VC capacitor voltage - UK Vintage Radio Repair and Restoration Discussion Forum

Malcolm T · 13th Mar 2023, 11:31 am

What is the approximate break down voltage of the common variable capacitor found in AM transistor radios usually from the 1960s / 70s for main tuning , the type that has the U bent steel frame and usually twin gang. ?

CambridgeWorks · 13th Mar 2023, 12:01 pm

In the past I tested some with a 500V megger. About half would spark across here and there. An inspection to see if plates could be bent slightly for a remedy sometimes worked if it was an outer one.
However, some of the sparking was also it seems due to dust accumulation.
I am referring to the two gang with a U frame about 50mm long.
Rob

Malcolm T · 13th Mar 2023, 1:06 pm

Thats the ones, Yes the dust and grease is a problem, theres no way to get them apart though, not that i can see. Thanks for the voltage info.

G6Tanuki · 15th Mar 2023, 11:29 am

They'll generally handle about 500V. One particular degradation I have come across with these is very small, localised raised lumps of whitish powdery oxide - possibly caused by some sort of inclusions of impurity in the aluminium. The raised bumps can be enough to cause a short between the plates, often intermittent, coming and going as the capacitor is adjusted.

A thin feeler-gauge worked in between the plates canburnish-away the peaks of hese little lumps and restore functionality.

Philips210 · 15th Mar 2023, 3:42 pm

Quote:

Originally Posted by G6Tanuki

A thin feeler-gauge worked in between the plates canburnish-away the peaks of hese little lumps and restore functionality.

Good tip that, thanks!

Regards,
Symon

kalee20 · 15th Mar 2023, 4:02 pm

Up to 500V breakdown, though in use I'd not want to expose to more than 200V.

And I'd want to ensure that in the event of a breakdown (more likely while the vanes are being turned), the HT line isn't shorted without there being a limiting resistor.

Variable capacitors for transmitters are generally made with much wider spacing, to handle DC plus peak RF voltage.

13th Mar 2023, 11:31 am	#1
Malcolm T Hexode Join Date: Dec 2018 Location: Lugo, Spain Posts: 483	VC capacitor voltage What is the approximate break down voltage of the common variable capacitor found in AM transistor radios usually from the 1960s / 70s for main tuning , the type that has the U bent steel frame and usually twin gang. ?

13th Mar 2023, 12:01 pm	#2
CambridgeWorks Nonode Join Date: Jun 2010 Location: Spalding, Peterborough, Cambridgeshire, UK. Posts: 2,859	Re: VC capacitor voltage In the past I tested some with a 500V megger. About half would spark across here and there. An inspection to see if plates could be bent slightly for a remedy sometimes worked if it was an outer one. However, some of the sparking was also it seems due to dust accumulation. I am referring to the two gang with a U frame about 50mm long. Rob __________________ Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd

13th Mar 2023, 1:06 pm	#3
Malcolm T Hexode Join Date: Dec 2018 Location: Lugo, Spain Posts: 483	Re: VC capacitor voltage Thats the ones, Yes the dust and grease is a problem, theres no way to get them apart though, not that i can see. Thanks for the voltage info.

15th Mar 2023, 11:29 am	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: VC capacitor voltage They'll generally handle about 500V. One particular degradation I have come across with these is very small, localised raised lumps of whitish powdery oxide - possibly caused by some sort of inclusions of impurity in the aluminium. The raised bumps can be enough to cause a short between the plates, often intermittent, coming and going as the capacitor is adjusted. A thin feeler-gauge worked in between the plates canburnish-away the peaks of hese little lumps and restore functionality. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

15th Mar 2023, 4:02 pm	#6
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: VC capacitor voltage Up to 500V breakdown, though in use I'd not want to expose to more than 200V. And I'd want to ensure that in the event of a breakdown (more likely while the vanes are being turned), the HT line isn't shorted without there being a limiting resistor. Variable capacitors for transmitters are generally made with much wider spacing, to handle DC plus peak RF voltage.