UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Reply
 
Thread Tools
Old 13th Oct 2019, 10:24 pm   #41
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 933
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.

Joe
Attached Thumbnails
Click image for larger version

Name:	switch.jpg
Views:	69
Size:	87.8 KB
ID:	191867  
joebog1 is offline   Reply With Quote
Old 14th Oct 2019, 10:21 am   #42
trsomian
Pentode
 
Join Date: Oct 2010
Location: Box End, Beds. UK.
Posts: 168
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".
trsomian is offline   Reply With Quote
Old 16th Oct 2019, 10:03 am   #43
Oldmadham
Triode
 
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 17
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
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.
Oldmadham is offline   Reply With Quote
Old 17th Oct 2019, 2:30 am   #44
Maarten
Nonode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 2,604
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.
Maarten is offline   Reply With Quote
Old 17th Oct 2019, 9:38 am   #45
ricard
Octode
 
ricard's Avatar
 
Join Date: Mar 2006
Location: Lund, Sweden
Posts: 1,356
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.
ricard is offline   Reply With Quote
Old 17th Oct 2019, 12:19 pm   #46
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,139
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.
kalee20 is online now   Reply With Quote
Old 17th Oct 2019, 12:41 pm   #47
Argus25
Nonode
 
Argus25's Avatar
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,434
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
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.
Argus25 is offline   Reply With Quote
Old 17th Oct 2019, 10:10 pm   #48
MotorBikeLes
Octode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 1,323
Default Re: Cretinous idea - The combined volume pot and mains SW.

I vividly remember those slowly receding spots in the middle. I stayed with an aunt back in 1953, and she had a TV; I saw the "interlude potter" a few times on that TV..
later(1964/5) I played with my first old TV (which I did get to work briefly), and read about angled guns and ion trap magnets. Adjusted them on sight for optimum picture.
later, I read about aluminised tubes, guns now in line again, and no bother with ion burn.
However, I have seen (and caused at least once) a burned out central spot on a colour TV tube when the scanning stopped, but the beam did not (coils unplugged?).
Les.
MotorBikeLes is offline   Reply With Quote
Old 18th Oct 2019, 1:37 am   #49
joebog1
Heptode
 
joebog1's Avatar
 
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 933
Default Re: Cretinous idea - The combined volume pot and mains SW.

Back to the title of this thread!!

I have worked on guitar amps that have the tremelo pot ganged with the mains switch.
This is mounted smack bang in the middle of ALL the pots.
It does cause hum. My method of curing it ( ALMOST completely ) is to wire the on/off switch in balanced microphone cable. The two conductors obviously carry the mains voltage, and the screen is connected to ground at the feed end. ( IEC socket/mains cable etc )

Joe
joebog1 is offline   Reply With Quote
Old 18th Oct 2019, 11:56 am   #50
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,666
Default Re: Cretinous idea - The combined volume pot and mains SW.

If we are concerned about hum resulting from mains live wiring then twin-core mainscable with an in-built screen is available. This has the advantage over mic cable that it meets the regulatory requirements for mains operation. Otherwise sliding copper braid over the outside of ordinary mains cable also works. It doesn't fix the problem of noise from the switch mechanism itself leaking into the volume pot track, of course.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 18th Oct 2019, 6:51 pm   #51
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 7,774
Default Re: Cretinous idea - The combined volume pot and mains SW.

In times-past, when doing stuff for clients-best-unmentioned we bought a load of seriously-screened moulded 13A-to-IEC309 leads from a company called "Rendar" with the option for the foil/overbraid tied to protective-ground at either the IEC309 end, the 13A end, or both.

[Yes it was horribly expensive - think modern 'audiophile' pricing for power-leads - but it was a "Cost-plus" contract and the UK taxpayers were paying for it]

I'm sure that if you had loads of money available you could source suitable foil/braid-shielded 2-core cable for wiring modern single-pole mains-switching-pots. Or you could just redesign the audio stages to be less-sensitive below 100Hz?
G6Tanuki is online now   Reply With Quote
Old 18th Oct 2019, 7:46 pm   #52
GrimJosef
Nonode
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 2,666
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
Originally Posted by G6Tanuki View Post
... I'm sure that if you had loads of money available you could source suitable foil/braid-shielded 2-core cable for wiring modern single-pole mains-switching-pots. Or you could just redesign the audio stages to be less-sensitive below 100Hz?
You could indeed roll off the low-frequency performance, if that was unimportant to you. It might save you 1.48 plus VAT and P&P https://tm3connections.com//index.ph...l-screen-mains.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline   Reply With Quote
Old 19th Oct 2019, 2:29 am   #53
Maarten
Nonode
 
Join Date: Dec 2007
Location: Haarlem, Netherlands
Posts: 2,604
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
Originally Posted by kalee20 View Post
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.
Philips used those sometimes.
Maarten is offline   Reply With Quote
Old 19th Oct 2019, 7:34 am   #54
Electronpusher0
Hexode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 455
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
Originally Posted by GrimJosef View Post
Otherwise sliding copper braid over the outside of ordinary mains cable also works. It doesn't fix the problem of noise from the switch mechanism itself leaking into the volume pot track, of course.

GJ
When I restored a valve Armstrong amp it had screened audio cable carrying mains to the switch on the volume control. I replaced it with ordinary mains cable over which I slid some braid screening and some heatshrink insulation. I felt safer that way.

Peter
Electronpusher0 is online now   Reply With Quote
Old 23rd Oct 2019, 6:15 am   #55
Oldmadham
Triode
 
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 17
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
Originally Posted by Argus25 View Post
Quote:
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.
OK, prewar CRTs are certainly a different story.
They were mostly originally similar to, or in fact, were, CRTs designed for Oscilloscope use.
Even with much more modern 'scopes, I well remember the adjuration to "turn down the intensity!"

Pretty much anything recognisable as a "TV" CRT from the 1950s on will not normally have a problem.

The kind of "burn in" I have seen over the years, is when a tube is deliberately underscanned to ensure a correct 3:4 ratio picture, or those tubes which have displayed a "quad screen" display for a long time.

I had a big Philips which had the latter problem, although it was really only obvious when the picture background was static -----fairly rapidly moving backgrounds masked it enough to be tolerable.
Oldmadham is offline   Reply With Quote
Old 23rd Oct 2019, 7:11 am   #56
G4YVM David
Heptode
 
G4YVM David's Avatar
 
Join Date: May 2014
Location: Salisbury, Wiltshire, UK.
Posts: 853
Default Re: Cretinous idea - The combined volume pot and mains SW.

Quote:
Originally Posted by Radio Wrangler View Post
Absolutely cretinous. stupid in two ways.

I realy do want a new word adding to the English language: "Pessimised"

Like optimised, but in the opposite direction.

David
Many decades ago I had a dream wherein I created the word "colomendy". I spent years searching for it but it doesn't exist except in my dream where it means "doing something for someone which they badly need but doing it in the worst possible way, on purpose" . For example, a librarian filing every book in the wrong place, a car mechanic disconnecting everything so the fix takes far, far longer or an electrical engineer deliberately mixing mains wires and volume pots...all are colomendic behaviours.

I will let you have my word for free.
__________________
David, G4YVM.
G4YVM David is offline   Reply With Quote
Old 23rd Oct 2019, 10:01 am   #57
Hybrid tellies
Nonode
 
Hybrid tellies's Avatar
 
Join Date: Jan 2004
Location: Bristol, UK
Posts: 2,404
Default Re: Cretinous idea - The combined volume pot and mains SW.

Most televisions from the 1960's and 1970's used combined on/off volume controls. It did not seem to cause to many problems with hum pickup even on some of the televisions that had a decent size and quality speaker fitted.
From the servicing point of view they could be a pain to change when the on off switch failed.
__________________
Simon
BVWS member
Hybrid tellies is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 8:29 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.