Unusual problem? Mains switch...

[kestrelmusic]

I've been repairing an Ultra Coronation Twin for a friend. It was dead - no HT or LT. Turned out to be both poles of the mains switch (ganged with volume control). Has anyone come across this problem before?

Incidentally, any advice on getting a 1Meg Lin pot with DPST switch? Looking on Ebay they seem to be a silly price.

[Boater Sam]

Try injecting switch cleaner and working it. Drill a small hole if you have to.
Or replace just the switch with one off an old pot.
Often they are just stuck, I leave all mine in the on position.

[Craig Sawyers]

I had exactly that with a Quad 33 preamp. Basically the contacts had burned through.

[julie_m]

Pot switches can fail short (usually due to welded contacts) or open (usually due to grease drying out and going sticky). The first case probably requires replacement; even if you can successfully prise the contacts apart, they now have roughened surfaces and eventually, inevitably will stick again. The second case is fixable with care. WD40 will appear to work at first; but will evaporate sooner or later, leaving the grease as sticky as ever.

Potentiometers are pretty modular, with many parts being interchangeable within a manufacturer's range, so it is not unfeasible to assemble a "Frankenstein's Monster" pot with the track taken from one and the switching mechanism, spindle and cam from another, different value one from the same range. So if a 1MΩ pot without a switch (are you sure it's linear? Volume controls are usually logarithmic) and a similar switched pot of another value work out cheaper, you could buy them and swap the bits over. You'd even have enough left over to make an unswitched pot of the original value!

[Graham G3ZVT]

When I got turned on a Cossor television set for the first time in 50 years smoke poured out of the on/off volume control. I thought it was the pot's track burning up, but no, it was the switch. After a minute or so it cleared up and the switch has been fine ever since.

The DP switch in my Bush TR82C had one pole of the switch that closes only when it feels like it, maybe every 5th operation. Obviously the cure was to bypass the faulty pole.

I suppose this is what endears these Bush sets to us, valve build ethic in an all transistor set.

[Nuvistor]

Try bowood electronics for a control.
https://www.bowood-electronics.co.uk

[emeritus]

I remember my power engineering lecturer observing that the switches fitted to pots had been designed for American 110V mains and so were not likely to be reliable on 240V mains. However, I haven't had any fail on any of my stuff.

[crackle]

I have found that a faulty on/off switch is fairly common.

Mike

[Nuvistor]

Originally Posted by emeritus

I remember my power engineering lecturer observing that the switches fitted to pots had been designed for American 110V mains and so were not likely to be reliable on 240V mains. However, I haven't had any fail on any of my stuff.

Unless I misunderstood, all the V/C with switch that I came across in fixing radios, TV’s had the switches labelled 250v, and very often made in the U.K.

[Nanozeugma]

Originally Posted by Nuvistor

Try bowood electronics for a control.
https://www.bowood-electronics.co.uk

Seconded. The only place I could source a 1 Meg Log pot with a DPST switch and a decent length shaft for a fair price.
When I was on the hunt for one, I quickly discovered they are now like hen's teeth with prices to match. Back in the day, they were as common as muck.

[Station X]

I've come across this, but with the switch stuck on rather than off. Easily cured by careful dismantling, cleaning and reassembling.

[emeritus]

Unfortunately I didn't enquire further about my my lecturer's comments, which were directed at the switches fitted to contemporary UK TVs, so don't know if he had actually done any research into it or whether his view was based on the mode of construction. This was in the late 1960's.

[AC/HL]

Anything mechanical will have wear problems eventually. The last couple of switching faults I encountered were relays, much the same problem.

[HamishBoxer]

Yes,cleaner works Most times on o/c mains switches.

[Nuvistor]

I changed many more volume controls/on off switches due to worn out noisy volume controls than failed switches.
The main reason I had for a faulty switch was damage due to the capacitor across the mains input going s/c.

[mark2collection]

Originally Posted by kestrelmusic

Incidentally, any advice on getting a 1Meg Lin pot with DPST switch? Looking on Ebay they seem to be a silly price.

RS have them, look up part number 387-880 or manufacturers part number 1624204-2, made by TE Connectivity. They were around £5 ...

Have used these a few times. Occasionally you have to modify the locating/locking lug on the pot, as it may not align with the original hole on the chassis.

Mark

[G0HFCFrank]

Why fit a double pole switch at all? It has been standard practice for some years now to only switch the live side of the mains.
Frank C.

[Silicon]

Another failure mode of the mains switches attached to the back of vintage potentiometers is displacement of the quick make/ quick break toggle mechanism.

The range of movement is usually limited by metal tangs pressed out of the metal cover of the potentiometer.

Sometimes the moving parts slip past the tangs and that stops the switch from working.

I have removed the plastic cover of the switch, flipped the toggle mechanism over and re-attached the plastic cover. This fixed the problem in the short term.

I don’t know why the moving parts slipped past the metal tangs.

[ms660]

Re: Post 17. Looking at a schematic, one's the neutral/-ve switch the other is a filament supply switch.

Lawrence.

[G0HFCFrank]

Thanks Lawrence, I had seen "mains switch on" several earlier posts and forgot to engage brain before typing.

Frank C.