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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 13th Oct 2019, 10:24 pm   #41
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Its easy enough to do. I did it like that for input switching of RCA sockets.
All the bits came from the junk box. Amplifier was 4 X 807 per channel and built in twin power supplies with large choke filters. I couldnt lift it Hence it now being stripped out.

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Old 14th Oct 2019, 10:21 am   #42
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I started using the word "pessimise" some 25 years ago when taking items from work to the EMC test house. The blokes there would scan the frequencies looking for peaks of noise, and then rotate the antenna, the work piece etc, to, in their words, "optimise" the height of the peak on the analyser. I wanted the thing to pass, so my name for their "optimise" was "pessimise".
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Old Yesterday, 10:03 am   #43
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Originally Posted by G6Tanuki View Post
The TV "combine brightness and on/off to avoid spot-burn" thing seems like a good idea, but I'm wondering - once we got tellies with push-button on/off switches in the 60s and then the 1970s arrived and TVs had remote-controls with separate brightness and on/off functions, was there ever any attempt at 'spot-blanking' applied ?

I certainly don't remember seeing a central switch-off-spot on any push-button-on/off or remote-controlled TV when switching off using the remote.

Was the whole 'switch-off-spot' thing a bit of a red-herring? I can understand it being an issue with old-fashioned power-supplies using a mains-transformer with an EHT winding and significant smoothing-capacitance so there was a bit of stored energy, but surely not an issue with flyback-derived EHT and 'coated' tubes with only a few hundred pF of capacitance to store EHT-energy?
It was never that big a problem .

The electron beam, for that short while, didn't really do much harm.

What did cause the classic burnt "spot in the middle of the screen" were the negative ions, which were also, unfortunately, emitted, but not deflected by the magnetic deflection, so continued on their merry way to hit the screen "dead centre", all the while the CRT was in use.

They travel slower than electrons, but are far more massive, & are much more prone to damage the phosphor.

Earlier CRTs had "ion traps" where the electron gun was initially pointed off centre, then the beam was deflected back into the right path by a fixed magnet.
the ions were not so deflected, & hit the side of the tube.

This was a complex system, & was later made obsolete by "Aluminised" screens, which were sufficiently "ion burn" resistant to do without.
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Old Today, 2:30 am   #44
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Maybe not a big problem, but it was definitely possible to have a spot burn in from the electron bundle if it was still focussed at switch off.
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Old Today, 9:38 am   #45
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I remember an old television at my grandparents house which was originally bought in New York in the 1960s. It's the only TV I've ever seen that had a 'turn-off-spot' which lasted for up to a minute or so as I recall (it was a long time ago). I never noted any burn-out effect in the area where the spot persisted. But it's the only TV I've ever seen to exhibit that effect, so I'm assuming that most televisions of later date either had a purposely designed circuit to avoid the spot (i.e. a 'spot killer' as mentioned earlier in the thread), or that by design the EHT dies down fast enough so the spot doesn't appear.

Getting back on topic, one advantage of the on/off switch + volume control is that the unit basically has one single everyday control, so it's a single-control user interface in more modern parlance. Logically it makes sense, you turn the volume down, and when it's so low you can't hear it you might as well turn it off completely.

Certain members of the general public would be scared stiff if they had more than one control to operate - I've heard stories which I don't think are myths of people coming in to the dealer to have them select a different channel on their radio, or a different volume level. So having the power switch and volume control in one forces them to understand how the volume control actually works.

Not having the unit potentially burst out at maximum volume when switched on would be another advantage, on the other hand with older valved equipment, the sound would come up gradually giving the user (assuming he or she was still close to the unit) time to manually turn down the volume if indeed necessary.
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Old Today, 12:19 pm   #46
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Default Re: Cretinous idea - The combined volume pot and mains SW.

There was the combined volume control/on-off switch which solved part of the problems. Can't remember the manufacturer, but you turned it to alter the volume, and push/pulled it to operate the switch.

It saved the wearing out of the track, and meant that volume could be as last-used.

Of course, it still needed the mains wiring to be brought close to the signal, and it introduced the new problem of knobs being out of line in one position - it it was positioned within a group of similarly-knobbed controls, that is. Though, surprisingly, with about 1/8" of movement, one knob sitting proud (or back) did not look obvious.
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Old Today, 12:41 pm   #47
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Originally Posted by Oldmadham View Post

The electron beam, for that short while, didn't really do much harm.
Actually it did and is a very significant problem, that is on CRT's that have non-aluminized screens.

Not only did the aluminization protect from ion burns but it made the phosphor much more resistant to high beam current burns at switch off with an intense central spot.

I have evaluated a large number of non-aluminised pre WW2 CRT's and I can assure you the phosphor is extremely easily damaged by the turn off spot. I assume it is better with aluminization due to the thermal conductivity of the thin layer of aluminium.

For example I was able to detect screen phosphor desensitization in my 904 after only 3 or four episodes of turn off spot in an NOS 5FP4, which is why I modified the circuit to suppress the beam current at turn off.
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