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Old 30th Mar 2015, 9:46 pm   #1
vaslejean
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Default Seeking help for Avo VCM mk II

Hello everyone,

I have found an AVO VCM MK II in an antique shop where it was used as a door stoper (Never seen an AVO VCM in Canada before). The owner did not know what it was intended for but knew it had someting to do with old radio tubes. I have not been able to test it toroughly and I am still reading the various info I could find on it,. It does switch on and the meeter needles seems to move nicely as it should. And a quick verification seems to indicate voltages (filaments for instance) as it should be.The EMER documents offered for download on VMARS seems most interesting I just wish the schematics were more easy to read. I am tempted to see if I can order the Radiobygones articles on the subject.

I beleive I may need to replace set MA/V potentiometer as it is stiff beyond all hope. I was able to find reference to various dedicated people who have been involved in the restoration of these and would be interested to hear of anyone who might have had experience with them and the common problems to expect.

Any input will be greatly appreciated.
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Old 30th Mar 2015, 10:00 pm   #2
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Default Re: Seeking help for Avo vcm mk II

Just search around this site and you'll find tons of info already written.

Probably the most valuable thing you could do is to add a modification to protect the meter movement.

They are fragile and can easily be destroyed by unfavourable settings of controls. You'll find a lot of material on people trying to find alternative meter movements to replace damaged originals.

The VCMII is a very effective instrument and will give you emission and Gm measurement results as well as tests for heaters, soft vacuum etc.

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Old 30th Mar 2015, 10:35 pm   #3
vaslejean
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Default Re: Seeking help for Avo vcm mk II

Thank you David ,

I did add diodes to protect the meter of an old Hickock 600A tester meter in the past. It is certainly the first thing to do on these old analog meters. The VCM MK II is indeed a very complete instruments. For the moment, I am wondering if the MA/V potentiometer could not be replaced with a multi steps shallco type switch resistive assembly to wich I would add in serie a variable resistor to possibly achieve a fine adjustment ?
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Old 31st Mar 2015, 1:14 am   #4
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Default Re: Seeking help for Avo VCM mk II

There's one pot in that instrument with a fairly odd law it may be that one...but stiffness ought to be fixable.

Have a look for Martin (Dekatron as nom-de-ploom) He's collected just about all the info on these beasties. I had a MKII in the 1960s (£10 from Jim Fish's shop) but an error on my behalf killed the meter. Nothing was found to replace it, so it got boiled down for bits and made an interesting PSU.

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Old 31st Mar 2015, 3:36 pm   #5
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Default Re: Seeking help for Avo VCM mk II

...

Dekatron seems to be very much involved on anything AVO indeed. I went trough a few threads. Mines of worthy info there for sure. I will see if I can order the articles mentioned on some of them (was it from Radiobygonnes or Radiophil ....?) I'll have to make ascertain of which issue.

I have read something about using gentle heat (hair dryer possibly) to try to release stuck potentiometer. But since these are wirewound, I am worried the damage may already be done. If some of the winding burnt-down in it, the first one to fiddle with the pot would already have had the resistance (rotational resistance) to increased.

All that I could establish at this time is that the meter seems fine , there is voltage , and it would probably a Late MK II. I need to make sure which schematic is reliable , the one on the manual, or the EMER one that seems more detailed but for which I have not been able to find one with a resolution that allows to read component values...

In the next dew days I will dismantle part of it to see If can access to the back of the control panel.

Thank you David
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Old 31st Mar 2015, 6:41 pm   #6
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Default Re: Seeking help for Avo VCM mk II

Hi Vaslejean,

Thanks for the kind words (you too David)!

The Radio Bygones article is found in number 134 Christmas 2011 magazine, the Radiophile articles are found in Summer and Autumn 2000 magazines.

The article found in Radio Bygones contain a corrected schematic.

If you remove the potentiometer from the tester you can clean it more easily. I usually use isopropyl alcohol on cotton swabs and press them gently to the stuck shaft. I also use methylated spirits and kerosene to loosen shafts.

The usual damage to these potentiometers is that the wire has come loose on the track, the tracks are tapered in some of these potentiometers so the wire tends to move. If you have to repair these potentiometers you can use some type of araldite glue on the sides or the bottom of the track to keep the wire in place. Otherwise people have had to replace them with an arrangement of a rotary switch together with a potentiometer to simulate the tapered track.
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Old 1st Apr 2015, 5:29 am   #7
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Default Re: Seeking help for Avo VCM mk II

Thank you Martin.

Using the technique as suggested by you (took the knobs off and carefully dampen the shaft with alcohol using cotton swabs) I was able to get some progress even without taking the potentiometer off. No more sore fingers in attempting to rotate this knob.

I will however wait for these articles before attempting anything. I did all 3 controls and find out that the set zero control that is made of two potentiometer linked by an extension shaft has the extension undone or broken (could not make sure just by looking) as the second potentiometer shaft does not rotate while turning that knob.

I was somehow hoping I could have managed a quick attempt at testing a EL84 just to test the things out but I guess it will have to go trough a major overhaull before anything can be safely attempted. I will gather all the information I can for the time being as I never had any experience with valve testers other than Starks & Hickocks. The Avo while more of a chalenge at this time, is obviously more interesting and valuable as an instrument.

I have found a few threads where your were participating, and I can just praise your contribution on the subject
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Old 1st Apr 2015, 10:12 am   #8
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Default Re: Seeking help for Avo VCM mk II

The Zero pot on mine had the same problem. The rear section had gone stiff and the previous user had just used more and more force until the coupling broke.
The shafts are quite small (1/8"?) and I managed to replace it with a small 1/4" flexible coupling with reducer inserts made by a friendly forum member with a lathe and drillpress.
The only real effect of the second pot is to make zeroing easier. But it works well with just the front pot (R1).

And one major advantage of the Mk2 over the later models is that it has a fairly robust 1mA meter movement rather than the very fragile 50uA later movement. All I had to do to mine (other than the Zero pot) was to clean the switches with Servisol, replace the capacitors and clean up the valve sockets.

I also put 1N5407 diodes in series with the existing metal rectifiers in case they decided to fail later on!
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Old 1st Apr 2015, 1:22 pm   #9
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Default Re: Seeking help for Avo VCM mk II

Thank You Jeremy,

That is rather encouraging.

I was somehow pleasantly surprised by the effectiveness of the alcohol treatement on the reluctant potentiometer. I had not seen the damaged coupling then on the other control then.

I will see if I can undo the rear one on the set zero control, and bring it work again though I supspect it must be solidly stiff. The first potentiomter seems to have very little rotational resistance for a short part of it's travel in the middle of it. I may not be done with that one (could excessive force have bent or twist it's shaft ?)

The meter is the good news on this thing for it still works as it should, or so it seems, the needle register dead on the ~ scale symbol and return properly to zero. If I am to refurbish this VCM , I will probably send the meter to Herts to have it clean & remagnitized...The diodes in series is a clever idea.

Wasn't there a website solely dedicated to these Avo valve tester back in the early 2000's?

Thank you again.
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Old 1st Apr 2015, 1:31 pm   #10
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Default Re: Seeking help for Avo VCM mk II

If the meter is working smoothly and settling on the ~ correctly it is probably not worth spending £100+ with postage to get it cleaned (not to mention the risk of loss or damage in transit from Canada!).

But It would be worthwhile making sure the valve holders are clean. I have had problems in the past with poor contact, especially on the most used ones (B9A, Octal). If they are loose or corroded, it would be worthwhile to replace them. Martin (Dekatron) has a neat trick for this. Cut off the valve base pins underneath with the wires still soldered to them. Then resolder the pins to the new base pins. Saves a lot of unsightly soldering iron burns on the wiring!

The other thing to check is the little cutout relay. The setting of this is a bit critical, and the contacts could probably do with a quick clean with a VERY fine abrasive board.

The JacMusic website has a lot on valve testers, but not a huge aount on the Mk2 Avo.

If you PM me with your email address I'll send you everything I have.

All the best
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Old 1st Apr 2015, 2:47 pm   #11
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Default Re: Seeking help for Avo VCM mk II

...PM sent, Thank you very much Jeremy.

a Few pictures:

in order;

zero - set pot seen from behind

the plastic coupling , plastic part intact from what I can see , metal part of it torn apart ...

And a cracked (on both side symmetrically) D77 valve heat shield symmetrically never seen that before
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Old 4th Apr 2015, 3:48 pm   #12
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Default Re: Seeking help for Avo VCM mk II

As has been mentioned, replace the gm (or other odd law) pots with a rotary switch and resistors.

This is another of my projects of the future, got as far as finding an ex-military, gold plated, 30 way switch, clean it up to see if possible. Immediate problem is balancing the contact pressure of the wiper, only solution so far is to use two wafers with the used wiper 180 degees apart.

This switch is 45mm centre to centre hole spacing, might be too large to fit. Yet to take my Mk2 to pieces to look inside.
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Old 4th Apr 2015, 4:38 pm   #13
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Default Re: Seeking help for Avo VCM mk II

That looks like a Painton 'Winkler' switch.

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Old 4th Apr 2015, 5:56 pm   #14
vaslejean
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Default Re: Seeking help for Avo VCM mk II

Hi Woodships,

It might be a tight fit but it may still be possible by using extensions (stands-off) from the mounting point on switch. That is what I had on mind myself and I have , salvaged from an old tektronix scope, one or two switches that has a middle shaft to allow fine adjust trough a potentiometer in tandem. Have yet to see if the switch has enough steps to permit a working replacement.

The releasing effect of the alcohol treatement on the pots has vanished overnight (it was to be expected too). They are stuck again. There is no way I can test this instruments as it is now. The old grease as to be cleaned-off, taking the pots apart will permit that and to assess of the situation wth them.

Anyone know which schematic is Late MK II? Also, was the flying top cap lead using an Avo specifc connector, or is there any hope to find some suitable connector to make one ?
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Old 4th Apr 2015, 5:59 pm   #15
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Default Re: Seeking help for Avo VCM mk II

Hello David.

Would that be close enough...?

http://www.ebay.de/itm/GOLD-PLATED-R...-/301175351712
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Old 4th Apr 2015, 6:15 pm   #16
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Default Re: Seeking help for Avo VCM mk II

That's usual, penetrating fluids free something up, then vanish, taking their effect with them. The next phase is to use some penetrating fluid with some lubricant mixed in, to leave things movable when the volatile fractions quit the scene.

Yes, that's the sort of switch. There is a huge family of them. I thought they were rather special and had all sorts of future uses, so i collected a boxful of different types.

There are two groups. One sort has a detent mechanism which can click onto each stud in turn. Because of the style of the moving contact, these switches short adjacent studs as they are turned, so they would not be a good choice for transformer tap switching.

The other sort have detents only for every other stud, so you only get half as many ways, but you get break-before-make switching.

I just thought I'd chuck in the keywords so you could find parts like the one woodchips posted.

If your pots are electrically OK on their windings, then a bit of thorough re-lubrication should be all you need.

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Old 4th Apr 2015, 8:19 pm   #17
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Default Re: Seeking help for Avo VCM mk II

Paignton, or Elcon on that particular wafer. Also available in 24 way.

Couple more similar switches.

Blue one is 32 way with 40mm centre mounting holes. This is a normal type switch with the stamped and formed contacts, also break before make which isn't probably of any use as a gm switch.

White one is 24 way with 25mm centre mounting holes. Again gold plated.

Last photo shows the relative size.
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Old 5th Apr 2015, 12:01 am   #18
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Default Re: Seeking help for Avo VCM mk II

Maybe it is possible to do something like Mike Phelan did with the CT160 potentiometer that he wrote about here: https://www.vintage-radio.net/forum/...ad.php?p=47426 .

I haven't checked the AVO VCM Mk I/II in detail so I don't know if that approach is possible, maybe you need two or more potentiometers and perhaps a set of switches at the end of the shaft to adjust for the different laws of the track. Using a ganged potentiometer where you tap the track like Mike did and then a set of microswitches with a wheel that actuates them in turn might be a solution worth looking into.

I don't own a Mk I/II so unfortunately I can't be of much help with testing such a solution.

/Martin
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Old 5th Apr 2015, 5:28 pm   #19
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Default Re: Seeking help for Avo VCM mk II

For top-cap connectors, the original connectors in the VCM deck must have been 0.1" but I have found that a 2.5mm plug is a good enough fit.
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Old 7th Apr 2015, 3:57 pm   #20
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Default Re: Seeking help for Avo VCM mk II

Thanks to all of you for the numerous suggestions , thank you Jeremy for the dimension of the connector...

I have managed to take the pots out, one still remain on the tester (neg. grid volts), I will attack it when I am done with the others.

My observations on them ;

1 - While I have yet to completely make sure the windings on them are good, visual inspection (can not find my magnifier!) seems to suggest they are not too bad.

2 - The second potentiometer on the SET-ZERO control was so stuck, that while someone did succeed at breaking the coupling between it and the first potentiometer it apparently did not prevent the stop post from being bent.

...Jeremy, you suggested I may be able to do without it, I may end-up not using it.

3 - Some mechanical constraint did not permit proprer riveting of this post , this post is riveted to the center terminal (whiper) and there is hardly any rivet material holding it. Or did they assembled them off from defective parts ?

One of the terminal (wiper 1st pot.) came off with hardly any tension to take it off. I will have to find a way to repair this...

This component should be have been rejected off any decent assembly line.

I will pack these with grease (lithium white grease?) and put them back in
once I can make sure they measure properly.

I can find it, but I beleive someone have measured the thickness of the different enameld wires used to wound the SET MA/V pot . Someone can point me out to the info on this matter?
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