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Old 16th Dec 2018, 6:57 pm   #1
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Default Avo 8 mk3 magnetic shunt adjustment.

Hello, when i got this Avo mk3 (1966), i realized that the magnetic shunt was fixed with a celo tape to the back plate.


Today i read the fantastic thread:

Meter Suggestions for Basic Alignment/Calibration

https://www.vintage-radio.net/forum/...d.php?t=147143


So, i follow Philpotts indications, and tried to adjust the magnetic shunt in the correct position. I only got a correct reading for 10 volts without the shunt, and 9 volts with the shunt situated totally at the left.


I wonder if the magnetic shunt should be in a special position, I mean if the right side is different to the left one or even the up or down side

Any idea of what is happening?

Fortunately i have another AVO 8 mk3 (1965) that is perfectly calibrated.

Excuse for the mistake in the title of the thread. I mean adjustment.

Last edited by Guest; 16th Dec 2018 at 7:22 pm.
 
Old 16th Dec 2018, 8:12 pm   #2
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Default Re: Avo 8 mk3 magnetic shunt adjunstment

I just tried the shunt in the 4 possible positions and the same, so the problema might be in the circuit or the movement.

Also i did the test indicated by Philpotts in the others working AVOs from my collection and all are perfect.
 
Old 16th Dec 2018, 8:57 pm   #3
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Default Re: Avo 8 mk3 magnetic shunt adjunstment

The magnetic shunt. This one is has been moved right across, I think they are normally further over. Adjustment is from left to right or right to left, the screw is loosened slightly and using a lolly stick and something to tap it with (I use a very small hammer)

It will try to twist slightly, I try to keep it parallel for best results. A movement of a millimetre will give a noticeable change in the reading- and the screw must be tightened every time you check the reading as a loose screw can distort the reading. The wooden stick I use can fit into an Avo 8 Mk 3 from the side if you are careful of any wires.

Dave
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Old 16th Dec 2018, 8:59 pm   #4
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Default Re: Avo 8 mk3 magnetic shunt adjunstment

I am not sure if lolly stick translates properly. This is what i use:

It is unusual for an Avo 8 Mk III to require more than 2 or 3mm movement on the shunt to compensate for old age.

Dave
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Old 16th Dec 2018, 8:59 pm   #5
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Default Re: Avo 8 mk3 magnetic shunt adjunstment

Yes, mine is in the same position as in your photo, maximum to the left but I just read 9 volts instead of 10. If I move it to the other side, I receive less than 9 volts
 
Old 16th Dec 2018, 9:08 pm   #6
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Oh i see! I wonder if the magnets have been touched with steel in the past and lost too much flux to be adjusted. 10% is a huge error, does this appear on other ranges?

If the outer part of one of the movement coil springs has got hung up on it's attachment leg this could create an under-read error of maybe 10 or 12%. I have had this happen on an avominor.

Dave
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Old 16th Dec 2018, 9:19 pm   #7
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

The movement looks original and untouched (the welding is original).


I dissasemble the shunt, clean it because i realize that it has some rests of glue from the celo tape that was fixing it out of the movement, then I trie few minutes ago with 1,5 volts, 3, 5 , 8 ,10, 12 volts, and now with the shunt situated on the maximum left, i can read for example 10 volts when i inject with the power supply 10,2 volts. It is better but not perfect, on other ranges it seems to work better, but is not exactly. The others AVO are completely exact.
 
Old 16th Dec 2018, 9:22 pm   #8
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Once the magnetic shunt has been set when the movement was calibrated, it should not need to be adjusted unless there is a loss of magnetism.

In general, in my experience, Avo magnets usually retain their magnetism unless subjected to some external influence. It would seem that your movement has lost magnetism to such an extent that it can only be brought into calibration with the shunt removed entirely. In this situation, there is obviously no provision for adjustment, which is not at all ideal, but if by chance the calibration is correct at least the meter is usable.

The ideal solution is the have the magnet remagnetised and "artificially aged", which means magnetised more than necessary than slightly demagnetised until the magnetic field strength is as needed for the magnetic shunt to give the correct range of adjustment.

At one time, any instrument repair workshop would have been able to do this but very few still have this facility, even if they still exist at all. Some people have reported acceptable results by adding small circular magnets to the main magnet to overcome this problem. One difficulty might be in keeping the scale linear, but this does not seem to have been the case.

It is best to check the sensitivity of the movement (37.5 microamperes full scale deflection[FSD]) with it disconnected from the rest of the circuit. This ensures that it is not the external circuit which is drawing more than 12.5 microamperes at FSD. This check is best done with the movement mounted in a glass fronted, draft-proof box so that air currents do not affect the position of the pointer.

To set up the instrument correctly, it is essential that the movement has 37.5 microampere FSD and a total resistance of movement and swamp of 3333 ohms.

If you need any technical information, service manuals etc. please send me a private message and I should be able to send you what you need.

PMM
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Old 16th Dec 2018, 9:32 pm   #9
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Quote:
It would seem that your movement has lost magnetism to such an extent that it can only be brought into calibration with the shunt removed entirely. In this situation, there is obviously no provision for adjustment, which is not at all ideal, but if by chance the calibration is correct at least the meter is usable.
Exactly, when i open the Avo for the first time, i found that the shunt was dissasembled and fixed with celo tape on the back of the scale plate, fortunately the screw was in its place.

So, i think the loosing of magnetism should be the reason, i send you a PM about that technical information.


Thanks
 
Old 17th Dec 2018, 2:29 pm   #10
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Sorry Aitor, i didn't read your original post properly!

It sounds like your meter will be ok for the moment.

Dave
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Old 17th Dec 2018, 2:43 pm   #11
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

I am not sure if it is ok, because it has an error that is impossible to correct unless taking apart the shunt. Only without the shunt, the AVO works perfect. So as you say, i think the magnets are weak.

Last edited by Guest; 17th Dec 2018 at 3:03 pm.
 
Old 17th Dec 2018, 2:48 pm   #12
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

I have an Avo 8 which seems to have acquired very fine iron filings in the movement. The movement still moves, but sticks erratically.

If I remove the magnets will this of itself damage their strength such there is then no hope of calibrating the sensitivity? I realise this is only the start of the adventure, but if it will wreck the meter there's no point in trying.

Best rgds for the season, and thanks for your help in anticipation.

Simon
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Old 17th Dec 2018, 6:30 pm   #13
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Yes Simon, i am glad you asked because dismantling in this way is guaranteed to mess the magnets up!

Debris has to be removed using blue-tak (which can be made stickier by kneading it between the fingers) A good modus operandi is a 'worm' of blu-tak on the end of a cocktail stick- as a blob won't always be slim enough to get into the gaps. This has to be done very delicately and the worm has to be changed regularly so that the flakes of steel don't get pulled back into the magnet in subsequent probings. Needless to say this has to be done very delicately, with the hands of a bomb disposal man and the eyes of a watchmaker. (x10 and x5 jewellers loupes needed) Due care is needed to avoid touching the coil springs, and steel or iron tools/tweezers can't be used near the magnets (hence the use of a cocktail stick)

The ferrous flakes that you are encountering are very often blade shaped, such that from the wrong angle they are virtually invisible- so more than one viewing angle and good light is important. The position of the moving coil can obscure flakes, so the coil sometimes has to be rotated to expose them.

There is often ferrous 'fur' covering the outside of the magnets, and this can be removed very easily by rubbing over with blu-tak.

Dave
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Old 17th Dec 2018, 9:21 pm   #14
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Simon,

The entire moving coil assembly and its frame can be removed from the magnet assembly of Model 8 Avometers up to Mark IV but of course it has to be done very carefully. Above all it is important not to disturb or knock the magnet assembly. The screws which hold the magnet assembly together must not be loosened as otherwise it will very likely need to be remagnetised.

The procedure is described in the service manual for the Model 8 Marks I & II on page 7, with references to Plates 2 & 3 (Photographs).

PMM
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Old 18th Dec 2018, 11:45 am   #15
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Default Re: Avo 8 mk3 magnetic shunt adjustment.

Thank you gents for your advice, I like the image of the bomb disposal man with blu-tac and (non magnetic) tweezers. I'll wear a full face helmet as well....


Sounds like a good game for Boxing day!

Many thanks and Seasons' whatsits

Simon
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