Avometer Heavy Duty High Sensitivity Setting

[pthomas]

Hello, I recently acquired and restored one of these. Its a very nice meter and a lot smaller than the other models. I have a question about the 10ma = 100mV high sensitivity position. This implies a 10ohm resistance on the meter. According to my maths based on the attached (previously on this forum) diagram I get around 14 ohms. The instructions imply that this is for use with current shunts. Does anyone know more about this?

[Phil G4SPZ]

Your 14 ohms is correct, and I think the “10mA/100mV” is possibly a “nominal” value.

Consider 10mA flowing through the main terminals. This creates a voltage drop of 140mA across the 14 ohms. The current splits in the ratio of 1:14, with a nominal 700uA flowing through the movement and 9.3mA flowing through the shunt.

Now consider 100mV applied to the terminals. This drives around 7mA into the meter, which splits in the same ratio of 1:14, with a nominal 500uA flowing through the movement and 6.5mA going through the shunt.

So 10mA and 100mV would appear at different places on the scaleplate. FSD actually corresponds to 9mA or 127mV; do the scales suggest this? Or possibly the error is taken account of in the design of the external accessories. That’s my take on it, but I don’t own a HD Avometer so I’m open to correction!

[Superscope]

Never really thought about this conumdrum before!

So I got the Calculator out.

First problem for me was, I can't find a value for RV1 anywhere.
So had to work it out.

So if my calculations are correct, RV1 is 66.66 Ohms.
R11 is shorted out when selecting 10mA=100mV.

So total resistance between the Terminals is 13.7 Ohms.
Which is 15 Ohms ( internal Shunt Chain) in parallel with
the Movement/Shunt and RV1 which = 166.66 Ohms.

When fed with 100mV approx 7.3mA flows.
That is split approx 11.1:1 so 600uA flows through the Movement
and 6.65mA approx flows through the Shunt Chain.

The External Shunts as far as I know are 100mV at Max Current,
and yes, the user Manual states that the 10mA=100mV needs
to be selected when using external Shunts.

Basically, thats the long winded way of saying what Phil said.





Ian

[Phil G4SPZ]

Originally Posted by Superscope

Basically, thats the long winded way of saying what Phil said.

No, Ian, you went one better and got the sums right! I made the mistake of thinking that the movement and swamp (not shown…) was 100 ohms, then I added the 45 ohms. As as result, as you correctly worked out, the current splits in the ratio of 1:11, rather than the 1:14.1.

In my defence, I was taking RV1 to be 50 ohms as per the circuit in the original posting, but with the 16.7 ohms in series the figures work out the same anyway.

[Superscope]

Phil,

I have no idea what value RV1 really is, as it isn't shown in any of
my Schematics.

My calculations suggest it is 66.66 Ohms and everything seems to
work with that Value.
Maybe if somebody actually knows, they can jump in.

I didn't notice RV1 labeled on the original post.
None of my Schematics show that.

I will have to run the Math again!

Ian

[Phil G4SPZ]

Does your schematic show the separate 16.7 ohm resistor at the right-hand end of the shunt chain, as in the O.P?

[Superscope]

The Schematic I'm looking at, is from the Mk5 Operating Instructions.

R10, which is what I think you are refering too, is 26.7 Ohms in my version.

So there are clearly some changes going on in the Heavy Duty design.



Ian

[Phil G4SPZ]

As long as VR1 and R10 together equal 66.66 ohms, in any combination, the sums still work.

Are there many other differences between the two versions of the schematic?

[Superscope]

The only other difference I can see, is in the original post, R18
seems to have two values: 198 Ohms and 207 Ohms.

In my one, R18 is only 198 Ohms.




Ian

[Phil G4SPZ]

Perhaps R18 was selected on test to match the individual rectifier fitted?

[Superscope]

In Summary then, the answer to the original question in post #1 is:

The 10mA=100mV Label is refering to the fact that the 10mA Range has
changed to a 100mV Range instead, essentially for use with external Shunts.

It is not indicating the electrical characteristics (Resistance) of the Meter, so
the 10 Ohms calculated from the V&I values, is a Red Herring.





Ian

[Phil G4SPZ]

I’ve just checked the figures. With the sensitivity switch in the ‘normal’ position, and the range switch set to 10mA DC, 600uA flows through the movement and 9.4mA is bypassed through the shunt. The voltage across the instrument is 141mV at FSD.

With the sensitivity switch in the - rather misleadingly named - ‘10mA=100mV’ position, the 10mA range becomes a 100mV DC voltage range with a sensitivity of about 140 ohms per volt.

If inadvertently left in the ‘10mA=100mV’ position, this would give rise to errors on the other DC current ranges too.

[pthomas]

Interesting. Here is another twist. I have a much older Avometer HD which is an abandonded restoration project. The current shunt is 115 ohms and the ohms potentiometer is 35.5 ohms compared to 50 ohms and 100 ohms for my more recent fully restored version. Attached are pictures plus a marked up diagram. As you can see the ohms potentiometer and control is missing. It seems impossible to restore this due to its rareity and complex construction plus the movement reset (which works) is no longer connected to its front panel control. The movement is fine since I removed it from the chassis and did a current FSD test. It is best described as work in abeyance so I have no plans for it at the moment.

[Phil G4SPZ]

Mmm, it is a bit cramped in there. I’m glad I haven’t got one!

[Superscope]

That HD appears to be quite different from the very early 1948 Meters, so
I assume it's a Mk 2, based purely on the assumption that the 1948 Model
must be the Mk 1.

I never really paid the Heavy Duty Avometer any attention before.
The one I had, I gave away to a Railway enthusiast.

Regarding restoring a rare Meter with missing parts, the first thing I try to do,
is locate a photo of the original part if possible.
I can then determine if it is possible to make a repro part or not.

Could a spare pot from a later HD be made to work?
Or anything else from the Avo parts Bin?

The Control Knob on the outside could always be re-cast in Resin once you've
tracked down one to copy.

A few Forum members have HD's from around this Time period.





Ian

[pthomas]

Yes, It really only needs the knob and spindle. I think any of the other models like 7 and 40 where there is an open potentiometer could be made to fit. I am reluctant to put more into this one since it is very difficult to work on. I have the wiper just the knob and spindle is missing from the ohms zero function. Judging by the serial number this is a 1952 model. Also having a metal case where the range is upto 1000V is not a good idea!

[pthomas]

I noticed that the HD models have a double wound transformer for the AC ranges with a separate secondary to feed the rectifier which differs from the model 8.