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Old 19th Oct 2017, 3:24 pm   #1
Neil Purling
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Default Kit radio potentiometer question

I have one of those anonymous kit radios.
The switch on the volume control arcs on switch-off.
The track is only 10K as it works in the RF amp stage.
Would it then the a linear track, or logrithmic?
Those of you who have one could perhaps tell me.
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Old 19th Oct 2017, 3:29 pm   #2
crackle
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Default Re: Kit radio potentiometer question

Will the switch come apart to clean the contacts.

Mike
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Old 19th Oct 2017, 4:39 pm   #3
Neil Purling
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Default Re: Kit radio potentiometer question

It is still in the radio at present. The back of the switch is what looks like paxolin.
I don't know if there are any natural holes or if I can drill the paxolin to introduce the spray.
That would be the last resort in case it introduced further contamination.
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Old 19th Oct 2017, 4:48 pm   #4
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Default Re: Kit radio potentiometer question

If cleaning doesn't solve the problem with the arcing you could always connect a 'snubber' R-C network across the switch contacts.
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Old 19th Oct 2017, 5:04 pm   #5
Neil Purling
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Default Re: Kit radio potentiometer question

I just got the radio chassis out of the loft. Here is the switch.
Does a logrithmic track have a fast increase at the early part of the track that slows.
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Old 19th Oct 2017, 5:05 pm   #6
Boater Sam
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Default Re: Kit radio potentiometer question

I have found these paxolin backed QMB mains switches to be anything but and quite poor.
I think you may be lucky if you spray it but let it dry out completely before applying mains else the paxolin may track and burn. The pot can be lin.
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Old 19th Oct 2017, 5:14 pm   #7
M0FYA Andy
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Default Re: Kit radio potentiometer question

I think RF gain controls are usually linear.
Andy
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Old 19th Oct 2017, 6:40 pm   #8
Neil Purling
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Default Re: Kit radio potentiometer question

Apparently if you turn the shaft half way through it's travel and measure the resistance a linear track obviously give 50% of the stated value of the pot. A logarithmic pot will be only 10-20% of the total.
As seen by actual measurement the pot has a linear track..
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Old 19th Oct 2017, 6:54 pm   #9
bigfathairyvika
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Default Re: Kit radio potentiometer question

I thought snubbers fitted across the coil?

Name or source of kit would help.

Mark
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Old 19th Oct 2017, 6:56 pm   #10
Boater Sam
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Default Re: Kit radio potentiometer question

If an AC Quick Make and Break switch is arcing, I would consider it Knac***ed.

But is it switching AC or DC?

DC arcing is always a possibility unless there is a snubber to quench the arc.

Last edited by Boater Sam; 19th Oct 2017 at 6:59 pm. Reason: added
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Old 19th Oct 2017, 7:04 pm   #11
Neil Purling
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Default Re: Kit radio potentiometer question

Squirting the switch seems to have worked - for now. I sprayed the track to get rid of the crackles long ago.

Now i'll have to get off my bum and make a case. The bakelite original was smashed beyond repair

Last edited by Neil Purling; 19th Oct 2017 at 7:10 pm.
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Old 19th Oct 2017, 7:20 pm   #12
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Default Re: Kit radio potentiometer question

Quote:
Originally Posted by bigfathairyvika View Post
I thought snubbers fitted across the coil?
I've always put snubbers across the place where the arcing is taking place [it's anti-back-EMF diodes that go across the coil]

Something like: http://uk.rs-online.com/web/p/rc-net...itors/6727032/

Avoid the "KEMET" ones - they're made using the same failure-prone technology as the eternally-problematic "RIFA" filter-capacitors whose baleful influence is documented on this forum with only-too-common frequency.
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Old 19th Oct 2017, 7:38 pm   #13
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Default Re: Kit radio potentiometer question

Ah.. yes across the switch for DC and across the inductor for a/c.
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Old 19th Oct 2017, 8:44 pm   #14
G8HQP Dave
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Default Re: Kit radio potentiometer question

Not necessarily. A capacitor across an AC mains transformer primary can be just as effective as one across the switch, but with the advantage that it lasts longer because it only is exposed to AC and spikes when the equipment is switched on. It also means that Off means Off.
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Old 19th Oct 2017, 9:18 pm   #15
bigfathairyvika
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Default Re: Kit radio potentiometer question

Yup i agree , exactly what i said.
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Old 20th Oct 2017, 11:35 am   #16
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Default Re: Kit radio potentiometer question

Neil, if it is a bakelite pilot little maestro look-alike, I have a case you can have foc. Getting it to you could be a problem as I doubt it is worth the cost/risk of posting.

ken
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Old 20th Oct 2017, 1:01 pm   #17
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Default Re: Kit radio potentiometer question

A very big, stout, double-walled cardboard box nabbed from a shop's recycling bin, plenty of tightly scrunched-up newspaper balls and a booking with a cheap courier like Hermes should get it to Neil for well under a tenner without too much risk of it being ruined, especially if you double-box.
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Old 20th Oct 2017, 1:56 pm   #18
Neil Purling
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Default Re: Kit radio potentiometer question

Ambientnoise: You have a PM.
With regard to what G8HQP Dave said in post #14: I already have a few 0.1uf 1000V PIO and some 0.1 uf X2.
I was thinking that arcing was not helped by the inductance of the heater transformer primary winding & lazy contact springs, but it has gone quiet now.
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