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Old 1st Mar 2005, 1:44 pm   #1
Duke_Nukem
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Default AVO Model 8 FAQ

This thread was compiled from contributions made by may people across many threads.
Quote:
I've picked up an AVO with a damaged PCB shunt board and damage to the needle damping assembly. After some 'phoning around, I was kindly mailed a salvaged shunt board free of charge from a company down south (England!).

Can anyone help in terms of a source for bits and instructions to enable repair to the damping assembly?
  1. Have you tried Avo Megger at Dover for help ?
  2. Name of repair co. was Dowding & Mills, tel no. 01623 555110. contact name at D&M is alan ellerby.
  3. May I make all owners of poorly Avo's aware of an independant repairer of Avo meters of all ages for pocket money prices. He also carries vast stocks of B121 batteries at bargain prices too.

    Mr Avo, also known as Bill Barton is based in Bolton, Lancashire and can be reached on (Details removed, please see post #2).

    For those of you whom may visit the Vintage Valve Technology Fair at Haydock, Bill normally has a stall and is pleased to deliver advice, take meters for repair and recalibration and even sell you an AVO.

Quote:
Just obtained one of above but the meter is sticky, and requires tapping lightly to return to zero or read accurately on the scale. I presume the meter movement is at fault. Would this be an easy job, or is it best left alone for the experts?
[list=1][*]If the problem is rust in the pivots, I've heard that one can feed low-frequency AC to the meter which makes it vibrate to loosen the rust. I've never had occasion to try this but it seems an easy first step.[*] In my experience the most common problems are:-

a. The needle is bent and catches on the scale at some point or the counter weight on the rear is fouling.

b. Dirt or small fragments of metal between the moving coil and the magnet.

c. Damaged or distorted hair springs.

All need care to correct - I have rebuilt meter movements successfully in the past but I have also destroyed a lot on the way![*]it could be turns of the hairspring crossing, which would be obvious on inspection.

It's difficult to blow magnetic particles and corrosion out of the gap without also distorting the hairsprings, so I wouldn't try it. Bits of masking tape and non-magnetic tools are safer, but you need good close eyesight or a binocular microscope. Usually magnetic particles affect the meter in one or a few places across the scale, not everywhere.
[*] A note on servicing meter movements can be found at
http://www.thevalvepage.com/servdata/testeq/avo.htm
Although this is for the meter movement of an AVO characteristic meter the same method applies to analogue meters in general.

Quote:
When doing a measurement the pointer oscillates around the correct reading for ages so it looks as if the damping has gone.
  1. some of these movements had a " swamp " resistor across the coil, also used to set the sensitivity. If it is reading high this could be a suspect.
    Some Avo movements used silicone oil damping on the pivots, difficult to see or replace!
  2. The fault which you describe is almost invariably caused by an open-circuit universal shunt chain. In the AVO 8, in fact nearly all Avometers, the current shunt chain is permanently connected across the movement. In the Model 8 the basic sensitivity of the movement is 37.5 microamperes and this is modified to 50 microamperes by the shunt chain.

    In a Model 8 Mk I to IV, look for a burnt or damaged winding on the flat bakelite board at the bottom of the panel; in a Model 8 Mk V, 6 or 7 the shunts are printed circuits.

    The shunts can still be bought from Avo (Megger) in Dover but the wire wound type are a 'site' repair calling for accurate calibration.

    The swamp is a resistor used to set the movement resistance to an accurate value which determines the division of current between the movement and the universal shunt chain. (2500R total in Mks I to IV and 3333R in Mk V, 6 and 7, from memory). It also compensates for temperature drift in the earlier type - thermistors are used in Mk's III and IV. In the later marks, the swamp is a thick film 'tile' which has a tendency to drift in value. An effective repair can be made by fitting a ten-turn cermet pot. in place of the original.

Quote:
I have just acquired an Avometer Mark 8. Can anyone tell me what leads it needs, whether I can buy them anywhere (UK), or whether I can make them up myself ?
  1. RS sell the original type leads with hook connectors to go under the terminal posts on the AVO, and the insulated plug in probes on the other. Alternativly, most later AVOs have holes for 4mm plugs in the terminal posts, so any cheap leads with either unshrouded plugs, or those with retractable shrouds (eg Fluke type) could be used, check out the major component suppliers like RS, Farnell and Maplin (all have good websites), you may also find some in your local component supplier.
  2. You could try :-
    Anchor Surplus,The Cattle Market,Nottingham
    They used to get a lot of ex-MOD AVO leads, and sell them at reasonable prices.
  3. I picked up some leads at Birketts in Lincoln today [Jul2004] for 3.50.
  4. You can buy replacement leads, clips and probes from either RS components (rswww.com - stock No. 203-5632 - 45.00 pounds ) or Farnell (www.farnell.co.uk) if you want the genuine article. They are also available from Megger at Dover (www.megger.com/uk). If you choose to buy from Megger, you might like to buy replacement terminal caps at the same time.
Quote:
Does anyone know where I can get a copy of the manual ?
  1. have posted a PDF version at www.g1jbg.co.uk/service.htm. It's nearer the AVO8 Mk1 or Mk2, but it's the best I can find at the moment.
  2. Circuit diagram and manual can be downloaded from : http://www.richardsradios.co.uk/avo8.html
(To do : Check links)

Quote:
Does anybody know where I can get hold of the 15V cell for an AVO Model 8? Or is it discontinued, in which case I'll make do with a couple of PP3's and a 3.3k resistor!
  1. Alkaline BLR121's (early AVO 8's) and BLR154's are available from Rapid Electronics
  2. Maplin List BLR121 on their no FM07H
  3. The BLR121 batteries available are rather poor, and have tiny button cells inside. I use a stack of 5 lithium batteries (the large ones which are about the size of an old penny). These are available on market stalls at about 30p each, and last really well.

    If you tape the 5 into a stack, and make a spacer to fit the distance between the clips, they go in very neatly. These cells seem to be very stable, and the " set Ohms x 100 " control needs far less tweaking.

Quote:
Dose anybody know if you can still get the plastic terminal covers (red / black) anywhere?
  1. It's possible to use the end caps used for closures in some plastice tubes for medicines and other products; I've seen plenty in red but not black (magic marker?) and will need a hole drilling in them. Not 100% but better than getting a belt.
  2. The Avometer terminal caps are still available from AVO (Megger) at Dover. The last batch I bought cost 0.35 each, but I don't know if there is a minimum order charge.

    You could also try any of the official AVO repair agents - list available from AVO.

    AVO International became Megger Ltd. Same address in Dover.
    http://www.megger.com/
Quote:
My AVO 8 seems quite different to another one I saw. Were there different versions ?
  • There were many variants with a lot of minor changes. To find out more refer to the following thread :-
    Avo 8 Versions

Quote:
...my first AVO 8 it works well on voltage ranges but none of the resistance ranges work.
  • It might be worth checking that the cams on the range switches are working correctly. On the left hand switch (viewed from the front) the cam should lift the (lower) leaf switches in the RESISTANCE position only. On the right hand switch (ditto) the cam should lift and drop several times as you switch through LR ... ohms ... INS. (I can give you the sequencwe if you need it).

    It's not unknown for a cam to come loose from its spindle and just sit there. If that's the case, repositioning the cam and tightening the fixing screw is all that's needed.

Quote:
Where can I get manuals for AVO meters?

Last edited by Darren-UK; 1st Dec 2011 at 3:22 pm. Reason: Info updated.
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Old 1st Dec 2011, 3:21 pm   #2
Darren-UK
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Default Re: AVO Model 8 FAQ

We have today been advised that Bill Barton is no longer carrying out AVO repairs, this information having originated from Bill's wife and kindly passed to us by another member.
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