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Old 2nd Jul 2019, 12:59 pm   #1
G4CMY_Tony
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Default Looking For a Potentiometer With Switch ...

… where the slider reaches the end of the track before the switch starts to operate.

I am repairing a vintage radio receiver that is now working very well. However I have replaced the combined volume control and on/off switch with a modern pot and, unfortunately, the switch operates before the slider reaches the end of the track so minimum volume is not low enough.

Does anyone know where I can obtain a 750K (ideally) or 1M log pot with a shaft that is at least 35mm long and a switch that waits until the slider reaches the end before operating?

I would be grateful as this is driving me nuts!
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Old 2nd Jul 2019, 1:38 pm   #2
Ambientnoise
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Default Re: Looking For a Potentiometer With Switch ...

Can you try a 1M in series with the pot ? It will drop the signal by 6dB (min and max) which maybe acceptable while you search for a pot.

Ken
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Old 2nd Jul 2019, 1:44 pm   #3
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Default Re: Looking For a Potentiometer With Switch ...

try bowood https://www.bowood-electronics.co.uk...165ed24e986a52
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Old 2nd Jul 2019, 1:55 pm   #4
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Default Re: Looking For a Potentiometer With Switch ...

If you look at “where to get sets and parts” I have put the information there.
You will find Blore Edwards and or Dansette.com supply them.
Cheers
John
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Old 2nd Jul 2019, 1:56 pm   #5
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

Thanks to both for your suggestions.

I have put a resistor in series with the pot as a temporary solution. It helps but it still does not reduce the volume enough.

I bought the replacement pot from Bowood, who seem to be the only ones who can supply a suitable pot with a switch. However, the switches seem to be attached to the back of a standard pot so the switch mechanism needs to engage before the end of the track.

Shorting the slider terminal to the earthy end of the pot does kill the volume so I am convinced this is the problem.
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Old 2nd Jul 2019, 1:57 pm   #6
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Default Re: Looking For a Potentiometer With Switch ...

John - thank you for your reply - crossed in the post with my previous post.

I'll check that out.
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Old 2nd Jul 2019, 2:00 pm   #7
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Default Re: Looking For a Potentiometer With Switch ...

Blore Edwards are in South Wales worth looking at their web.
Dansett.com keep a small supply and are easy to deal with.
Cheers
John
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Old 2nd Jul 2019, 2:03 pm   #8
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

John

I have just ordered one from Dansette.

I checked the "Where to get parts" forum a few weeks ago and should have checked again as I must have just missed your post.

Thanks once again for your help.

I love this forum!

Tony
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Old 2nd Jul 2019, 2:12 pm   #9
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Default Re: Looking For a Potentiometer With Switch ...

Just a thought, the replacement pot was it a log or lin taper that you fitted?
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Old 2nd Jul 2019, 2:56 pm   #10
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

It was log - I have made that mistake before
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Old 2nd Jul 2019, 3:28 pm   #11
Ambientnoise
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Default Re: Looking For a Potentiometer With Switch ...

I wonder if it not the pot per se but crosstalk around it ? It may be worth checking/dressing the wiring layout. Have you measured the minimum useable resistance, could be useful to check against a replacement.

Ken
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Old 2nd Jul 2019, 3:46 pm   #12
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Default Re: Looking For a Potentiometer With Switch ...

The moulded carbon track pots with a carbon ‘ brush’, Morganite I think made them, suffered from this problem.
TV sets is the 1960’s that used them, RBM, the volume would not turn completely off but the volume was very low and it wasn’t an issue.
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Old 2nd Jul 2019, 3:48 pm   #13
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

Hi Ken

Thank you for your input.

The audio wiring around the pot is well screened and I have deliberately not disturbed that. I have replaced all of the wax decoupling capacitors but ensured that I wired up the replacements as closely as possible to the originals.

When you turn the volume down to minimum and then gently turn it further against the tension of the switch spring, the volume decreases further just before the switch activates. This leads me to believe that if the switch activated slightly later, the volume might reduce sufficiently by then.

In addition if you short the slider to the earthy end of the pot (as would be the case if the slider went all the way to the end) the volume is killed.

I have ordered a replacement so we will see if that cures the problem.

Tony
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Old 2nd Jul 2019, 4:34 pm   #14
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

Quote:
Originally Posted by Nuvistor View Post
The moulded carbon track pots with a carbon ‘ brush’, Morganite I think made them, suffered from this problem.
TV sets is the 1960’s that used them, RBM, the volume would not turn completely off but the volume was very low and it wasn’t an issue.
If the volume reduced just a little more on this one then it would be acceptable but it is not quite there.
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Old 2nd Jul 2019, 4:44 pm   #15
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

Quote:
Originally Posted by Ambientnoise View Post
I wonder if it not the pot per se but crosstalk around it ? It may be worth checking/dressing the wiring layout. Have you measured the minimum useable resistance, could be useful to check against a replacement.

Ken
I have just measured the minimum useable resistance and it is 7K (i.e. just before the switch actuates).

With the switch in the OFF position it measures as 0.
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Old 2nd Jul 2019, 5:49 pm   #16
Ambientnoise
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Default Re: Looking For a Potentiometer With Switch ...

So, 0.7% of the total 1M which does not sound high but useful data to compare against others found. On the other hand, it means the pot only gives a max attenuation of about 43dB, which does sound poor!

Ken
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Old 2nd Jul 2019, 6:16 pm   #17
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Default Re: Looking For a Potentiometer With Switch ...

Just checked some of my 1M switched pots ! Two linears and one log showed a few ohms, one other log showed about 500R. You are welcome to the good log (radiospares, long round plastic shaft, pcb pins on the pot, normal tags on the switch) if it helps.

Ken
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Old 2nd Jul 2019, 9:27 pm   #18
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Default Re: Looking For a Potentiometer With Switch ...

Hi Tony, I just wondered which receiver this is? If it has a diode-triode detector valve then sometimes if the cathode biassing is out it can be difficult to get a decent min volume level. Usually this is down to a bad cathode bypass electrolytic. Cheers, Jerry
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Old 3rd Jul 2019, 5:11 pm   #19
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

Ken - thank you for your kind offer. I have ordered a new one which I am hoping will cure the problem. However, if it does not then I'll send you a PM.

Jerry - the radio is a Pilot model U355 1937 vintage. It does have a diode / triode detector and I have replaced the cathode bypass capacitor. The cathode bias seems to be OK and the valves have been tested and check out OK . Thanks for your suggestion - all advice is very welcome.

Tony
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Old 7th Jul 2019, 9:43 am   #20
G4CMY_Tony
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Default Re: Looking For a Potentiometer With Switch ...

I am pleased to report that the pot from Dansette.com has cured the problem

Thanks to all for advice and information.

Tony
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