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Old 31st May 2020, 3:13 pm   #1
RogerWalker
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Default AVO Wide range signal generator

I am considering restoring one of these but upon initial examination, mine has a mechanical problem that I can't for the life of me work out how it could have occurred - or how it could be fixed. If anyone on here is familiar with the workings of the beast, I'd be grateful for some advice. I bought the generator off e-Bay some years ago and it has lain untouched on my pending shelf ever since. I know nothing of its history or previous ownership. Turning the tuning knob does not move the tuning scale.


The generator tuning scale employs a large disc roughly 150mm in diameter that is rotated at its perifery by a small diameter pulley attached to the tuning knob spindle. I seem to recall this type of slo-mo drive being known as "epicyclic". For this to work, the pulley has to be in contact with both sides of the disc. And here's the problem: The pulley is not in contact with the disc - with a 2mm gap, both sides.



As I see it this could only occur if a) the drive spindle or the tuning capacitor to which the disc is attached have moved apart, or b) the disc has reduced in size somehow or c) there was a rubber 'tyre' around the disc that has perished and disappeared. In reverse order, there are no traces of a rubber tyre, the outer edge of the disc looks original and not worn or scuffed and finally, the spindle and the tuning capacitor mountings are 'battleship grade' and have no means of moving or being moved by the provision of elongated mounting holes.



Looking closely at the 'pulley' it is not actually V-shaped, giving credence to the tyre concept. I have added a picture of the offending drive spindle - if anyone can shed some light on this mystery, I would be much obliged! The chain in the picture is nothing to do with the tuning drive - it actuates the frequency range change turret and its knob is coaxial with the tuning knob.
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Old 31st May 2020, 3:28 pm   #2
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Default Re: Avo Wide range signal generator

This is a wild guess but isn't there a rubber drive wheel missing from the the drive spindle?

Alan
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Old 31st May 2020, 3:30 pm   #3
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Default Re: Avo Wide range signal generator

Hi!

Any help?

https://www.google.com/url?sa=t&sour...elMewvSked_THZ

Chris Williams
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Old 1st Jun 2020, 10:20 am   #4
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Default Re: Avo Wide range signal generator

Thanks Andy - I had the manual - but frustratingly, in the only picture that could have helped, the large tuning knob masks the drive spindle. That photo does at least snuff out the theory of a rubber tyre around the big disc, and gives weight to the suggestion that there might have been a 'tyre' on the drive pulley. Only someone who has one of these can answer that.
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Old 1st Jun 2020, 10:23 am   #5
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Default Re: Avo Wide range signal generator

Thank Chris, but I already had the manual. It doesn't shed much light on why the drive doesn't work, sadly.
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Old 1st Jun 2020, 9:00 pm   #6
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Default Re: Avo Wide range signal generator

Hi Roger,

As discussed, I'm not sure it's a complete tyre this looks like a cut and glue thing. Hope it helps anyway

Regards - Andrew
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Old 2nd Jun 2020, 10:52 am   #7
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Default Re: Avo Wide range signal generator

Thanks, Andrew - very kind of you to open it up for me.

I'll rummage through my grommets box - but your tyre doesn't look like rubber. I have seen that brownish stuff somewhere before - maybe on anti-microphonic tuning capacitor mounts? It will have to be fairly squishy to fit under the disc and pretty indestructible to withstand the 'bacon slicer' effect of the disc. Any ideas anybody for a suitable material?

Surely I am not the first person to have this problem?
Regards Roger
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Old 2nd Jun 2020, 11:14 am   #8
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Default Re: Avo Wide range signal generator

Would a couple of layers of heat shrink tubing work?

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Old 2nd Jun 2020, 11:23 am   #9
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Default Re: Avo Wide range signal generator

If you're very lucky with the sizes it might be possible to use a few thick o-rings stacked so there's a 'V' between two of them at the height of the disc.

Feeding the disc into that 'V' would spring them apart slightly to provide the grip.

Cheers
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Old 2nd Jun 2020, 1:02 pm   #10
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Default Re: Avo Wide range signal generator

Quote:
Originally Posted by RogerWalker View Post
I'll rummage through my grommets box - but your tyre doesn't look like rubber. I have seen that brownish stuff somewhere before -
Regards Roger
Hi Roger, maybe you've seen it on a gas bottle hose and so on, it's actually a brick red and not that soft to be honest, likely ageing but not hard either. Suggestions from me would be section from a bicycle inner tube doubled up to suit; a 3/4 tap washer; section of suitable malleable hose.

Regards _ Andrew
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Old 6th Jun 2020, 7:52 pm   #11
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Default Re: AVO Wide range signal generator

Oops just saw this post. You repair it like this. Look around 12 minutes in.

https://youtu.be/R3q5QJ53DVI
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Old 6th Jun 2020, 9:34 pm   #12
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Default Re: AVO Wide range signal generator

It looks like a strip of leather.

I wonder if the knob assembly has a deliberate bit of slop and it allows the chain tension to put pressure on between the tyre and the tuning scale disc?

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Old 10th Jun 2020, 11:30 am   #13
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Default Re: AVO Wide range signal generator

Quote:
Originally Posted by ChristianFletcher View Post
Oops just saw this post. You repair it like this. Look around 12 minutes in.

https://youtu.be/R3q5QJ53DVI
Strange title for a video
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Old 10th Jun 2020, 8:25 pm   #14
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Default Re: AVO Wide range signal generator

Some odd spoken phrases as well, unless very carefully kept in context.
You 'need to grease your shaft', and 'The end of my knob is very dirty so I'll go do some cleaning'

Make sure no-one else is in earshot when you watch this video. He's not trying double entendre deliberately, but Dick Emery would have been proud to have written those lines.

It's actually a good video showing what goes wrong and its repair. But use Christian's link, don't try typing the title of this video into you tube's search window!

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Old 11th Jun 2020, 6:37 pm   #15
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Default Re: AVO Wide range signal generator

Someone should measure the diameter and width of the recess that houses the rubber 'tyre'.
Also measure the thickness of the rubber tyre.

ChristianFletcher used a cut down electrical grommet to make a temporary repair.
I don't think that PVC will work in the long term.

I have seen rubber rings in ink-jet printers that may be suitable.
They are on the drive shafts that move the paper through the printer.
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Old 12th Jun 2020, 11:25 pm   #16
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Default Re: AVO Wide range signal generator

Thanks everyone for the ingenious replies. I will try a few of the ideas floated and revert.
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Old 13th Jun 2020, 12:09 am   #17
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Default Re: AVO Wide range signal generator

Something like the orange/brown coloured rubber bunsen burner tube would be ideal, cut 5 or 6mm off the end and use that.
https://www.ebay.co.uk/itm/BUNSEN-BU...MAAOSwfdFaig3x

would have been easy to get hold of during school days ...
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Old 13th Jun 2020, 12:22 am   #18
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Default Re: AVO Wide range signal generator

Rummaging in my rubber items box, I found a couple of sizes of rubber tubing that might work. The smaller size is quite stretchy and you could double it if you need more diameter.
Happy to put a short piece of each in an envelope and post to you if you PM me your address.
Best Regards
Chris
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Old 13th Jun 2020, 4:37 pm   #19
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Default Re: AVO Wide range signal generator

I don't know how long my grommet will last either. I think the issue will be the glue rather than the rubber failing. I would have tried silicon hose as a good stretch fit on the wheel but didn't have any available. It may last a while for equipment that does not get a lot of use or mainly used around the same frequency for IF alignment etc. Glad the video was useful unfortunately I found the HT on the output transform open circuit on my signal generator and I have not got around to rewinding it yet.

A note on the Title: Bum Holes was the most polite thing that came to mind after finding the HT open circuit!

Thanks Regards Chris
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Old 13th Jun 2020, 5:27 pm   #20
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Default Re: AVO Wide range signal generator

Good afternoon,
Looking at the first picture carefully it does look to me is if something soft was placed around the edge of the dial plate. The groove in the knob spindle there to ensure the dial plate doesn’t deviate around. Something like the cut to size grommet material (or rubberised sharp edge protector) may fit over the edge and work.

This could last a bit longer mechanically as turning the tuning from one end stop to the other, it only goes past the spindle shaft and get squashed once. A rubber grommet on the spindle would be squashed maybe 8-10 times (or however many rotations of the spindle shaft is needed to go from one end to the other) as it passes the dial plate.

Christopher Capener
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