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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Jan 2020, 8:48 am   #1
Diabolical Artificer
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Default Display is alive! No power.

Going through a drawer of junk PCB's yesterday I noticed there was a noise coming from the drawer found to be a display off a VCR. It's like the sort of noise you hear in a factory of an engine in a cellar, very strange.

There is no hidden DC source like a battery, the display is a vacuum type, but not a classic VFD. Is this normal?

Andy.
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Old 12th Jan 2020, 10:09 am   #2
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Default Re: Display is alive! No power.

You didn't crack the seal on the display while rummaging did you?

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Old 12th Jan 2020, 7:07 pm   #3
G4YVM David
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Default Re: Display is alive! No power.

Why would that make an noise? Air ingress you mean?
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Old 12th Jan 2020, 7:41 pm   #4
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Default Re: Display is alive! No power.

Quote:
Originally Posted by G4YVM David View Post
Why would that make an noise? Air ingress you mean?
That's what I was thinking, Andy only noticed the noise while "going through a drawer of junk" and the display is "a vacuum type", if he cracked the seal air leaking in could make a noise or move the internals?

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Old 12th Jan 2020, 7:52 pm   #5
Lloyd 1985
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Default Re: Display is alive! No power.

Maybe something else is making it's internals resonate?

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Old 13th Jan 2020, 6:41 am   #6
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Default Re: Display is alive! No power.

Display not cracked, but it does resonate when picked up like an acoustic guitar, however it does make a noise with a steady pitch if I carefully pick it up and put it next to my ear. I doubt I could pick it up with a mic but will try.

Andy.
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Old 13th Jan 2020, 4:15 pm   #7
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Default Re: Display is alive! No power.

You haven't got a high ac magnetic field anywhere near?
Or, another thought, its not a sub-harmonic resonance with an ultrasonic sound such as a TV line whistle?

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Old 14th Jan 2020, 7:08 am   #8
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Default Re: Display is alive! No power.

Thinking about this it has to be resonating with some nearby source and coupled inductively or similar as perpetual motion doesn't exist. there is a PC monitor nearby . The first sound I heard had to have been caused by the PCB getting knocked, there must be some mechanical structure in there that lends itself to long term vibration, were talking minutes after it was out of the draw and loud enough to hear at 6ft.

I'll investigate more, Andy.
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Old 14th Jan 2020, 2:33 pm   #9
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Default Re: Display is alive! No power.

It's amazing how little energy is needed to keep some things resonating. In our house in Cambridge there's a picture hanging on a rather flimsy partition wall. It's wide but short, sort of letterbox format. One corner of it often sits there tapping to itself. The lightest of touches on the picture stops it, but it starts again when released. I can only assume it's driven by warm draughts or vibrations from the road or something similar.

Chris
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Old 15th Jan 2020, 3:00 am   #10
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Default Re: Display is alive! No power.

If it's not a VFD, what is it? It's the only kind of vacuum display that I know of.
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Old 15th Jan 2020, 6:55 am   #11
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Default Re: Display is alive! No power.

Here's a pic... Sorry about the quality.

A.
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Old 15th Jan 2020, 7:00 am   #12
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Default Re: Display is alive! No power.

That looks like a VFD. What makes it not one?

David
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Old 15th Jan 2020, 11:22 am   #13
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Default Re: Display is alive! No power.

It's the epitome of what I think of as a VFD! Some (like the one Andy's pictured) are almost things of intricate beauty, multi-coloured and highly detailed- though they're also highly specific and obviously bespoke things only viable for large production runs of consumer stuff.

I've often noticed that they can "ring", with anything ranging from a high-pitched buzz through a whine to a whistle- I assume that the frequently multiple directly-heated cathode wires twang as the energy reserve and the various intricately-shaped metallic supports transmit the energy to the glass and the outside world. Being thin wires in a vacuum, there's probably little internal or external damping so something trivial will set them singing and it can continue for a long while. Even a heating pump or extractor fan some way away might hit a resonance, or a busy road or passing aircraft!,

Colin
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Old 15th Jan 2020, 7:18 pm   #14
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Default Re: Display is alive! No power.

Don't these have an internal grid as part of th structure? I can see that vibrating in the audible range.
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Old 17th Jan 2020, 7:38 am   #15
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Default Re: Display is alive! No power.

Right, so it's a VFD. Not having mucked about with or seen many of these things I wasn't sure.

"I've often noticed that they can "ring"," Ahah! I'm not going mad, thanks Colin.

Andy.
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