AVO Multimeter survey

[sparkymike]

Photos now here at last.
Mike.

[The Philpott]

I have forgotten what the R denotes at the end of the serial number..someone will know.
What does the text on the battery cover read? I think the transformer traditionally goes in with wingnuts uppermost. (But you knew that!) Those copper oxide rectifiers are remarkably stable by and large, the 1947 one i have just used is good at mains voltage, and only loses about 300mV when shown 12vAC. The internal transformer is probably fine as well, but i find some of the low current terminations are easily knocked when working inside the meter.
Let us know how you get on, perhaps you could do a restoration thread.

Dave

[sparkymike]

Hi Dave, no text on battery cover, although it might be under a stuck on label hand written by a previous owner detailing battery use.
Now after a good clean with nothing but a damp dish cloth, the meter itself now looks pretty good.Two small splits on the wooden box have been repaired and need to buy some more Ruskins button polish.
Mike.

[sparkymike]

There is a serial number inside the case lid. 10-656-143-R.
If that is original then it is same as the meter numbers and case is original.
No idea what the nameplate numbers mean though.?
Mike.

[sparkymike]

Now had a chance to check out the meter and seems to be untouched inside and at first glance all coils look to be intact. Only minor problem I can see is that the "P" control track has lifted at one end out of its circular recess in the rear of the bakelite front panel. Can this just be secured with a spot of superglue or similar? (or was the track sprung slightly, so that it was a tight fit in the recess.?)
The pots by the way, were not stuck as first thought. The "R" can only be rotated after lifting the knob.( I had not realised that.) The "P" control was only lightly stuck.
The case has now been repaired where needed, all old varnish stripped off and now a coat of walnut stain has brought it back looking ok. Once that has dried off I will button polish it. Re. the "R" in the serial number, could this have been the team or workman who built it ? Could it have meant "reconditioned" ?
Mike.

[The Philpott]

No, i think 'R' is a variant- it probably means one or more of the internals specs are different. (knowing ACWEECO it will be a subtle change)

Pot Q can only be turned after being lifted off it's 'bed' however Pot R should turn freely through about 280 degrees (or it will keep rotating beyond this if the screw on the rear is slightly loose)

I did have a P pot winding that had lifted slightly out of it's groove, you are correct i think they were just shoved in and relied upon being slightly springy to retain them. When i had this issue i put a couple of little blobs of araldite where the winding meets the bakelite, which held it in well. (Not superglue, as it's a brilliant insulator and through low surface tension it creeps across surfaces and into crevices way too much)

Dave

[sparkymike]

Hi Dave,
you are correct. I made a mistake re. R and Q pot. The "Q" pot has to be lifted.
Mike.

[The Philpott]

Mike, i was working on a Model 40 earlier and it occurs to me to say at this point in your resto. that the 'divide by two' button (although having only 2 moving parts) can over the years lose a bit of it's springiness. Before giving a meter with a DB2 facility a clean bill of health i repeatedly check it's operation as any issues can only be remedied with the movement out of the way! I have had 2 occasions where the spring leaf has had to have it's slight bend increased so that the meter can return properly to it's ordinary ranges. If it's working ok it can likely be left alone but if the movement ever has to come out the DB2 operation should be checked beforehand. (in my opinion)

Generally if sporadic/erratic readings should ever present themselves on these meters the first thing to jiggle is the Q pot. If it's not operating properly everything else goes to pot (pun intended)

The serial number on the box, as you surmised, indicates the box originally held another meter- mine has the same discrepancy. 48A is the Admiralty Pattern Number. The senior service were very particular about giving every last assembly a pattern number, be it a pulley, a meter or a morse key. Now if we could find the meter that was ORIGINALLY in your box that would be a coup- but i guess we have to be resigned to the fact that it's either under the ocean or in tiny pieces under 20 feet of soil!

Dave

[sparkymike]

One other point Dave and thanks for above hints. I will take them on board although I do not intend to remove the movement if I can avoid it !!
Do you know what type of material was used for the box padding strips ? Was it just a white felt ?
Mike.

[sparkymike]

Jumping the gun a bit, I just found some 3/4mm thick felt in natural ecru colour,
on that well known auction site, that may well fit the bill, going by photos I have seen of pristine examples.
Mike

[The Philpott]

I have seen black, green, or off-white felt used. At least with thin stuff you can sandwich bits together to bulk it out if needed. One of those wooden boxes was coming apart so badly (and the top was soaked through in machine oil) that i ended up dismantling it totally, soaking the oily bit in petrol for days, then drying, gluing and reassembling with marconi s/s hinges. I did find the occasional steel pin through the comb jointing that tried to ruin the project.

I think i mentioned in the 'where to get parts' section the online store where i got 55 inch leather blanks for cutting out new handles. They're never going to exactly match the originals but they are strong.

Do you have a precision reference voltage source Mike? I have an imported 2.5/5/7.5/10v unit which would let you calibrate the magnetic flux at room temp. using the 12vDC and 6vDC ranges.

Dave

[Barnmead]

The felt in mine is grey

[Barnmead]

New acquisition an 8 Mk3 22208-865 for the database
movement OK
problems so far:
red terminal cap and back panel instructions missing
contacts across ohms pot not closing when they should
a 5.6K 10% wired in series with meter movement - WHY?
Bent the former
Removed the latter

[sparkymike]

Hi Dave,
I do have a Farnell stabilised DC voltage supply unit which should be reasonable accurate I would think. Can you explain in more detail re. mag.flux ?
I just noticed three hairline cracks about 1" long radiating from one of the leather handle rivets. I did not notice them on outside of case, so as that is on the end of the meter where there there is plenty of clearance, I will just laminate a small piece of Paxolin to the inside wall, making sure I drill a hole to take the rounded end of the rivet which will also enable any future owner to remove rivet if needed.
Battery post and fork now removed and all verdigris removed. The post did not want to come out ,but a fair pressure on the thread end inside the box removed it.
Mike.

[The Philpott]

If the horseshoe type magnet has deteriorated over the decades by, say, 1%, all the voltage and current readings given by the meter will be slightly low. it is generally possible to modify the magnetic field by swinging the shunt arm around- up to 45 degrees away from it's minimum position. Any more than 45 degrees and the effect reduces. If not enough increase is available it is possible to experiment by removing this arm altogether.

Dave

[sparkymike]

Hi Dave, thanks for that. One other small problem is that the 6BA case screws all seem a bit loose. A common problem I know, but is there a good solution ? Longer screws are out of the question, as depth of holes will not allow that. I did wonder if a liquid engineering flller would do the job if the screws are reinserted after coating them with vaseline or similar before the stuff sets. I have some of that filler that can be used to build up worn parts etc.
Mike.

[Sinewave]

Here's a new one for me. This is a first of its type into my collection. I'm not entirely sure which model this is, as this type of battery terminal box is usually model 2-6?

The movement needle is bent which is fine, but the scale plate has written in it Hendon Technical Institute, so it's an Avo with some provenance. It also has a fuse in the place of the mechanical cut out.

The serial is No.69-8608.

[The Philpott]

I suggest a very late Model 5..?

Dave

[The Philpott]

Mike, i wonder if Loctite thread-locker in a bottle would manage that job..especially if Vaseline was on the screw threads and you just nipped up the screw lightly as the Loctite was curing.

The magnetic shunt i referred to earlier can be thought of as a suppressor-

1) Minimum (shunt is reducing the flux as much as it can)
2) Maximum (shunt is out of the way thus reducing the flux much less)
3) Turbo setting! (magnet failing to meet it's target so shunt removed altogether-not always successful as this can raise the flux too much)

I would say 4 times out of 5 the magnets are still strong enough to achieve accuracy on a 1940's meter (some don't even need adjustment)

Dave

[Sinewave]

Originally Posted by The Philpott

I suggest a very late Model 5..?

Yes, it certainly is a 36 range as the Model 5 is. Do you say a late 5 due to the zero knobs being plastic rather than brass screws?