Repairs to Avo valve tester meters

[johnneyt]

Regarding weak magnets in Avo meters, I came accross this site detailing one way of effecting a repair that might be of interest. Apologies if this has already been posted elsewhere in the past. It's part English part Japanese, with pictures !!

http://www6.wind.ne.jp/yutak/avo_ct160/index.htm

http://www6.wind.ne.jp/yutak/avo_ct160/mk4-2.htm

Regards, John.

[Dekatron]

Nice that you put those links up here for everyone to read about!

If you use www.google.com and enter the web-page addresses and search for the pages you can then press "translate this page" at the end of the found link and get a quite good English translation (you can even change the language to the one you prefer but the translation quality differs between languages)!

The index page then looks like this in English:

http://translate.google.se/translate...%3Den%26sa%3DG

Putting a magnet on one side as is done in the Japanese article is sometimes the only way to get a meter to read better. It is of course best to have the magnet re-magnetized as that will better keep the magnetization over the years, if it is not remagnetized the field strength will diminish over the years and you will have to change the added magnet to keep the reading correct. Another drawback of putting a magnet on the side is that you will have a magnetic field that is not uniform affecting the linearity of the meter (this has been explained to me by a meter manufacturer and is not anything I have discovered nor tested myself).

Herts Meters UK can re-magnetize the magnet for you and in my opinion they do a nice job of refurbishing the meter including re-magnetizing the magnet. I've always been happy with their job done, and so have several friends of mine. They also remove the static electricity introduced by the plastic scale and they can change the hair springs and fix the needle bearings and generally cleaning the meter for you.

If you can't have your magnet re-magnetized and you don't want to put a small magnet inside the meter housing the best way in my opinion is to place a meter OP-AMP amplifier in the tester as you can then adjust the sensitivity as the original magnet gets weaker over the years and by that means keeping your measurements correct.

[David G4EBT]

Interesting method of curing a sick meter, compared to what I imagine would be a prohibitively expensive professional repair by the experts.

I notice from the pics that to calibrate the analogue AVO meter movement they aren't using an AVO8 or whatever, but a modern digital multimeter! I do admit that small things amuse small minds, but I found that a tad ironic and had a quiet chuckle, (though not a chortle - which would be unkind), given the evangelising about the merits of AVOs above DMMs;-)

Ho hum.

David.

[Dekatron]

Last time Herts Meters repaired and refurbished an AVO MKIV meter for me it cost GBP £70 plus shipping to me in Sweden and that is a lot less than buying a used working meter on eBay (if it is possible to find one at all).

My meter got new hair springs, new needle bearings, was antistatically treated, cleaned inside and out, had the glass re-glued and had the magnet re-magnetized. It was also tested and the FSD and internal reistance corrected!

So I think it was well spent money for a meter that looks and works like a new meter.

[johnneyt]

Martin, regarding the diodes this is from Martin Scobies web site:

http://www.marie34.freeserve.co.uk/page9.htm

A quick tip on the safety of the AVO meter movements. They are particularly fragile in the AVO 160 model, the full scale deflection is only 30 micro amps. It is so easy to overload the meter movements on these models. There is a simple mod that can help protect the meters, I use 2 silicon diodes in parallel but opposite ( cathode to anodes ) connect across the meter terminals. Any serious overloads will be shunted away by the diodes. a switch must be wired in, so the mod can be disabled during the Mutual Conductance check as it will affect the readings. I bring the wires to the switch out of one of the unused valve holders, and insulate it well, as there are very High voltages present.

Martin Scobie makes standardised valves for checking calibration.

Regards, John.

[Dekatron]

Johnneyt,

Thanks for the link to Martin Scobies website, I did not know he wrote about it there as the text was more about the MKII valve tester. I'll ask him directly where he got that information from, he has helped me before, so I hope he has a good explanation.

Observe that the oscilloscope picture of the waves is inverted to what the curves are inside the tester, Vg voltage is negative and not positive like in the scope picture on Martins webpage! Anode and screen voltages are positive though.

We currently have almost the same discussion in this thread, where we have developed it some more: https://www.vintage-radio.net/forum/...ad.php?t=55782

[johnneyt]

Thanks Martin I'm following that at the moment, its getting very interesting. Protection beats trying to locate a new meter. You mention the Herts Meter Company having repaired your meter, do they tackle repairs to meters with damaged coils.

Regars, John.

[Dekatron]

Unfortunately Herts Meters have declined all repairs to the moving coil since you can't buy that thin enamelled copper wire any more and the hours to complete such an operation will be very high. But if you have two meters, one with a working coil and the other with a broken coil, I know they can (or atleast could) swap the coil from one meter to the other, but I do not know the cost for that operation.

Here is a nice webpage from Joges Röhrenbode where he repairs a meter movement in a TV-7 A/U: http://www.jogis-roehrenbude.de/Roeh...stauration.htm (unfortunately the pictures do not show if it is translated with google translate).

[johnneyt]

Thats what I call a professional restoration, it looks new which I suppose it is in a way. I wouldn't mind sending him my Hickok !

[Dekatron]

Unfortunately I missed to credit Dieter Wächter for the restoration, the articel was only published on Jogis Röhrenbode.

[David G4EBT]

Breathtaking!

Notice how he applied a gas blowtorch to the glass dial - presumably to melt the mastic adhesive holding the glass face of the meter - not a task for the feint-hearted! He also appeared to use an ultrasonic bath to clean some of the switchery.

He used what I think is the best method for infilling lettering etched into panels or knobs. Namely, clear out the grooves, paint in with white enamel gloss model paint, not worrying too much about going outside of the grooves. Then when the paint has all but dried, wipe off any surface surplus with a cloth damped with enamel thinners, (not cellulose!) which won’t melt other paint surfaces or plastic knobs.

Once more, I notice that for precision, a digital multi-meter - not an AVO, is being used to set up, check and calibrate the analogue valve tester meter.

Presumably that step-by-step total restoration wasn't done on a commercial basis - the cost orf so many hours of highly skilled work would surely be utterly prohibitive. As to translating the German to English, I find babelfish as good as anything for translating most languages into other languages.

http://uk.babelfish.yahoo.com/translate_txt

It isn't perfect because it's a literal translation, and due to different grammar and word order, you have to tidy it up. (Translate 'the quick brown fox jumps over the lazy dog from English to any other language, and back again to see what sense it makes of it!).

However, here's a clip of what babelfish made of the intro to the Jogis Röhrenbode website, where it outlines copyright info. It's easy enough to follow what is meant:

Clip:

Reference! This site is certain excluding only for the private and not for the commercial use. No photos and texts without my previous agreement may be used…

End clip.

As to the availabilty of fine wires for rewinding meter movements, in the UK, ‘wires.co.uk’ is the retail arm of what used to be known as The Scientific Wire Company. It sells a wide range of wires – enamelled, silk covered, tinned, gold, silver and nickel plated, resistance wires and so on. The smallest diameter enamelled copper wire that it sells is 0.032mm to 0.090mm 'Solderable Enamelled Copper Magnet Wire’ which equates to No 48 Standard Wire Gauge as used in the UK. That is extremely fine wire, but maybe still not be quite fine enough. Out of curiosity, I’ve just sacrificed one of my wispy silver surfer hairs to measure its thickness – it’s .06mm – almost twice as thick as 48 SWG wire! (How I suffer for my art!)

Link:

http://wires.co.uk/acatalog/SX_0032_0090.html

As a footnote, I’ve often wondered whether those who pay very high prices for valve-testers ever recoup their investment, but then I bought my Taylor 45D for ten Pounds in 1987, which equates to £21 today, so it soon paid for itself. (It had sat on a stand at a radio rally for hours unsold, and with little interest).

David

[John M0GLN]

Originally Posted by David G4EBT

As to the availabilty of fine wires for rewinding meter movements, in the UK, ‘wires.co.uk’ is the retail arm of what used to be known as The Scientific Wire Company. It sells a wide range of wires – enamelled, silk covered, tinned, gold, silver and nickel plated, resistance wires and so on. The smallest diameter enamelled copper wire that it sells is 0.032mm to 0.090mm 'Solderable Enamelled Copper Magnet Wire’ which equates to No 48 Standard Wire Gauge as used in the UK. That is extremely fine wire, but maybe still not be quite fine enough. Out of curiosity, I’ve just sacrificed one of my wispy silver surfer hairs to measure its thickness – it’s .06mm – almost twice as thick as 48 SWG wire! (How I suffer for my art!)
David

I have a small reel of 50 SWG Lewcosol HC Copper wire, if someone has an odd moment to spare and would like to try and rewind a meter,.

John

[Dekatron]

John, that is very tempting!

I have a selection of AVO meters at home where the only problem is the broken meter movement wire. I'll see if I can set up some kind of jig for the removal of all the parts and see if I can measure the wire to se if 50 SWG is thin enough!

I have only seen the wire with my eyes one time and then I thought it was a piece of dust, but I could see that it was connected to the soldering spot just under the needle so the moment before I had dusted it away I stopped my hand and that saved that meter movement! It was just the rest of the metal parts not making contact with the soldering point closes to the needle, after repairing that the movement works again!

[John M0GLN]

Originally Posted by Dekatron

I have a selection of AVO meters at home where the only problem is the broken meter movement wire. I'll see if I can set up some kind of jig for the removal of all the parts and see if I can measure the wire to se if 50 SWG is thin enough!

If you want it, PM me your address and I'll send you the reel.

John