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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 Oct 2019, 6:41 pm   #21
Electronpusher0
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Originally Posted by Argus25 View Post

Some manufacturers like Tektronix & Philips got around this issue in their instruments by having the power button connected to a long plastic arm and the mains switch itself nearer the rear of the instrument.
I plan to do something similar in my next preamp project. Mount a rotary mains switch on an L shaped bracket near the back of the enclosure. Use a section of 6mm plastic knitting needle as the extension connected by sleeve to the switch.
Bring the plastic shaft out of the front panel by dismantling an old pot and just use the front panel mounting part with the 6mm hole in it to guide the knitting needle. Mount a knob on the needle to match the other controls.

Peter
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Old 12th Oct 2019, 6:51 pm   #22
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I agree - combined power switches and volume controls are a stupid idea. One glorious side effect on a hi-fi amplifier is that you can never turn the volume fully down.

Still, a friend and I considered the situation a few years back and concluded that we couldnít come up with the name of any decent amplifiers that use this daft switch system!
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Old 12th Oct 2019, 6:54 pm   #23
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Quad managed to put a mains switch ganged to the volume control in the 33, and right next to the disc input circuit board (admittedly with an aluminium sheet screen) without any hum issues.

The Quad 22 valved control unit did the same thing, but had a soldered on tubular extension to the pot around the switch, so effective screening seemed to be more of an issue in that case.

But that is the exception rather than the rule.

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Old 12th Oct 2019, 7:09 pm   #24
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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... The Quad 22 valved control unit did the same thing, but had a soldered on tubular extension to the pot around the switch, so effective screening seemed to be more of an issue in that case ...
Don't tell anyone but that's the unit I was referring to in my post #8 above. It only switches one pole of the mains and if you plug the (unpolarised) 5A connector in the 'wrong' way round i.e. with the Live switched rather than the Neutral then the unit doesn't meet its hum spec. The circuit's here http://www.geocities.ws/ResearchTria...quad22cirb.jpg.

Cheers,

GJ
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Old 12th Oct 2019, 7:27 pm   #25
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Going back to the original statement I would like to comment upon a not unrelated issue associated with my Musical Fidelity M3Si amp. In this I use the line out as the input on my Dynaco ST-70 valve amp but it creates a hum with power on to both units but with the MF3Si not yet switched on. Itís not a particularly loud hum but I dare not remove the cover to see if anything could be rearranged as it will compromise the warranty.
I contacted MF who responded suggesting I box up the unit and send it back to them at my cost as they were unaware of the reported issue. I decided instead to invest in a NAD 1020B preamp I saw on the internet; there is no hum whatsoever from that.
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Old 12th Oct 2019, 8:16 pm   #26
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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?
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Old 12th Oct 2019, 8:20 pm   #27
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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I plan to do something similar in my next preamp project. Mount a rotary mains switch on an L shaped bracket near the back of the enclosure. Use a section of 6mm plastic knitting needle as the extension connected by sleeve to the switch.
Bring the plastic shaft out of the front panel by dismantling an old pot and just use the front panel mounting part with the 6mm hole in it to guide the knitting needle. Mount a knob on the needle to match the other controls.

Peter
I don't think it's necessary to butcher a pot to do this, especially if the pot has some life in it. You could cobble together a "bearing" for the knitting needle out of a suitable piece of tubing, metal or plastic would do. This can be an interference fit or maybe glued into the front panel with a bit of Araldite. I did a similar thing years ago when I made a 'scope using a VCR97 crt and a circuit from "Practical Wireless" when I was at school (yonks ago). Both the focus and intensity controls were connected this way.

Colin.
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Old 12th Oct 2019, 10:44 pm   #28
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Default Re: Cretinous idea - The combined volume pot and mains SW.

As you say, there's no need for butchering. In fact there isn't even any need for cobbling. Control shaft extenders have been available to buy for ages. Here's one https://www.hificollective.co.uk/pot...nsion_kit.html. There are lots more on eBay.

Cheers,

GJ
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Old 13th Oct 2019, 1:12 pm   #29
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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As you say, there's no need for butchering. In fact there isn't even any need for cobbling. Control shaft extenders have been available to buy for ages. Here's one https://www.hificollective.co.uk/pot...nsion_kit.html. There are lots more on eBay.

Cheers,

GJ
Hmm, yes GJ, at a price (£13.50 + VAT + P&P. Whenever I see "HiFi Collective" I hold my wallet close to my chest.
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Old 13th Oct 2019, 2:37 pm   #30
Craig Sawyers
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Well, you get a brass coupling collar, a 300mm extension shaft, an L bracket for the pot etc with pressed in M3 bushes, and a brass front panel bush to support the shaft.

Now you might be able to find the bits for this by shopping around (I tried Farnell, RS, Mouser, Rapid, Digikey and failed), but for a kit of bits to do the job I don't think that don't think that HiFi Collective's price is at all ridiculous.

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Old 13th Oct 2019, 4:32 pm   #31
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I can't help thinking that the repurposing of an otherwise scrap old pot is a good way for at least part of it to cheat the WEEE monster. It would have made a good item for one of the old magazine hints and tips features for those of us who remember them.
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Old 13th Oct 2019, 5:04 pm   #32
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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Hmm, yes GJ, at a price (£13.50 + VAT + P&P. Whenever I see "HiFi Collective" I hold my wallet close to my chest.
Colin.
That's why I mentioned the eBay ones Colin .

Even the HFC price is just 100 minutes at the minimum wage. True, plus VAT and P&P, but minus the value of any bits you had to source.

The point I was really making is that shaft extenders are a recognised approach. When all of the power and signals enter through the rear panel it really doesn't make sense to put the actual controls for them on the front. The actuators for those controls do need to be there though.

Cheers,

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Old 13th Oct 2019, 5:45 pm   #33
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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I can't help thinking that the repurposing of an otherwise scrap old pot is a good way for at least part of it to cheat the WEEE monster. It would have made a good item for one of the old magazine hints and tips features for those of us who remember them.
"Dick and Smithy in the Workshop" to the rescue!

In the past I've bored-out quite a few old potentiometer bushes for this sort of thing, and yes, knitting-needles are a good cheap source of suitably-sized shafting.

Another tip: a short length of the stiff Nylon tube used in industrial pneumatics/robotics makes a good spindle-coupler: heat it in boiling water to soften it and you can then force-fit 1/4-inch/6mm spindles into the ends. This is a good way to deal with diameter-mismatches when you've got a mix of 1/4-inch/6.3mm and 6mm - something that doesn't always work out to be smooth-running when you use solid couplers.
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Old 13th Oct 2019, 5:55 pm   #34
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Default Re: Cretinous idea - The combined volume pot and mains SW.

OK proof of concept. All from the odds and sods bin.
Old pot front mount as guide bush,
6mm aluminium rod from B&Q but a knitting needle will do just as well.
Standard on/off switch
Bracket from aluminium angle
Heatshrink tubing as coupler, the type that is lined by hot melt glue. This has just enough give to allow for slight mis-alignment but no rotational give if you have the rod and switch butted close together.

Peter
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Old 13th Oct 2019, 7:14 pm   #35
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Default Re: Cretinous idea - The combined volume pot and mains SW.

I found a nice rounded chromed bushing which had once been a part of a kit for mounting a short-shafted ordinary pot as a good-looking panel mounted trimmer in my junk box. It now supports a length of quarter inch nylon rod. The far end is drilled to fit onto the a push-push on/off switch where its button would have gone. The bushing is on the front panel of my preamp and the carefully polished end of the nylon rod protrudes as my mains on/off button. THe end result looks neat, it feels right and all the mains is kept in the PSU area near the back panel.

You don't have to use a rotary switch.

David
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Old 13th Oct 2019, 7:21 pm   #36
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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I found a nice rounded chromed bushing which had once been a part of a kit for mounting a short-shafted ordinary pot as a good-looking panel mounted trimmer in my junk box. It now supports a length of quarter inch nylon rod. The far end is drilled to fit onto the a push-push on/off switch where its button would have gone. The bushing is on the front panel of my preamp and the carefully polished end of the nylon rod protrudes as my mains on/off button. THe end result looks neat, it feels right and all the mains is kept in the PSU area near the back panel.
The geek in me would want to see if I could use the nylon rod as a light-guide, fitting LEDs around the behind-the-panel part, so the polished end glowed when the power was on.

Last edited by G6Tanuki; 13th Oct 2019 at 7:33 pm.
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Old 13th Oct 2019, 7:22 pm   #37
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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... panel mounted trimmer in my junk box.
David
Why would you have a trimmer mounted in your junk-box? Sorry, I'm being facetious - I'll get my coat..
Colin.
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Old 13th Oct 2019, 8:40 pm   #38
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Default Re: Cretinous idea - The combined volume pot and mains SW.

Yes, the idea of bringing 50Hz 240V right up to the signal is just plain daft! Irrespective of the extra wear on the potentiometer track.

I tend to use a switch on the power point to switch on and off any equipment with a combined switch and potentiometer. It avoids the second problem although there is still the first.

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So if there was ever a case where a rotary control should be connected with an on-off power switch, it would be to put the power switch on the CRT's brightness control, in a TV, so it was zero when the set was switched off to suppress the white spot.

Of course designers wised up to this, and in better designs had "spot killers" that take the CRT's grid very negative at turn off, to prevent the white spot and phosphor burn, regardless of the angular position of the brightness control.
Though, some TV's did exactly the opposite - take the CRT grid to zero bias, or even to a positive potential, at switch-off. I recall a Mullard article where the potential divider for the CRT grid, instead of going to circuit 0V, was taken to the supply-side of the two-pole mains switch in the neutral line. When the switch was turned off, the CRT grid would be fed with raw mains, albeit with a lot of circuit impedance in series.

The idea was, to get a really high beam current, maximum brightness, while there was still enough voltage left in the reservoir and smoothing capacitors to run the line and frame scan. So the tube's EHT capacitance would discharge rapidly due to the high beam current, and by the time the scans collapsed there would be no EHT left to produce a spot.
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Old 13th Oct 2019, 9:09 pm   #39
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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The geek in me would want to see if I could use the nylon rod as a light-guide, fitting LEDs around the behind-the-panel part, so the polished end glowed when the power was on.
Oh Crikey! that's torn it! I'm going to have to do that now, aren't I?

There just weren't bright enough LEDs in 1981

It's white nylon, but I ought to be able to get a glow going.... but I might have a bit of perspex rod somewhere.

David
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Old 13th Oct 2019, 9:13 pm   #40
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Default Re: Cretinous idea - The combined volume pot and mains SW.

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... panel mounted trimmer in my junk box.
David
Why would you have a trimmer mounted in your junk-box? Sorry, I'm being facetious - I'll get my coat..
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