AVO MULTIMINOR MK5 – Ohms Zero Adjust & Accuracy problems

[Johnmark]

Having read some great posts about AVO Multiminor MK5 issues I have progressed in checking my recent buy! The unit is in good condition considering its age. All original save for replacement of R11, R13 and R17. The main problems are Ohms Zero adjust RV1 does not correct over FSD on either X1 and X100. All ranges work but meter generally reads high.
I have cleaned the battery terminals and done some checking of resistors with a DVM. Although I have the circuit schematic, I don’t know what is acceptable in resistor tolerances and the effect on meter accuracy.
R11 should be 113R whereas the replacement is 109R6.
R13 should be 10R4 whereas the replacement is 10R1.
R17 should be 2R2 whereas the replacement is 2R3
With black meter wire disconnected and DCV selected R19 + meter coil should be 1667R whereas it is 1677R some 10ohms high.
I have now changed R11 for 150R which allows x1 range to zero set but not the X100 range. I have now also changed R10 which should be 11k36 whereas it was 11.32k; now changed to 13.5k. This solves the Zero adjust problem on both scales but not the high meter readings.
Meter coil seems OK but have not removed yet. Left side mechanical zero set just about zero sets
The resistor modules all seem OK with no O/C’s. I’m thinking that the Swamp resistor may have gone high but not certain if the extra 10ohms for swamp + moving coil would cause high readings. Swamp reads 572R and moving coil meter 1K105 which gives 1677R.
Examples of high readings are: 8.1v reads 10vDCon 10VDC range, DC25V reads 32V on 100v range. 2VDC reads 2.5V on 2.5v range. ACV 241.6 reads 250V + FSD on 250VAC scale. DC 63.1µA reads 77µA DC 0.1mA range.

Any pointers will be most welcome.

[Johnmark]

Avominor mk5 circuit.pdf

[Superscope]

Hi John,

Sorry nobody has jumped in to try and assist you yet.

The Multiminor is one of those strange Avo Meters that
never seems to gain much interest, which is a shame as
it is really quite a good Meter.

I don't know what the tolerances are off hand for the Multiminor,
but it is a precision instrument, so it wouldn't be a surprise if your
reading are all up the swany if your resistors are way off.

I don't know if I have a parts list for a Multiminor, but I will try to check
later.
Bottom line is, all the Resistors will need to be the correct value and within
tolerance.



Ian

[Superscope]

Ok,

I only have Service Data for the Mk 4 Multiminor, but I suspect the details
are very similar.
A warning through, it never pays to assumes anything with Avo meters!

The Mk4 states, pretty much most of the Resistors are +- 1%

So, R11 is around 3% low.
R13 is around 3% Low
R17 is around 3% Low

Assuming I did the Math in my Head correctly!

Another word of warning, changing a Resistor to a different Value in order to get one
function working right, will almost certainly change several others and make the fault finding
very difficult.


Now, I always advise checking the FSD Movement current before undertaking any
other analysis.
The FSD should be 60uA (100mV)
Check this first!

Actually, No, check that the equipment you are using to measure the Multiminor
is accurate first, then move on to the movement.

If that is all Ok, a clue, might be in your Current measurements on the 0.1mA Range.

If 63uA reads 77uA that is around 20% High, so what this suggests to me, is
that if the Swamp + Coil are 10 High, I would expect all readings to be slightly Low.
Since they are not, it looks like the combined Shunt Chain of Resistors across the movement are actually high, meaning they are not taking as much current as they should
be, and so the Movement takes the extra current and over reads as a result.

Try disconnecting the Shunt Chain and measuring the combined resistance.



Ian

[Superscope]

In fact, thinking about it, 20% high would suggest the Shunt Chain of Resistors is OC somewhere.
The Shunt Chain is : R14, R15, R16, R17, R18 and not forgetting RV1.

If there is no Shunt resistance across the movement, it will read around 20% High.

Once that is sorted, put R10 and R11 back to the values they should be.

Not being able to Zero, could also be because the movement is over sensitive.


Ian

[Bazz4CQJ]

My recollection is that the MkV was intended for the Services and the MkIV for the domestic market (or other way round)(I've owned both). There are some quite significant differences. Not sure whether the attached adds anything.

B

[Johnmark]

Thanks Ian and Bazz for your valued responses.
Belatedly I uploaded the manual to help with reading my post.
Things have moved on. I have solved the zero ohms adjust by changing R’s but understand I may need to revisit if I solve the fundamental high meter readings.
The swamp plus MC was 10ohms high at 1677 so I have added a parallel resistor and the now reads closer to spec 1662R. The high readings still persist.
I am now solely concentrating on the basic meter accuracy without which other checks are not meaningful. The meter I have been checking values with is a digital AVO M2006 and at your suggestion I have also checked the current with a trusty AVO 8 and it reads the same.
So with the black meter wire disconnected and on DC 100µA range I get a reading of FSD when passing 60µA . With the shunt R18 – R14 I get 1.703k and with RV1 + R18-R14 I get 2.496k which is close to the circuit value of 2.5k. RV1 is 797R rather than 800R.
With black meter wire connected and shunt in circuit 63µA reads 76µA on DC 100µA range.
Shunt seems reasonably close and meter + swamp shows basic meter accuracy. Add the two together and typically 20% over reading. I am totally puzzled.

[Superscope]

Hi John,

I think you definately have an OC (or High Resistance) connection
somewhere such that the Chain of Shunt Resistors is not actually
shunting the current presented to the movement.

Try re-freshing all the relevant solder joints.

If possible, check the current flow through the Movement and the
Shunt Chain at the same time.

When set on 0.1mA Range, and fed with 0.1mA
You would expect to see 60uA through the Movement and
40uA through the Chain of Shunt Resistors.



Ian

[WME_bill]

MultiMinor Mk5.
I am intrugued. Your testing is of each component separately but not the whole.
I agree with Ian Superscope, check out the meter first.
Have you measured the meter + swamp R19 at FSD which should be 60ua (measured with Avo8) and 100mV ( measure with your digital) at same time.
Then with the universal shunt connected (R18 to R14 +RV1). You can do this without unsoldering by switched to 100ua DC. This should give FSD with 100ua and 100mV.
If it does not, then the universal shunt connections are awry, or you have a poor switch contact Negative to 100ua.

Having written this, I see Ian Superscope is suggesting similar.
I had similar trouble with my Mark1 about 60 years ago - that was a dry joint.
wme_bill

[Johnmark]

Thanks again Ian and Bill for your value advice.
I have had another good round in the ring with the MK5. I have even invested in a very cheap digital multimeter to improve the testing.(I have checked to accuracy and its ok )
I have noticed a strange characteristic on the value of MC + Swamp. At the start of the round of testing its always low about 1642ohms but over 30 mins of testing rises to 1687ohms higher than the circuit value of 1667ohms. I have measured to MC and swamp which are 1142ohms and 545ohms respectively. It’s the MC that increases in value from to 1108 to 1142ohms.
I understand the ratio of current sharing between the Swamp + MC and the shut chain R18 – R14 + RV1 a ratio of 1:1.5 . My R18 – R14 + RV1 is 2497.8 Ohms close to the circuit value of 2500 ohms.
So on 100µA range with a separate digital meter in each leg I get 61.7µA through swamp + MC and 37.8µA through the shunt chain. I am producing the DC current with a PP9 cell and multiturn pot in series allowing me to accurately set 100µA. However the meter read FSD ++ i.e just a bit more than FSD. To get FSD I only need 80.5µA and that reads 80.4mV at the terminals.
I have checked the resistance from -ve terminal to each shunt resistor and there are no high resistance connections. I have desoldered and resoldered all connections in the shunt chain and MC + Swamp.
Could the MC + Swamp value of 1687ohms i.e 1.2% be the cause?
Any ideas welcome.
John

[Bazz4CQJ]

What does the swamp resistor look like? It's possible that I have one.

Also, re the use of your DMM, be cautious if you try to use that for measuring a wire-wound resistor as the inductance may influence the reading.

B

[Johnmark]

Just wanted to thank all who replied to my post. I have concluded that an uncertain history and some of the components in particular the MC & Swamp resistor have values out of spec. I now have a meter that works on all ranges but AC Volts read low. I can live with this and thanks to your help, one more instrument has been saved from the skip. John