Avo model 40

[sparkymike]

I have here an Avo meter model 40 and have a couple of queries.
On removing the meter from its metal clad case, I noticed in the bottom of the box, a 6BA nut and a small brass plate around 10mm long, with two holes in it.
One of the holes is 6 BA and the other is a plain hole Do they belong in this meter, and if so where.
There is what looks like a wire wound metal cased resistor across the relay terminals. This has opened up so, that the coil can be partly seen inside it, so needs changing.
What value should this be and rating ?
Apart from the above and some verdigris covered battery terminals, the insides look cleaner than any other Avo's I have seen. Maybe these were sealed better than the normal ones. There is still grease on the inside walls of the inner casing.
Mike.

[Barnmead]

Little plate to take the screw for battery box ?

[pmmunro]

Some photographs would help. There is no relay in an Avometer Model 40 so if we could see the component that you are refering to, we can identify it.

As always, if you need to do any work on your meter, a service manual is very much worth having, and readily available.

PMM

[The Philpott]

Almost certainly the part identified by Richard, if the brass plate has rounded ends. Sounds like you have a screw missing, which someone removed, allowing the nut and plate to fall in.

You refer to a relay....but these Avo's don't have a relay in them. (Pic?)

Dave

[sparkymike]

Not relay, wafer switch. The component in question is seen on bottom right hand corner of photo.( diode ?) Had problem with camera, but ok now.
Mike.

[The Philpott]

It's a Westector, Mike, an early type of diode. Possible that checking the accuracy of the AC ranges would tell you whether it's any good...? A germanium diode would be a suitable replacement (Germaniums are sensitive to heat so special care when soldering them in..and quite easy to solder them in the wrong way round.)

Dave

[sparkymike]

Hi Dave,
never heard of one of them !! Any idea of its purpose. Would a modern diode work instead ?
Or is it a sort of double purpose type of diode ?
I had at first thought it was a diode, but the heavy coil inside made me think resistor. However the red line on one end should have been a clue to my ageing mind !!
Mike.

[sparkymike]

Now removed the diode and I see it is marked "SA1" Was that an Avo id.?
Mike.

[The Philpott]

No, i think SA1 is the model/rating of the Westector. (I hadn't heard of them either until i found a couple of lazy ones in an AVO)

My 1948 AVO had two of them, acting as the rectifier for the AC readings. Their accuracy was marginal at 240vAC and very poor at 14vAC. I replaced them with a couple of Germanium diodes (cheap, from the usual sources) and got a good result. The meter they are in is open at the moment so i could get a photo later. (Notwithstanding that it is a totally different model.) I think they were 1N34 Germanium diodes.

If you search on Westector on here you will likely find the discussion about my old AVO.

The first time i soldered Germaniums in i got them back to front (and got reverse deflection from the needle) Whoops.

Dave

[sparkymike]

Now found the position for that small brass plate. It locates on the inside of the metal casing with a 6BA c/snk screw and provides a tapped hole for the bakelite
probe carrier when fitted on the end of the box.
Mike.

[The Philpott]

I see what you mean about the leaf switches looking like relays; in fact the inside of an AVO does have a similar feel to an old-school telephone exchange!

Germanium diodes pictured. I didn't have a hands-free shunt so a second bod had to hold the leads with pliers to carry soldering heat away.

Dave

[The Philpott]

Looking at your pics on the survey, the meter already has a copper oxide rectifier (short black cylinder with 4 connections)...and this westector is separate from that. Perhaps a modern little silicon diode would be ok to replace it. Others will know no doubt.

A very clean looking meter inside, good purchase.

D

[pmmunro]

Mike,

From your descriptions, it doesn't seem that you have a circuit diagram.

In the attached diagram you will see that there is an option to have a diode connected directly across the moving coil. This may be the diode which is connected across the leaf switch contacts in your meter, it just happens to be a convenient place to locate it.

The purpose of this diode is to protect the moving coil from a heavy alternating current overload while set to a direct current range. Without the diode, the moving coil has too slow a response to show any deflection despite a heavy alternating current flowing through it. For this reason the coil could be destroyed in such conditions.

In normal working conditions, the diode will not conduct to any significant extent but in the overload condition described above it will on alternative half cycles. The other half cycles of the waveform will then apply a considerable torque on the moving coil and the resulting pointer deflection will be sufficient to trip the mechanical cut-out.

The significant characteristic of the diode in this application is that it does not allow sufficient current to flow to affect the reading at any votage less than that required for full scale deflection. The Model 40 needs 90mV and 3mA at the moving coil for fsd so the diode must not allow more than a few microamperes to flow at fsd. You will see from the Westector characteristics that at 90mV the current in the diode is negligeable.

The cut-out setting for a Model 40, depending on which data you use, would be between 12 and 20 times fsd current, so possibly 40mA. If only half cycles of an AC waveform are applied, I would expect it to require 80mA rms to trip the cut-out at the end stop. Assuming the same amplitude of current would have to flow in the Westector, or substitute diode, this is well beyond the curves shown for the original type but, I would expect, not too much for it to handle. This should also be within the capabilities of a small signal germanium diode such an OA95.

It should be remembered however that the Model 40 has an acceleration cut-out and so may be expected to trip before the pointer reaches much more than 60% full scale in response to a heavy overload.

As some versions of the Model 40 did not have this diode, it is not essential for its operation if the lesser level of overload protection is accpetable.

PMM

[sparkymike]

Hi PPM,
thanks for circuit, which I had not got. I see that the type H2 rectifier (diode ) was not fitted in meters with B in serial number. I have soldered in a more modern diode for time being.
Mike.

Hello, i think i have a spare part of that diode in my avo parts box.

If you need it, just tell me and i will try ti find it and send it for free

I found it!!

[sparkymike]

Hi Aitor,
I have sent you a PM.

I have just been checking though a box of odds and ends from my neighbour and found two very small rubber bungs with "AVO" on the end. I don't know if it is correct application, but they fit very well in the PF sockets in the above meter.
Mike.

I think that rubber with "Avo" are for model 8 mk5/6. They are used to cover the screws on the right an left corner at the bottom