I've never come across a glass resistor before. Both were in the Kilohms range so may be OK.
The K switch : It made no difference to the OHMs range, it just changed the voltage reading for my battery from 0.6v to 1.2v.
BLAST! You know I've got to take another look now.

Regards ..... Gary

Just checked the glass resistor ...... 180k + 60k so looks good.

[The Philpott]

Okay, so K operates correctly but resistance ranges kaput..The windings for 1 kohm and 10kohm ranges are on the little board.. As you'd expect, their resistances should correspond to APPROX half scale on the respective ranges. It is quite common for one or more of them to have become cooked, or just become o/c through age,corrosion,embrittlement at the ends of the wire..all the usual gremlins.

The leaf switch stacks are quite often worn or abused and this can cause resistance ranges to fail.....in addition note that there is an extra leaf contact that closes when the 1 kohm range is selected.

As regards the little bobbin, it does not appear in either of my donor meters (and there's no hole to suggest one was ever there) so it's possible it's a factory tack-on repair from the 'fifties or a little later- a lot of them went back for repair after 10yrs service it appears-perhaps as a routine refurb. or when some were retired from the ministry (speculation)

It looks from the image that your bobbin has replaced the 150v board winding, but note that it only drops around 75v in itself...i'll get an exact value later if it's not on the schematic already provided earlier in the thread! There is certainly a winding missing from the end of the board, which supports this theory.

Dave

[sparkymike]

Had three Avo meters given to me the other day by a friend who knows I have a soft spot for them. I will take photos and post soon. Two are model 8's and the other is minor I think. Not the one with loads of probe sockets, but just two.
Mike.

[The Philpott]

I think (Peter's?) schematic posted by Phil shows 31.2 kohm which equates to 78v, so we're in the correct ballpark.

There are odd little trimmer windings along the board, either to compensate for an under-wind on the main windings, or on some meters to compensate for the difference between AC and DC measurements when low voltages are involved. For some reason these are a common point of failure, i theorise(d) that shorter board windings were more subject to mechanical stress on installation....and of course the shunts being in series, means one failure renders all ranges above it, u/s.

Note that your 7.5v AC winding is probably on a different board to the others..

Edit- I did find on a Model 7 that a failure in the transformer killed all the AC ranges...not just the AC CURRENT ranges. I don't know if the Model D is wired the same.

Dave

[sparkymike]

Photos of the three freebees.

[sparkymike]

And close up of serial number of last one in previous post.

[Sinewave]

From the serial numbers of the Model 2's, (seeing as the serial doesn't follow the date code on later meters) how certain can we be when our Model 2 meters reach 100 years this decade?

[Superscope]

In short, without a Date Code we can never be certain.

Although there are no Date Codes, the Serial Numbers are consecutive,
from the earliest Model 2 right up until the last Model 6.
So estimates can be made (Guessed).

Based on what we know, the best estimate is as described on Post # 2570

https://www.vintage-radio.net/forum/...=4343&page=129

From a Historic point of view, 1927 is probably the only Date of importance
for the series 1 Model 2 and 1930 for the second series.



Ian

[Sinewave]

Well at least it gives me some window, so at least I know it's not quite 100 years just yet.

Anyway, I've picked up another Power Factor and Wattage unit, it's for the Model 7, which I already have. I was hoping it would be for the Model 40. I don't think they ever had serial numbers on these units.

[Phil G4SPZ]

Originally Posted by Sinewave

…Power Factor and Wattage unit, it's for the Model 7…

I have one of those, too. It appears to be unused. I have not yet managed to find a use for it either!

[Sinewave]

Originally Posted by Phil G4SPZ

I have one of those, too. It appears to be unused. I have not yet managed to find a use for it either!

I might take it to work one day for a laugh. Usually I use the clamp on type for a quick rough measurement, but the Avo type needs to be placed in circuit.

In fact I'm wondering if this second unit I have, is ex Barnmead equipment? I bought it off a seller who we suspect bought a load at an auction house, then it all turned up online....

[The Philpott]

Yes, i believe it is....and the prices for that batch are descending to a more reasonable level. Lesson learned, i think.

Dave

[Sinewave]

Quite, it's at that point which I bought the unit.

[Superscope]

So, there really is a Model 7 variant of the type D.


I finally got round to looking at that usual Type D based on a Model 7 Panel, that Phil kindly donated to me a while back.

The main thing I wanted to ascertain, is whether the 1mA Movement was original to the Meter and thus making this a unique Model in it's own right.

I found the Serial Number penciled on the ends of the Horseshoe Magnet and it indeed matches the serial on the Type D Scaleplate. I had wondered
if somebody had just put a 1mA movement in and retained the Type D Scaleplate.
Evidently not, the Model 7 Movement seems original to this Meter instead of the usual 2.5mA one.

Sadly, the movement is Low Flux, it's about 20% down on sensitivity, so I won't go further on restoring this until I have worked out the best way to
fix the movement.
To date, I have had no success restoring a low flux movement to normal, but that is still a work in progress.

Due to the poor state of the meter, it will take time to work out the actual differences from a stock Type D.

What is clear though, is that AWEECO used Model 7 parts to fabricate some Air Ministry Type D Meters.
We know of at least two that survive in the Forum, so it's fair to say many must have been made this way.

Perhaps that indicates just how much pressure AWEECO were under to get Air Ministry orders out of the Door in the early 40's



Ian

[Phil G4SPZ]

Good to hear you’ve made some progress on this instrument, Ian. It came to me in that state!

It would only have made sense for Avo to do this if they could use all the standard multipliers and shunts from the Type D, so they must have adapted the 1mA movement to read 2.5mA FSD whilst exhibiting the correct overall resistance for the Type D, by inserting specially-made series and shunt resistances. I didn’t get as far as tracing out that part of the circuit, so hopefully you’ll have more persistence and success!

[pmmunro]

Ian,

The Air Ministry designation on the scale plate is 10A/10610 which I thought corresponded to the earliest Type D, similar to the 36-range Universal Avometer. These were protected by a fuse. The later Type D (10S/10610) is apparently derived from the Model 40 and has the inertia/end-stop cutout.

I only have one 10A/10610 which is not operational at present. I had assumed that it had a 2.5mA fsd movement as in the later Type D, but it might be worth checking if this is in fact the case. This could be confirmed from the values of the other circuit components.

PMM

[The Philpott]

20% down on sensitivity suggests to me the magnet might be from a bad batch..I've boosted one with a 7% shortfall using a Neodymium magnet that is APPROX 9mm diam and 4mm thickness. You would think that two magnets, one either side, would be required-but in this case i got lucky.

Dave

[Superscope]

My previous attempts at fixing Low Flux movements, have mostly centered around
trying to re-magnetize the Magnet.

To date, I have never been able to return any movement Magnet to a normal condition.

I have played around with Neodymium magnets, but again, results have been poor
together with the unwanted visual appearance.
I like repairs to be as invisible as possible.

This Morning, I was playing around with some very small Neodymium Magnets I bought
a couple of years ago, and actually had some success.

I have managed to restore the 1 mA Movement, to guess what? 1 mA.
When the movement is bolted back in to the Meter, the Neodymium Magnets should be
virtually invisible.

Points of note are:
The Magnetic Shunt has a much smaller effect, so most of the adjustment needs to be
carried out with the Neo's.

Also, I found it was important to have the Neodymium's attached in such a way that they
are magnetically attached to the ends of the Horseshoe, but they must be repelling their
neighboring Neo's. Otherwise, it seems to reduce the Flux improvement.

I haven't re-searched what's going on Magnetically here yet.

The Magnets I used are N52 (6mm x 2mm ) Discs.
4 on each side have returned the movement to functionality, although, it will need striping
down for proper cleaning before it goes back in.

I have not needed to glue these in, They are held on with their own magnetism.

Ian

[Phil G4SPZ]

Ian, that’s really helpful. I have an Air Ministry Model 7 that reads 4% low, so I’m tempted to order a few of those discs and have a bash myself!