Broken pot switch bodge fix, is it safe?

[Vakito227]

Hello, recently the neutral switch on the top of the volume potentiometer has stopped working. I'm not really sure how to get inside it without breaking it so I have permanently bridged the gap. The live switch is still operational so it works as it did before but I don't know if it will be causing damage to the circuitry while it is off since I still keep it plugged in and switched on because the socket is behind a cabinet. Common sense would tell you that if the live switch is off then the circuit is not complete and nothing will happen but having a separate switch for both live and neutral must have a reason...

[Peter.N.]

I can't see that being a problem as long as you remember you only have a single pole switch, a mains switch only normally uses a live side single pole.

Some old TVs had the bottom of the brightness control connected to the neutral contacts to rapidly discharge the EHT by causing a sharp brightness rise but I can't see that being a problem in your case.

Peter

[Vakito227]

Originally Posted by Peter.N.

Some old TVs had the bottom of the brightness control connected to the neutral contacts to rapidly discharge the EHT by causing a sharp brightness rise but I can't see that being a problem in your case.

Oh no, it's for for an amplifier on a record player. Thanks for the help!

- Peter

[paulsherwin]

Nothing will be damaged by doing this but it isn't very good practice. You should really replace the pot, or dismantle it and repair it properly. It may just need some Servisol switch cleaner on the contacts.

[Argus25]

There was a reason two switches were used. It depends on the power plug systems and the chance of phase and neutral getting reversed. With two switches the phase always gets turned off even if the P & N fed to the set get swapped. If it is guaranteed that P & N will never get swapped, a switch just on the phase is fine. So the repair is fine only if it is definite that the remaining working switch is switching the phase always, and there is no chance it could be the neutral.

If neutral gets switched alone it is not good as phase is then still connected to the transformer primaries etc and if the chassis earth fails the chassis could go high even with the set's power supposedly turned off. Or a shock hazard still remains with the power switch in the off position which could confuse someone working on the set.

The fact that one switch has failed means most likely the other one is on the way out, so overall I think you should replace it anyway.

[broadgage]

Presuming UK use and connection via a 13 amp or other reliably polarised plug, I would not worry much.
Most UK appliances have only single pole switching in any case.

Reinstating double pole switching might be prudent if overseas use is contemplated.

Do double check that the functioning pole of the switch is in fact in the live and not in the neutral ! if the mains flex is terminated on the switch, then both cores look the same colour.

[HamishBoxer]

I agree Servisol or De-Oxit has cured plenty of o/c switches for me in the past.

[Vakito227]

I have used plenty of servisol super 10 on it and it hasn't made any difference. As for replacing it, do you know where to source them from? I don't know what make they are but it is a double stacked 0.5M ohm

Thanks

[paulsherwin]

You need to get the Servisol inside on the switch contacts. This often requires some dismantling. You may need to drill out some rivets, replacing them with small nuts and bolts.

I wouldn't worry too much about this - your repair isn't actually unsafe, just a bit of a bodge (but not a dangerous bodge so long as you are aware of it and don't do anything silly).

[Argus25]

Originally Posted by Vakito227

As for replacing it, do you know where to source them from? I don't know what make they are but it is a double stacked 0.5M ohm

If you do decide to replace it, sometimes it can be difficult to find the combination of the correct resistances, the switch type and the physical shaft dimensions of the original double gang control. One way is to just replace the switch assembly on the back of the control. Usually these come off complete with the metal shell. So one trick is to look around on ebay etc for any type of the same brand of pot (single or double gang any value) that has the same switch assembly, to transplant onto the double gang control you have.

[cosmocat]

I have fixed something similar before by using TE/CITEC pots sold by Farnell/CPC etc. The 23 / 27 range pots can be easily dismantled into their sub parts by bending back the tabs that hold them together. Any combination can be built up this way and fixed back together by re-bending the tabs. By a Pot with a switched fitted, and a double gang pot to get the joining shell and pots with the correct track values and types.
This way I built up a Dual Gang 1M log with mains switch.

I never trust a pot switch to turn off, I unplug the thing before doing any work. Have you noticed that modern power switches are marked ON only, that is because you can't ensure it is off.

[ms660]

Very wise.

Lawrence.

[dseymo1]

I've often thought that it would be useful to have a pot with a shaft extension at the rear, together with a cam and bracket system which allowed operation of microswitches as required, whether for mains or anything else.
Too over-engineered for the mass market, I suppose!

[Herald1360]

Sounds like the sort of exercise Tektronix might have got up to......

[Boater Sam]

How about a relay to switch the mains, use the remaining pot contact to switch the coil?

[Mooly]

I've seen the bodge of shorting one side of a switch far to many times as my days as a field tech. Its poor practice on both safety grounds and also on the fact that the other side of the switch probably isn't far behind.

Sorry, but I can't endorse this as good practice, and if its still the same kind of stuff as it used to be then flooding a switch with Servisol or similar is also a no no in my book.

I've seen the consequences of both to many times. Don't go there would be my advice.

[Aerodyne]

There's some good advice here and I urge you to take it. Do not leave this wiring in place. Either replace the pot complete or, if possible, do as suggested and exchange the switch section. Pots like this are hard to come by but there are firms that still make them, or make them to order, OMEG being one of them.
However if the thing has concentric control spindles in lieu of a balance control, you may have little choice and have to replace the switch unit. You might consider fitting a separate on-off switch.
Another less immediate possibility is to find a suitable replacement at one of the vintage radio meetings, as offered by BVWS and Radiophile.

Tony

[Argus25]

Originally Posted by Mooly

I've seen the bodge of shorting one side of a switch far to many times as my days as a field tech. Its poor practice on both safety grounds and also on the fact that the other side of the switch probably isn't far behind.

Bravo Mooly & Aerodyne, sanity prevails. As in my previous post above about the reasons why a double pole switch was used in the first instance. It is definitely a repair or replace scenario.