avometer 8 reads high

[M3VUV51]

Hi all,i have an avo meter 8 mk1 this came to me with the meter coil o/c,i have replaced the meter with a good working one but it seems to read about %30 high on the dc volts ranges,any idea where to start looking,all resistors and shunts look fine,the values compair well to a good working unit i have,i have tried moving the magnetic shunt and swapped the shunt resistor/bobbin in series with the movement but still the high readings,thanks in advance,m3vuv.

[The Philpott]

Very likely to be misalignment or wear within the leafswitch stacks which sit behind the rotary selector switches. 30% high sounds like a classic case of this. With age sometimes a contact comes to be closed when it should be open, or vice versa. They can be carefully adjusted (usually) but first try cleaning with a piece of absorbent card and contact cleaner. The type of card in the middle of a loo roll is a suitable texture for cleaning- dry, then wet, then dry again.

Dave

[6SN7WGTB]

I might be talking nonsense here, but were some AVO 8 movements 37.5µA and some 50µA?

Only ask as 50/37.5 = 33% over...

[The Philpott]

The 37.5uA movement on such a Model 8 is shunted up to a round number of 50uA- this being the lowest DC current range.

[The Philpott]

Damn- In post No.2 i may well be talking out of my eras as these switching issues (in my experience) lead to a LOW reading not a high one. Sorry.
Dave

[pmmunro]

M3,

As you don't say, can I ask if you've checked the basic sensitivity of the movement, which shold be 37.5 microamps? That will tell you if the problem relates to the movement or the shunts/multipliers.

PMM

[M3VUV51]

Originally Posted by The Philpott

The 37.5uA movement on such a Model 8 is shunted up to a round number of 50uA- this being the lowest DC current range.

You may have a point there as the movement was out of another meter, can I measure the fsd current using another working accurate Mk8?

[The Philpott]

As you have quite a sizeable discrepancy of 30%, there's a quick way of doing a rough check on the DC 50uA range without any eqpt. apart from a PP3 battery (or two) and your other Model 8.

Running the current from the battery through your body (eg from hand to hand) will deliver enough current to deflect both Model 8 meters- which you will have wired in series. If you have sweaty palms start on the 250uA range just in case- if you have dry hands you might need the second PP3 to give more push from the voltage. You can vary the current quite smoothly by varying the pressure and/or contact area, the Holdtite croc clips will give higher current, small probes much less.

I've just tried this with one meter, it should work with two so you can run them up and down their scales and compare them.

Needless to say if you get it wrong and connect the battery direct to the meter/s bad things can happen!

Dave

[M3VUV51]

i have a homebrew esr meter with a 50uA movement,i suppose if i zero that with the probes shorted for fsd then feed the avo movement with the esr meter feed without adjusting the esr meter,that should give me a good idea,also should i include the meter swamp resistor in circuit or feed the meter directly?

[M3VUV51]

well i found the bobbin no 10 an the pic way off so substituted it for a 10 turn 500 ohm trimer in series with a 300 ohm fixed resisistor,hasnt made a jot of difference no matter where i set the trimmer,can someone tell me what to check on the schematic? attached and where it is on the pic attached,i have cleaned all the switch contacts and checked the cam alignments,they are spot on.,thanks in advance.

[M3VUV51]

are the mk1 and 2 the same electricaly ?,i have a good working mk2,just wondering if i can compair resistor/shunt values between the good unit and the one reading high?

[M3VUV51]

Evening all,i have just checked the meter in series with my good avo 8 meters in series with a pp3 battery as suggested earlyer in this post,both read the same on the 50ua scale when i touch the pp3,ie they both track the same with the same finger presure on the
pp3 battery,infact i did it with both of my good meters in series with the iffy one as well and all read the same,so guess its the correct ua fsd movement!,best regards Paul m3vuv.

[M3VUV51]

well i think i am getting somewhere,one of the wire wound shunts measures 11kohm+,it should be 1200 ohm,although it physicaly looks fine,wierd.

[The Philpott]

Unusual but not unheard of. Particularly with bobbin or board wound shunts made of very thin wire, corrosion sets in at a particular point (usually hidden from view) and they go high resistance rather than simply open circuit. You can even get a diode or semiconductor effect, where the apparent resistance varies according to which way the test current is flowing. This could i suppose be due to the older types of fabric (cotton?) insulation becoming loaded with microscopic layers of the aforementioned corrosion.

Dave

[M3VUV51]

given up on this at the moment as life has got in the way.

[The Philpott]

-There's no dishonour in shelving things for a rainier day. (just need to cover or box up to keep the dust out- once it's in you can't get it all out!)
Dave

30% sounds a lot like 50uA vs 37.5uA, perhaps the shunt is open circuit.

[Techman]

Originally Posted by M3VUV51

Hi all,i have an avo meter 8 mk1 this came to me with the meter coil o/c,i have replaced the meter with a good working one

These are known for this fault - it's a bit of a weakness with them unfortunately. Quite a few got scrapped where I once worked due to this fault - and once new replacement movements became no longer available.

I didn't think you could use a movement from another meter that wasn't exactly the same model and mark, but I could be wrong on that one.

[Superscope]

You can pretty much swap any Movement between any Model 8
Mk I to Mk IV if you wanted too,
and between any Mk V to Mk VII.


However, the Scaleplate and Series Shunt Resistor must
all remain together with any Movement used for it to have any chance of accuracy.

So basically you must replace the entire Movement/Scaleplate/Shunt and all
from the Donor Meter.



Ian

[M3VUV51]

thats what i did,i have just found wirewound resistors 2,3 and 4 o/c,can these be replaced with 10 turn trim pots?