AVO 8 Mk. V: low d.c. volts reading

[Skywave]

Reads low - only on d.c. volts.
Example: a verified 300-v.d.c. input produces scale reading of 290-v.; selected FSD of 300-v.d.c. Same error exists on all d.c.v. ranges and is proportional to the selected FSD.
I suspect a resistor, common to all d.c.v. ranges, has gone high.

There are many members here who have the experience of fixing faults like this with this model of AVO 8, whereas I don't.
Any advice will be appreciated: thank you.

Al.

[pmmunro]

Looking at the circuit diagram from the service manual, resistor R16, 58Kohms is at the bottom end of the multiplier chain and will affect all direct voltage ranges. On the 3V range, it has to drop 2.9 volts from the input terminal to give 100mV across the movement in parallel with the universal shunt. As the current for full scale deflection is 50 microamperes, the value has to be 2.9V divided by 50 microamperes, hence R16's value of 58Kohms.

However, R16 also affects the 30 to 1000V alternating voltage ranges, so may not be the cause of your fault. Selector switch contacts 1 & 4 and 2 & 3 set up the meter for DC ranges and contacts 7 & 9 are used on the direct voltage ranges. These contacts are substatial print pads on the circuit boards and the moving contacts are plated copper segments on the switch rotors so faults are unlikely. One possible cause of trouble might be corrosion of one of the print pads. This might be related to battery leakage but I am guessing here as it depends on the relative positions of the battery box and the switch contacts.

All this assumes that the meter is in original condition and that no-one has changed any component values in such a way as to appear to fix one fault but alter other range sensitivities.

As always, the basic movements characteristics need to be correct to start with, i.e 37.5 microamperes, 100mV and 2667 ohms for the movement including swamp resistor and 8000 ohms for the total resistance of R9 to R15 (the universal shunt). When switched to DC ranges, the parallel combination of the movement and universal shunt should be 2000 ohms and there should be exactly 60Kohms between the meter's terminals when switched to 3V (DC), 200K on the 10V range and so on up the multiplier chain.

PMM

[Skywave]

Thank you. A comprehensive reply which I shall use to investigate further.

Al.

[Skywave]

It has been a long time since I've looked at the insides of an AVO 8 Mk. V and as soon as I did - and saw the printed circuit wiring that AVO have chosen - my heart sank. IMHO, the mechanical assembly of the AVO 8 has always struck me as a design chosen to make maintenance as difficult as possible - and that printed circuit wiring seems to complete that idea.

But to the substance of this thread: with the meter switched to 3 v.d.c., I do get the 60 kΩ and when switched to 10 v.d.c. I also get 200 kΩ as you have stated.
The printed circuit boards do not carry any legending to identify R16 (nor any other resistors) and in one location several resistors are 'sandwiched' between two boards, making finding R16 very difficult. However, I did get a reading of 58 kΩ across one resistor in that 'sandwich', albeit with that resistor in circuit. (The construction here is such that removing one end of that R to accurately measure its value is not possible).
As for measuring the basic characteristics of the movement, I was unable to do that on account of how this meter has been assembled. Since other ranges are correct, I can only assume that the basic characteristics of the movement are in order.
As for the switch print pads, they're not bright & shiny, sure, but they don't appear to be heavily tarnished, either. Overall, their condition is as I would expect from an instrument of this age and the design AVO have chosen for those switches.

Any further thoughts / ideas, please? Thanks.

Al.

[Sinewave]

Don't be phased by the printed ribbon cable. After a while you'll see how easy it is to work on a Mark 5. Out of all of the Avo's I work on, I've found the 8 Mk 5 and onwards to be the easiest of them all to work on.

[pmmunro]

Skywave,

Do you have the service manual for the Model 8 Mark V? If not send me a private message.

PMM

[Skywave]

Originally Posted by Sinewave

Don't be phased by the printed ribbon cable.

O.K. There are some blue resistors 'sandwiched' between 2 boards. To gain access to them, the top board needs to be removed. To do that, the delicate ribbon cable will need to be removed and also what looks like a printer circuit shunt. The ribbon cable will melt with the heat of the iron; the shunt seems to require more heat than even my Metcal will produce.
Perhaps there's a technique required here that I don't know of.

Al.

[Sinewave]

Is this the board above the movement? The ribbon cable won't melt if you're careful. I've removed a few with no issues.

[Skywave]

The board is 'above' the movement in a horizontal plane, not a vertical one.

Al.

[Sinewave]

I can take a movement out leaving that board connected just by carefully folding it back and supporting it. Unless absolutely necessary I don't desolder them. Otherwise to desolder them I use flux coated desolder braid, carefully and gently.

[Skywave]

A question related to the cct. diagram of the AVO 8 Mk. V.

On the cct. diag, there are a number of switches shown - drawn as single pole, three - or more - ways, with numbers in circles allocated to each switch contact. Where are these switches? And what is the significance of those numbers?
And I don't understand the representation of (what I assume to be) the polarity reversing switch: is that five contacts that I see? Is it indeed a 'switch'?
(Generally speaking, I'm having a bit of a hard time trying to decipher this cct. diag.!)

Al.

[Sinewave]

Those switches positions with numbers in circles refer to the contacts which are made when the two range switches (DC and AC switches on the front of the meter) are in a particular position.

[Skywave]

Thanks: but I'm still confused. Each of the AC and DC switches have selectable contacts on a printed circuit board and also a rotary selector switch beneath. Presumably, the circled numbers on the cct. diag. refer to the latter switch. If that is the case, how do I identify switch contact #4, for example? I cannot see any markings on those switches to determine that - or any other - contact number.

Al.

[Sinewave]

The only way is to trace them from the circuit diagram, noting the connections to components they lead to.

[pmmunro]

The switch knobs on an Avometer front panel control what are essentially two separate but coordinated switches.

The segmented switches right next to the front panel are the range switches which select the appropriate current, voltage or resistance value relating to the switch position. Inboard of these is the function switching which determines the connections within the meter to make it act as a voltmeter, (alternating or direct), an ammeter (again alternating or direct) or an ohmmeter.

These function switches are set up according to the table at the top right hand side of the diagram to switch the meter into the mode corresponding to the range knob position.

For example, the two contact groups shown on the diagram on either side of the movement have contacts 1,4,5 & 17 and 2,3,6 & 18. Now, referring to the table, contacts 1 & 4 on the left and 2 & 3 on the right are made when any DC range or an ohms range is selected. When the meter is switched to an AC range, the function contacts change to having 1 & 5 and 2 & 6 made bringing the rectifier into circuit.

When the left hand switch is in the "OFF" position, 1 & 17 and 2 & 18 are closed puting a short circuit across the movement, damping its movement for safe transit.

PMM

[Skywave]

Thanks: that helps.

And I think I'm making some headway with this 'low d.c. volts' indication, now that I've learnt a good deal more about the AVO 8 Mk.V. It seems that the resistance of all the high ohmic value resistors in the series chain R22 to R18 (and possibly 'below') are slightly high in value, thus producing a total cumulative error. For example, (according to the AVO cct. diag.), R22 is 6MΩ; mine reads 6.07MΩ. Similarly R22 800 kΩ (816kΩ); R21 4MΩ (4.18 MΩ) . . . and so on *. To test that thought, I paralleled R21 (R21 is for the 300 v.d.c. FSD setting) with a high-value resistor to get the effective value of R21 nearer to 4 MΩ and then applied an accurate 250 v.d.c. * with the AVO set for 300 v. FSD. The AVO meter scale then indicated 250 v.d.c.; prior to that above test, it was reading about 240 v.d.c., i.e. the fault.

Now I've owned this AVO 8 for so many years now, (possibly late -1970s ?) I cannot remember now from where, when or how I acquired it. It's certainly had a busy / hard life in my hands and probably those of its former owners (whoever they were). So perhaps its age, plus use / misuse might explain and validate my suspicions about the incremental change of the resistor values.

* The resistance values were measured in-circuit with a Fluke 73, a Fluke 75 and a Fluke 77, the calibration of each I know I can trust. The resistance readings (for each resistor) were the same for all three meters. These meters were also used to set the 'accurate 250 v.d.c.'

Any thoughts / ideas / comments along those lines will, of course, be useful to me.

Al.

[Sinewave]

The accuracy is best tested at full scale deflection rather than midway along the scale.

[Skywave]

Yes, fair comment. I will repeat the tests at FSD per range.

Al.

[Skywave]

Calibration problem, d,c, voltages, now fixed.

Several resistors in the voltage-dropping series resistor chain had gone high in resistance. Not by a lot, but enough to impair the calibration accuracy. These resistors were replaced: R16, R18, R20 and R21. In order to get the necessary precise values, I resorted to various series and or parallel combinations of resistors. The resulting calibration on all d.c.v. ranges is now very good.
Subsequently, I took the opportunity to check the calibration on all other ranges: they were O.K.

The aforesaid resistors are located on a pcb towards the top of the meter. A drawing is attached showing those resistors, which should be of assistance to anyone else needing to replace one or more of them.

Al.