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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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13th Aug 2019, 1:57 pm | #1 |
Heptode
Join Date: Feb 2011
Location: Southampton, Hampshire, UK.
Posts: 574
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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 |
13th Aug 2019, 2:55 pm | #2 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,766
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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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David. BVWS Member. G-QRP Club member 1339. |
13th Aug 2019, 2:59 pm | #3 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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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 |
14th Aug 2019, 7:41 pm | #4 |
Heptode
Join Date: Feb 2011
Location: Southampton, Hampshire, UK.
Posts: 574
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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 |
14th Aug 2019, 8:10 pm | #5 | |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
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Re: Avo 7 meter - no voltage reading
Quote:
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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14th Aug 2019, 8:51 pm | #6 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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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 |
16th Aug 2019, 3:42 pm | #7 |
Heptode
Join Date: Feb 2011
Location: Southampton, Hampshire, UK.
Posts: 574
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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 Last edited by Hammonds; 16th Aug 2019 at 3:44 pm. Reason: Typo |
16th Aug 2019, 4:12 pm | #8 |
Pentode
Join Date: Dec 2002
Location: Norfolk, UK.
Posts: 168
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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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Richard |
16th Aug 2019, 8:08 pm | #9 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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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 |
18th Aug 2019, 11:19 pm | #10 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
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Re: Avo 7 meter - no voltage reading
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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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 18th Aug 2019 at 11:21 pm. Reason: Punctuation |
18th Aug 2019, 11:31 pm | #11 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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Re: Avo 7 meter - no voltage reading
Seconded! (Or thirded..)
Dave |
8th Dec 2019, 5:14 pm | #12 |
Heptode
Join Date: Feb 2011
Location: Southampton, Hampshire, UK.
Posts: 574
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Re: Avo 7 meter - no voltage reading
Good evening to all the good folk have responded to my original post.
Despite my best efforts, I have been unable to identify where the problem lies as it's beyond my expertise. I've decided to call it a day and put it in the for sale section and see if anyone else wants to have a go. Many thanks anyway. Best regards Dave |
8th Dec 2019, 7:06 pm | #13 |
Dekatron
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,834
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Re: Avo 7 meter - no voltage reading
To me, an AVO 7 is of historical interest/value for collectors, it is not really accurate enough for modern fault finding as David has previously explained. I'm not knocking it - but it is what it is! On the other hand, the AVO 8 is a much better instrument and can be bought for as little as £30 for a good 'calibrated' one. As you have decided to do, I'd let the 7 go and then buy a working 8.
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