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Old 13th Aug 2019, 1:57 pm   #1
Hammonds
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Default Avo 7 meter - no voltage reading

Good afternoon, All
I've just bought an Avo 7 multimeter at an auction.
The DC amps range seems to be fine, but I can't get any readings on any DC voltage range.
Any ideas, please, before I take it apart. A circuit diagram might be useful, please.
Best regards and thanks in advance.
Dave
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Old 13th Aug 2019, 2:55 pm   #2
David G4EBT
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Default Re: Avo 7 meter - no voltage reading

You don't say whether or it's the MK1 or MK2.

The MK1 manual is here:

http://www.richardsradios.co.uk/AVO/...structions.pdf

The MK2 manual is here:

http://www.richardsradios.co.uk/AVO/avo7_mk2.pdf


AVOs have a huge fan base on here and elsewhere so I'll leave it to AVO 'aficionados' to suggest where the fault may lie (switchery?), but some cautionary comments, especially on the AVO7, when working on low voltages might not be out of order. Apologies if you already know about the limitations - not everyone does.

AVOs keep cropping up as the instrument of choice when it comes to analogue meters, which is fine for work on valved equipment with higher voltages, and was invariably what is quoted on maker’s data sheets. But Avo 7s and 8s or similar low impedance meters when measuring low voltages across potential dividers will not produce accurate results. True, the meters will accurately measure the actual voltage that they see, but that voltage will be lower than we'd expect to see because the Avo will act like a resistor in parallel with the circuit under test. EG if we have a potential divider using two 100k resistors across a 12 Volt supply, we know from Ohms Law that the calculated voltage at the junction of the two resistors will be 6 Volts.

An AVO 8 (or any 20,000 OPV meter) on the 10V range will be like plonking a 200K resistor in parallel with the 100K one in circuit, so will produce a reading of 4.8V. However, an FET or Valve Voltmeter (such as the Heathkit V7A), with an impedance of 11 Meg Ohm will show 5.97V, close to what we'd expect to see.

Worse still if we apply an AVO 7 (or any 1,000 OPV meter) on the 10 Volt range across the junction as it will be like putting a 10K resistor across the 100K one, so the reading will be 1 Volt - the actual voltage due to the loading of the AVO. Not so important when taking readings of higher Voltages, and there is the merit of using the same meter that was used when measurements were taken on valve radios by the manufacturers and shown on service sheets, so I do understand why many restorers like to use AVOs on old radios and TVs, but the lower the voltage being measured, the more significant this loading effect becomes.

I'm not knocking AVOs, just saying they have their limitations, which is fine if that's understood.

Good luck in sorting it Dave.
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Old 13th Aug 2019, 2:59 pm   #3
The Philpott
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Default Re: Avo 7 meter - no voltage reading

Prime candidates:-

-If the ACV ranges work- sounds like a switching problem with the leaf switches.
-If the ACV ranges DON'T work- the Voltage shunts are arranged in series so if the lowest value winding (or component resistor) is open circuit this renders all the voltage ranges dead. (Possibly the lowest ACV range might still work though- can't remember..)

What year of manufacture is it?

Circuit diagram easy to find- it's probably been represented on this forum if you look in the search facility.

Dave
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Old 14th Aug 2019, 7:41 pm   #4
Hammonds
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Default Re: Avo 7 meter - no voltage reading

Hi David and The Philpott
Thanks for all your help
I can't see any obvious marking to denote whether it's a mk1, but from the pictures on the manuals, it would appear to be a MK1.
I can't tell when it was made - help?!
On the ACV range, only the 10v range works. Is it worth while using some Deoxit on the leaf switches?
Cheers
Dave
Maybe I'll take it apart and have a look at those series resistors.

Last edited by Hammonds; 14th Aug 2019 at 7:41 pm. Reason: missing letter
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Old 14th Aug 2019, 8:10 pm   #5
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Default Re: Avo 7 meter - no voltage reading

Quote:
Originally Posted by David G4EBT View Post
Worse still if we apply an AVO 7 (or any 1,000 OPV meter) on the 10 Volt range across the junction as it will be like putting a 10K resistor across the 100K one, so the reading will be 1 Volt
It's worse, the AVO 7 is a 500 OPV job not a 1,000 OPV one, the total resistance across the input terminals when the divide by two function is selected is the same as it is when the divide by two function is not selected.

If you look at the schematic in the manual and select a range you can add up all the resistances, for instance take the 10 volt range.....4,500 + 450 + 50 which equals 5,000 Ohms which equals 500 OPV (5,000/10)

Lawrence.

Last edited by ms660; 14th Aug 2019 at 8:27 pm. Reason: Explanation example
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Old 14th Aug 2019, 8:51 pm   #6
The Philpott
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Default Re: Avo 7 meter - no voltage reading

The last 3 (or 4) digits in the serial number on the scale plate indicate the date of manufacture- it is a case of peering into the glass, looking slightly downwards.
3500-1147 would indicate instrument number 3500 made in Nov.1947.

If it's quite an early meter the voltage shunts are likely to be windings not resistors (and are thus more susceptible to failure.) A DMM with manual ranging is the best idea for checking them as a medium resistance range can be selected to avoid the possibility of *overloading the meter movement should it become energised by the DMM. (*Unlikely with a Model 7 Avo, but nevertheless..)

As regards the leaf switches, they can either be dirty, worn or misaligned. I don't know the constituents of Deoxit so can't comment on it. If contacts are merely dirty what i do is run a little strip of the cardboard from inside a loo roll thro' them, (dry) then soak it in contact cleaner such as isopropyl alcohol,run it thro' then repeat again with it dry. It's possible to rotate the selectors on the front of the Avo to close the contacts onto the card as they're being cleaned.

Should the contacts be misaligned or worn it's usually possible to carefully bend the offending leaf/leaves to make proper contact, but cleaning must be done first to exclude dirt from the equation.

It's not all bad news, some meters which have this sort of fault will have been sidelined and put aside decades ago, with the result that other moving parts have not worn out through over-use.

Dave
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Old 16th Aug 2019, 3:42 pm   #7
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Default Re: Avo 7 meter - no voltage reading

On the bottom right hand side of the scale plate is No 7945-23404 if that's any help.
It looks almost impossible to access the AC and DC selector switches. Any ideas, please?
Does anyone know if there's a service manual which identifies all the components, please?
Best regards
Dave
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Old 16th Aug 2019, 4:12 pm   #8
Barnmead
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Default Re: Avo 7 meter - no voltage reading

I suggest the first thing you should look at is the wirewound resistor R11 which should be 450 ohms. This is a favourite for going open circuit due to incorrect use of the range switch. The varnish may well be black!
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Old 16th Aug 2019, 8:08 pm   #9
The Philpott
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Default Re: Avo 7 meter - no voltage reading

They juggled with the serial numbers sometimes; reversing the date and instrument number.

7945-23404 is actually: Model 7, September 1945, Meter number 23404.

David has posted a link to a PDF doc, above, the last page of which has a circuit diagram. (Yours is a Mk1 meter)

A peculiarity of Model 7 is that the needle zeroing is carried out with the meter set to a DC range. When putting the meter away it is recommended not to leave the meter set to an AC range- as a trickle current is taken from the 1.5v cell to bias the rectifier for accuracy on low voltage AC readings. Admittedly it is a very small current.

Dave
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Old 18th Aug 2019, 11:19 pm   #10
Phil G4SPZ
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Default Re: Avo 7 meter - no voltage reading

Quote:
Originally Posted by Barnmead View Post
...look at R11 which should be 450 ohms...
I agree with Richard's diagnosis. Avometers are nothing if not logical. The 10V AC range has a separate multiplier (series resistor) of 470 ohms, which feeds a tapping on the current transformer, but all the other AC voltage ranges and all the DC voltage ranges share the common multiplier chain. Hence the combination of symptoms you report points strongly to R11 being open-circuit.

A standard resistor will do; select one from a batch of 470 ohm 10% components using a DMM and fit the nearest to 450 ohms that you can find. It can be bridged across the faulty bobbin. Hopefully, you'll find that all the voltage ranges start working again.

Avo rotary switches, in my experience, are very rarely at fault. The leaf switches, on the other hand, will almost always require cleaning in the way The Philpott has suggested.

Good luck!
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Old 18th Aug 2019, 11:31 pm   #11
The Philpott
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Default Re: Avo 7 meter - no voltage reading

Seconded! (Or thirded..)

Dave
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