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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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Old 7th Apr 2015, 7:28 pm   #21
Dekatron
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Default Re: Seeking help for Avo VCM mk II

It would be very nice of you if you have the time to take some more photos of the insides of these potentiometer that shows the track, it might spark some idea for someone on how to make new potentiometers without having to open theirs.

/Martin
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Old 7th Apr 2015, 9:35 pm   #22
vaslejean
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Default Re: Seeking help for Avo VCM mk II

Hello Martin,

I will do my best to take some pictures, but I am relying on my cell phone for that at this time.

If we could know the wire gauge used in these , alternatively, should the exact enough gauge be unavailable, it could probably be wired using a different shape form to achieve proper resistance for scale travel.

This being said I would rather have to simply grease, reassemble & reinstall the ones I have

The values of the potentiometers seems consitent (or just about) with what should be expected (that is if the schematic I use is the good one);

RV1 - 499K

RV2 - 1K279

RV4 - 289

RV1 from the SET ZERO tandem has the very end of it's travel wound somewhat sloppily as the picture might lead to beleive. (I think it was assembled like so since a continuous bead of some glue , or was it grease, aligned properly. Suggesting that this wound up section has not moved) the wire material is apparently the same has the one used on the external WW resistor (R5) mounted on RV4

Pictures in order;

RV1 , RV2 , RV4
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Old 7th Apr 2015, 9:43 pm   #23
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Default Re: Seeking help for Avo VCM mk II

...Erratum

... It is rv2 (as per second picture) that has the end of it's travel wound sloppily...
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Old 7th Apr 2015, 11:07 pm   #24
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Default Re: Seeking help for Avo VCM mk II

Thank you for the nice photos, they show quite well how the track is tapered so that the resistance varies along the track, they also show the different wire sizes quite well.

You should check how far the slider moves to see if the last part of the wire has become dislodged or not. I have been talking to a lot of people where the wire has come loose from the track, in what looked like it was the slider that had pushed it along. Some people have also had problems with the material that the track is wound on, it had started to disintegrate and the wire got loose - to me the base material looks like paper.

This is the reason why I suggested that you should try to glue it in place so that can't happen. You could use a thin piece of SRBP with a height of a millimeter less than the track top, extending to the bottom, that you glue on the inside of the track from end to end, making a full circle, to stabilise it. Preferably use a glue that can be removed in the future if you need to repair it but which is stable enough to keep the track and SRBP in place. It might not be an easy thing to do but it will hopefully keep the wire in place so the potentiometer lasts longer.

/Martin
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Old 7th Apr 2015, 11:10 pm   #25
vaslejean
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Default Re: Seeking help for Avo VCM mk II

Picture of RV1 with contrast deliberatly altered (sorry for the quality I can not do any better as I have insuficient lightning) to allow visualisation of the mean used to link all (3) track sections.

Wires twisted and tinned together.

The saw-tooth former is raised from the bottom phenolic pot housing using 3 to 4 mm thick tufnul...? ring.
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Old 8th Apr 2015, 4:14 am   #26
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Default Re: Seeking help for Avo VCM mk II

Martin,

Another picture of RV2, where it could indicated as you suggested that the wire is disloged. What bothers me here though, is that it's the very end of the travel (clockwise end) of the second potentiometer of the set-zero control pot pair, and the varnish bead perpendicular to the winding would not aligned with the rest of the winding at the left if it was to be tightly and evenly wound. Also if the wiper blade was to make contact on an uneven winding, would'nt the actual resistance be the only thing to matter ? I am not suggesting that dislodge winding does not occure, I just wonder if the tolerance of these pots was not achieved in a different fashion than we would today. The substrate used to wind these might be paper, I would think they would have used a phenolic impregnated material. I have not seen any overheathing trace, though the unit I have may have not been used extensively.

The measurements made on the pots I have on hand were made using a Tektronix DMM916, not the finest meter by any mean, now what were they getting as a measurement back then using possibly an AVO MK II ?

It even had me wondering if my meter was not in need of calibration.

I should measure each pots resistance again, and the resistance between each sections before putting them together. It may be usefull for anyone who might need these data.

RV2 close-up
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Old 8th Apr 2015, 9:35 am   #27
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Default Re: Seeking help for Avo VCM mk II

Unfortunately I don't own one of these testers so I've never had a chance to check what the potentiometers look like inside, but from remembering what the photos looked like that I have been sent over the years all of the windings had straight endings.

Maybe the reason for the bent stop post could be that the user tried to reach the end of the track so that the wiper could make proper contact?
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Old 8th Apr 2015, 10:30 am   #28
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Default Re: Seeking help for Avo VCM mk II

Not sure if this has been mentioned before, but the article in Radiophile offers some solutions to unrepairable Set Zero pots. As mentioned, one way is to replace the rear pot with a ~100ohm resistor. This gives very sharp tuning, but manageable with care. Alternatively, replace the whole control with a 500ohm single or multi-turn pot. The other is to replace it with a small 12 position switch and a variable control. You would need to drill a hole in the panel to accommodate the pot. There is also info n how to replace the mA/V pot.
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Old 8th Apr 2015, 12:08 pm   #29
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Default Re: Seeking help for Avo VCM mk II

Martin,

The bent post is on the front potentiometer. The soldering tag attached to this post even came off with no effort (thanks to a defective riveting,; middle tag on all 3 pots)

The coupling that links it to the rear pot is torn apart. When I managed to open the rear pot, it was stucked solid very near the end of it's travel, now if someone would used excessive force to release a stuck control very near it's track end, and if the coupling was to snap, the energy released then could certainly end-up brutaly on the stop post, causing it to bend.

I will see if I can try putting this winding back the way it should. Given the thickness of the wire I am sure I can ruin this thing very easily though, do ve know if glue was used on the former to hold it in place ? If the winding has moved then there as to be some play.

Thank you Jeff for these valuable informations, Easter was busy here, and my occupation sometime does not allow me to carry things as I would like to. I was hoping to contact the two British organisations responsible for these articles since last thursday.
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Old 8th Apr 2015, 1:10 pm   #30
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Default Re: Seeking help for Avo VCM mk II

...You were right Martin.
I have yet to figure out how to keep it place.

...The winding before & after ;
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Old 8th Apr 2015, 2:23 pm   #31
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Default Re: Seeking help for Avo VCM mk II

That looks nice!

The only solution people have told me that they have used is glue, some have even filled the whole compartment with glue, shaping it so it slants toward the center so the slider could turn and some have glued a SRBP right onto the track keeping it in place. I'd prefer the last solution as that would be easier to repair if the wire breaks.

Even a low piece of SRBP glued near the top will help keeping it in place but a piece that goes from the bottom almost up to the top will stabilise it and also keep it in place if the original material becomes soft.

/Martin
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